Hydroperoxide Oxidation of Difficult-to-Oxidize Substrates: An Unprecedented C-C Bond Cleavage in Alkanes and the Oxidation of Molecular Nitrogen

Article abstract of DOI:10.1023/B:KICA.0000016108.09724.69

The reactions of n-alkanes (C₅-C₂₀), isoalkanes (2,2,4-trimethylpentane (isooctane), 2-methylbutane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,2-dimethylbutane (neohexane), fatty acid methyl esters, and molecular nitrogen were studied to reveal the nature of processes that form the basis of alkane oxidation in the V^(V)/H₂O₂/AcOH system. The results were compared to previous data on cyclohexane oxidation. Molecular nitrogen underwent oxidation in the V^(V)H₂O₂/CF₃COOH catalytic system with the formation of N₂O. A vanadium complex that oxidizes molecular nitrogen was formed by the interaction of H₂O₂ with the catalyst. In the reaction of hydrogen peroxide with vanadium complexes, active intermediates were formed that could react with substrates that are difficult to oxidize with oxygen for thermodynamic reasons or because of a high activation barrier. The proposed reaction schemes were consistent with the experimental facts observed.

Full text of DOI:10.1023/B:KICA.0000016108.09724.69

Kinetics and Catalysis, Vol. 45, No. 1, 2004, pp. 40–60. Translated from Kinetika i Kataliz, Vol. 45, No. 1, 2004, pp. 45–66.
Original Russian Text Copyright © 2004 by Gekhman, Stolyarov, Ershova, Moiseeva, Moiseev.
Hydroperoxide Oxidation of Difficult-to-Oxidize Substrates:
An Unprecedented C–C Bond Cleavage in Alkanes
and the Oxidation of Molecular Nitrogen1
A. E. Gekhman*, I. P. Stolyarov*, N. V. Ershova*, N. I. Moiseeva**, and I. I. Moiseev*
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 117907 Russia
*
*
* Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991 Russia
Received December 20, 2002
Abstract—In the V(V)H O /AcOH system, C –C n-alkanes, isooctane, and neohexane undergo oxidation to
2
2
5
20
ketones and alcohols; the oxidation products of branched alkanes are indicative of a C–C bond cleavage in these
substrates. A concept is developed, according to which the peroxo complexes of vanadium(V) are responsible
+
.
for alkane oxidation. These complexes can transfer the oxygen atom or the O radical cation to a substrate.
(V)
The formation of nitrous oxide was found in the oxidation of molecular nitrogen in the H O /V /CF COOH
2
2
3
system.
1
INTRODUCTION
The oxidation of alkanes by hydroperoxides in the
presence of transition metals has been known since the
In a discussion of the nature of conceivable oxidiz-
ing agents responsible for reactions with organic sub-
(V)
strates in the V /H O /AcOH system, a number of
2
2
report of Fenton [1]; as a rule, it occurs via a radical vanadium(V) complexes should be considered. These
chain mechanism. However, the reactivity of hydroper- complexes appear in the course of hydrogen peroxide
oxides and metal complexes depends on the nature of
the solvent, the metal, and the ligand environment
decomposition in this system and include from one to
2
–
0
four peroxo groups O depending on the reaction con-
2
[
2−4]. Thus, the use of metals with the d nonbonding
configuration opens up possibilities for the molecular ditions [13–19]. Some of these complexes, such as
paths of peroxide activation [2–5].
(V)
–
monoperoxo ([(O) V (O )] ) and diperoxo complexes
2
2
In carboxylic acid media containing vanadium(V)
compounds, the one-electron reduction of the central
atom by hydrogen peroxide is not the main reaction
(V)
–
(
[O=V (O ) ] ), were detected on the decomposition
2 2
of H O in AcOH with the use of NMR spectroscopy of
2
2
2
path of H O decomposition in the absence of ligands vanadium nuclei [6, 7]. Moreover, kinetic data are
2
2
that stabilize vanadium(V) [6, 7].
It is well known that vanadium(V) compounds in with three peroxo groups, [V (O ) ] , which is con-
2 3
indicative of the intermediate formation of a complex
(V)
–
AcOH catalyze H O decomposition with the formation verted into a vanadium(V) complex with singlet dioxy-
2
2
of singlet dioxygen (reaction (I)) and small amounts of
3
gen at a rate-limiting step [9–11]. Singlet dioxygen is
3
ozone (reaction (II)) in addition to O [8–11]:
2
responsible for the oxidation of anthracene and its deriva-
tives to corresponding anthraquinones and for the oxidative
degradation of olefins to the corresponding carbonyl com-
1
2
3
H O
2H O + O ,
(I)
2
2
2
2
2
H O
2H O + O .
(II) pounds [6, 7, 9, 10, 20–23] (Scheme 1). Within the frame-
2
2
3
work of this scheme, a mechanism was proposed for ozone
formation; this mechanism includes the intermediate for-
The simultaneous decomposition of AcOOH and
2
H O in an AcOH solution in the presence of vana-
2
dium(V) complexes is the oxidation of H O by
2
2
2
There is no data on the composition of the coordination sphere,
AcOOH, which is a stronger oxidizing agent [12],
2
2
–
except for kinetic or spectroscopic evidence of O2– and O
H O + AcOOH O + H O + AcOH.
ligands. Therefore, the other ligands are omitted from the formu-
2
2
2
2
las of the complexes. It is likely that H O or AcOH molecules
2
occupy vacant sites in the coordination sphere of vanadium(V).
1
Based on the plenary lecture delivered by A.E. Gekhman, corre-
sponding member of the Russian Academy of Sciences, at the
VI Russian Conference “Catalytic Reaction Mechanisms” (Octo-
ber 1–5, 2002, Moscow).
3
Henceforth, the term vanadium(V) complex with singlet dioxy-
gen refers to an intermediate responsible for the transfer of the
1
O species to a substrate.
2
0
023-1584/04/4501-0040 © 2004 MAIK “Nauka /Interperiodica”

HYDROPEROXIDE OXIDATION OF DIFFICULT-TO-OXIDIZE SUBSTRATES
41
–
O
O O
slowly
O
V O
O
L
O3
(A)
–
–
–
O
O
O
O
V
L
O
O
O
O
H O
H O
O
O
O
2
2
2 2
–
V O + H O
O V O
V
2
2
–
H O
–H O
–H O
2
2
2
O
O
O
O
L
L
L
(B)
O
V
O–
slowly
O
O2
1O2
O
L
k2
, olefins
O
O
Scheme 1. Mechanism of H O decomposition with the formation of ozone.
2
2
mation of a vanadium(V) complex with the trioxo ligand alkanes (pentane, hexane, heptane, octane, nonane,
2
3
–
dodecane, hexadecane, and eicosane, all of 98% or bet-
ter purity according to GLC analysis), isooctane (refer-
ence, GOST 4374-48), 2-methylpentane, 3-methylpen-
tane, 2,3-dimethylbutane, 2,2-dimethylbutane (98–
O
before a rate-limiting step [9, 10].
The oxidation of saturated substrates such as cyclo-
hexane and cyclopentane, which are not prone to reac-
tions with ozone and singlet dioxygen, results in corre-
sponding cycloalkanols and cycloalkanones [24].
9
9%, Aldrich), NH VO (chemically pure), bis(trime-
4 3
thylsilyl)trifluoroacetamide (BSTFA, Merck), metha-
(pure), ionol (2,6-di-
V /H O /AcOH system complicates the mechanistic tert-butyl-4-methylphenol, chemically pure), and
A wide variety of active oxidizing agents in the nol (chemically pure), LiAlH
4
(V)
2
2
studies of reactions that occur in this system. In this molecular nitrogen (high-purity grade). Commercial
work, in order to reveal the nature of processes that VO(acac) was purified by recrystallization from
form the basis of alkane oxidation in the CHCl . Fatty acid methyl esters (FAMEs) were synthe-
V /H O /AcOH system, we studied the reactions of sized from the corresponding carboxylic acids (all of
2
3
(V)
2
2
normal alkanes (C –C ), isoalkanes (2,2,4-trimethyl- 98% or better purity according to GLC analysis)
5
20
pentane (isooctane), 2-methylbutane, 2-methylpentane, according to a published procedure [25].
3
-methylpentane, 2,3-dimethylbutane, and 2,2-dimeth-
ylbutane (neohexane)), fatty acid methyl esters, and
molecular nitrogen. The results of this study are com-
pared to previous data on cyclohexane oxidation.
Preparation of a Catalyst Solution
To prepare an AcOH solution containing vana-
dium(V) compounds, 100 mg of finely ground NH VO
4
3
was refluxed in 100 ml ofAcOH for 4–5 h. Undissolved
EXPERIMENTAL
NH VO was filtered off. According to ICP MS data,
4
3
Materials
the concentration of vanadium(V) in solution was 3.7 ×
–
3
The following chemicals were used without prelim- 10 mol/l. The ICP MS spectra were measured on an
inary purification: glacial acetic acid (chemically pure), HP 4500 ICP MS instrument (an aqueous solution of
hydrogen peroxide (analytical grade, a 16.2 mol/l aque- NH VO was used as an external standard). In some
4
3
ous solution), commercial trifluoroacetic acid, linear kinetic experiments, VO(acac) , which is more soluble
2
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

4
2
GEKHMAN et al.
in AcOH, was used as the starting compound for a cat- identified by coincidence between the chromatographic
alyst. According to EPR data, the signal of V(IV) from retention times and mass spectra of sample components
VO(acac) disappeared immediately after the addition and standard substances.
2
of the first portion of H O . The replacement of
2
2
To study complex mixtures (in the case of 3-methyl-
pentane or isooctane oxidation), a solution of oxidation
VO(acac) by NH VO had no effect on the composi-
2
4
3
tion and distribution of reaction products and on the
rate of the reaction.
products in diethyl ether was exposed to LiAlH for the
4
exhaustive reduction of ketones and esters to the corre-
sponding alcohols. Based on a subsequent comparison
of the chromatograms of initial and reduced samples
obtained using chromatographic columns of different
polarity, as well as the mass spectra, conclusions on the
Experimental Procedure for Oxidation
and Analysis of Products
The oxidation of linear and branched alkanes and structures of sample components were made. In the
FAMEs was performed in a thermostatted jacketed cases when the retention times and mass spectra of
glass reactor with a magnetic stirrer at 30°C. In a typical components were similar (for example, for 2,4,4-trime-
experiment, 0.3 ml (4.86 mmol) of aqueous H O
thyl-1-pentanol and 2,2,4-trimethyl-1-pentanol), an
16.2 mol/l) was added to a solution of VO(acac)2 alpha-Dex 120 enantioselective chromatographic col-
5 mg, 0.019 mmol) and n-hexane (43 mg, 0.5 mmol) in umn was used. Because oxidation is not enantioselec-
ml of glacial acetic acid. The decomposition of H O
2
2
(
(
5
tive and, of the above two isomers, only 2,4,4-trime-
2
2
was monitored by iodometry. The conversion of sub- thyl-1-pentanol has an asymmetric center, the chro-
strates and the yields and compositions of reaction matogram of this alcohol exhibits two chromatographic
products were determined by GLC and gas chromatog- peaks with equal intensities, and the mass spectra of
raphy–mass spectrometry (GC–MS).
these peaks are identical.
To determine the substrate conversion, 0.1 ml of an
An AUTOMASS 150 GC–MS instrument (Delsy
internal standard solution (0.1 mol/l acetonitrile in ace- Nermag, France) was used for obtaining mass spectra
tic acid) was added to 0.1 ml of the reaction mixture. under the following conditions: EI = 70 eV; the ion
The resulting mixture (0.5 µl) was analyzed by chroma- source temperature was 100°C; the mass spectrometer
tography.
was adjusted using FC43 (reference peaks of m/z 18,
8, 69, 100, 131, 219, 264, 414, and 502); the spectrum
range was 30–300 amu; the scan time was 300 ms; a
fused-silica capillary column (0.25 mm × 25 m; d =
2
Sample preparation for the identification of alkane
oxidation products was performed as follows: After the
complete consumption of H O , 1 ml of CHCl (or
f
2
2
3
0
.3 µm) (Rescom) with OV-1, CPWax-58, or alpha-
diethyl ether) and 5 ml of water were added to 1 ml of
the reaction mixture. The organic phase was separated,
washed with a 5% aqueous Na CO solution (1 ml) and
Dex 120 was used; the split ratio was 1/40; Pinj
.6 atm (at Tcol = 50°C); He; Tinj = 250°C. The Lucy
=
0
(
2
3
ver. 2.10) andAMDIS (ver. 2.1) software and the NIST
water (1 ml), and analyzed by chromatography.
mass-spectral database (1987, 1995) were used for
GC–MS data acquisition and processing.
As an example, Fig. 1 demonstrates a portion of the
chromatogram of isooctane oxidation products. Figure 2
illustrates the identification of one of the oxidation
products with a retention time of 159 s (4,4-dimethyl-
-pentanol) with the use of a reference spectrum (from
the NIST mass-spectral database (1987, 1995)).
The quantitative GLC analysis of the initial sub-
The above procedure for the extraction of reaction
products from reaction solutions did not allow us to
determine quantitatively the concentrations of all com-
ponents because it is impossible to extract completely
water-soluble components (such as ethanol and 2-pro-
panol) from aqueous solutions into diethyl ether. In
experiments with isooctane, the above sample prepara-
tion process was changed. After the extraction of oxida-
tion products with diethyl ether, the resulting extract
2
was concentrated by slowly evaporating the ether at stances and oxidation products was performed on a
~
20°C in a flow of nitrogen; this prevented the evapo- 3700 chromatograph (Russia) equipped with an HP-1
ration of highly volatile components as far as possible. capillary column (0.25 mm × 25 m; d = 0.3 µm); the
f
split ratio was 1/40; P = 1 atm (at Tinitial = 50°C); He;
Tinitial was varied depending on the substrate; the heat-
ing rate was 10 K/min; Tfinal = 250°C; FID. An L-241
analog-to-digital converter (20 bit) and the Multichrom
For the analysis of FAME oxidation products, a
-ml sample was diluted with 5 ml of water and
extracted with 1 ml of chloroform. The extract was
dried with Ca5A molecular sieves, and 0.1 ml of
BSTFA was added to the dry solution. The mixture was
placed in an airtight microreactor and held at 60°C for
1
1
.39 software (Ampersand Ltd) were used for chro-
matographic data acquisition and processing.
1
h to perform silylation. The resulting solution was
examined by GC–MS.
-Propanol, 2-methyl-2-propanol,
-butanone, 2-methyl-1-propanol, and ethyl acetate
Table 1 summarizes the internal standards and Tinj
for the GLC analysis of oxidation products of various
substrates.
2
2-butanol,
2
The GLC analysis of the starting isooctane (analyti-
from a reaction mixture were separated with the use of cal grade) was performed on a 3700 chromatograph
chromatographic columns of different polarity and (Russia) equipped with an SPB-1 capillary column
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROPEROXIDE OXIDATION OF DIFFICULT-TO-OXIDIZE SUBSTRATES
43
304
00
3
2
09
1
59
2
47
1
47
1
89
1
37
315
1
79
120
240
Retention time, s
Fig. 1. Chromatogram of a chloroform extract of the reaction mixture of isooctane oxidation in the V(V)/H O /AcOH system, 30°C;
2
2
[
V(V)] = 4 × 10–3 mol/l; [Sub] = 0.1 mol/l; [H O ] = 1 mol/l.
0 2 2 0
5
7
1
00
(‡)
4
5
5
5
0
0
4
1
3
1
4
3
53
50
3
9
83
80
101
100
7
0
3
0
40
60
7
70
90
110
120
5
1
00
(b)
75
4
3
OH
5
0
0
4
1
4
7
0
8
3
101
100
3
1
53
50
3
0
40
60
70
80
90
110
120
(c)
1
–
00
5
0
0
3
9 44
54
3
1
70
4
1
5
8
50
45
–
100
3
0
40
50
60
70
80
90
100
110
120
m/z
Fig. 2. (a) Mass spectrum corresponding to the chromatographic peak with a retention time 159 s in Fig. 1, (b) the reference mass
spectrum of 4,4'-dimethylpentanol, and (c) difference between the spectra.
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

4
4
GEKHMAN et al.
Oxidation products
H O + O , O
amount of oxidized alkane depended on the initial H O
2 2
Alkane
concentration ([H O ] ) and monotonically increased
2
2 0
H O
2
2
as this concentration was increased (Fig. 3). More than
2
2
3
a tenfold excess of H O was required for attaining
2
2
~
90% conversion of an alkane because hydrogen per-
Scheme 2. Reaction paths of hydrogen peroxide con-
sumption under conditions of n-alkane oxidation.
oxide was consumed in at least two parallel reaction
paths (Scheme 2); in this case, the major portion of
H O decomposed into H O and O .
2
2
2
2
In terms of Scheme 2, the higher the reactivity of an
alkane, the higher its conversion, all other factors
[H O ] etc.) being the same. It follows from Fig. 3 that
the reactivity of n-alkanes somewhat decreases with
carbon chain length in the order pentane > hexane >
decane > dodecane.
(
0.25 mm × 100 m; d = 0.25 µm) (Supelco); the split
f
ratio was 1/40; P = 0.32 atm; T = 250°C; and T
=
inj
col
(
6
9
0°C. According to GLC data, the isooctane purity was
9.86 ± 0.04%. The following impurities were identi-
2
2 0
fied based on the retention times: 2,3-dimethylpentane
0.06%), 3-ethylpentane (0.02%), and 2,2-dimethyl-
(
hexane (0.05%) [26]. The total concentration of other
Secondary alcohols and ketones with carbon chain
lengths equal to the chain length of the starting hydro-
impurities was no higher than 0.01%.
The oxidation of N was performed by passing a carbon were detected in hydrocarbon oxidation prod-
2
flow of N (30–60 ml/min) through a reactor with a ucts (Table 2). The oxidation of linear alkanes affects
2
–2
solution of H O (1 mol/l) and VO(acac) (10 mol/l) only Csec–H bonds, and it is accompanied by neither the
2
2
2
in trifluoroacetic acid at 0–20°C. The gas mixture was oxidative degradation of the substrate nor the oxidation
collected for 5–20 min over an aqueous NaCl or NaOH of terminal methyl groups. Thus, 2-heptanol, 3-hep-
solution and analyzed by GC–MS under the following tanol, 4-heptanol, 2-heptanone, 3-heptanone, and
conditions: AUTOMASS 150 GC–MS instrument; 4-heptanone were detected in the oxidation of n-hep-
Supel-QPlot column (30 m × 0.32 mm) (Supelco); tane (Scheme 3). Neither 1-heptanol nor heptanal was
−
10°C; P = 0.5 atm; split ratio of 1/40; sample volume detected in the oxidation products of n-heptane. The
inj
of 100 µl; EI = 70 eV; mass range of 25–100 amu; and products of skeletal isomerization were also not found.
scan time of 50 ms.
The total yield of alcohols and ketones was no
higher than 60% at a substrate conversion of ~35%. The
ketone content of oxidation products was much higher
than the alcohol content (Table 2). Keto alcohols were
not found among the oxidation products of n-heptane
and lower alkanes.
RESULTS AND DISCUSSION
Oxidation of Linear Alkanes
Normal C –C alkanes and branched C –C alkanes
5
20
6
8
were chosen as substrates for comparing their reactivity
The buildup of ketones and alcohols in the oxidation
with that of cyclohexane [24]. In the absence of a cata- of alkanes (Fig. 4) occurs without an induction period;
lyst from an H O /AcOH solution, no detectable con- the kinetic curves exhibit no singular points. Tradi-
2
2
sumption of the above substrates at room temperature tional free-radical reaction inhibitors, such as ionol and
within several hours was found, as well as in the oxida- para-benzoquinone, have no effect on the composition
tion of cyclohexane. After the addition of a catalyst of alkane oxidation products (Table 2, nos. 1, 2) or the
–
2
(
~10 mol/l), the consumption of H O began, which shape of the kinetic curve. The yield of oxidation prod-
2 2
was practically complete after ~1 h at 30°C. The ucts, the ratio [ketone]/[alcohol], and the yield of posi-
CH CHCH CH CH CH CH (4%)
3
2
2
2
2
3
OH
CH CCH CH CH CH CH (35%)
3
2
2
2
2
3
O
CH CH CHCH CH CH CH (6%)
3
2
2
2
2
3
CH CH CH CH CH CH CH
3
OH
CH CH CCH CH CH CH (33%)
3
2
2
2
2
2
3
2
2
2
2
3
O
CH CH CH CHCH CH CH (3%)
3
2
2
2
2
3
OH
CH CH CH CCH CH CH (19%)
3
2
2
2
2
3
O
(V)
Scheme 3. Identified products of n-heptane oxidation in the V /H O /AcOH system (relative product yields are
2
2
given in parentheses).
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROPEROXIDE OXIDATION OF DIFFICULT-TO-OXIDIZE SUBSTRATES
45
1
5
4
3
2
1
0
2
3
7
6
5
4
3
2
1
1
2
4
1
2
3
4
5
0
100
Time, min
150
200
250
0
1
2
3
4
5
6
7
[
H O ] /[Sub]
2 2 0 0
Fig. 4. Accumulation of (1) pentanones and (2) pentanols in
the oxidation of n-pentane in the V( /H O /AcOH system;
0°C; [V(V)] = 4 × 10–3 mol/l; [Sub]0 = 0.1 mol/l;
H O ] = 1 mol/l.
V)
2
2
Fig. 3. Limiting n-alkane conversion as a function of
H O ] /[Sub] in the oxidation of (1) n-pentane, (2) n-hex-
3
[
[
2
2 0
0
ane, (3) n-decane, or (4) n-dodecane.
2 2 0
tional isomers are also independent of the presence or
(3)Alcohols and ketones are formed in parallel reac-
absence of an inhibitor. These data exclude a free-radi- tions.
cal chain mechanism of alkane oxidation in the
(4) Linear alkanes and cyclohexane are oxidized by
(V)
V /H O /AcOH system.
2
2
a nonchain mechanism.
It would be assumed that ketones, the amount of
All these data allow us to consider the oxidation of
which is greater than the amount of alcohols, are formed cyclohexane and acyclic alkanes in terms of a single
in the consecutive oxidation reactions alkane
alco- mechanism with the participation of the same active
hol
ketone. To test this hypothesis, we studied the intermediates. This provides an opportunity to reveal
simultaneous oxidation of n-octane and 3-pentanol. In the relative reactivities of lower linear alkanes and
the simultaneous oxidation of 3-pentanol (0.01 mol/l) cyclohexane by studying their competitive oxidation.
and n-octane (0.1 mol/l) at [H O ] = 1 mol/l, the con-
If the same vanadium complex is responsible for the
2
2 0
version of 3-pentanol was negligibly small. In experi- oxidation of both alkanes and cyclohexane, the ratio
ments when 2-butanol (0.01 mol/l) was oxidized simul- between the rate constants of substrate reactions with
taneously with n-octane (0.1 mol/l), the final concentra- an active intermediate can be calculated from the equa-
tion of 2-butanone was no higher than 0.001 mol/l even tion [27]
after the complete decomposition of H O ([H O ] =
2
2
2
2 0
k
ln(100 – x )
A
A
1
mol/l). All these facts suggest that alcohols are not
-
-------------------- = ---------------------------------------------- ,
k
ln(100 – x
)
cyclohexane
intermediates in the formation of ketones. Alcohols and
ketones are formed in parallel reactions, and they are
the final products of alkane oxidation.
cyclohexane
where x and x
are the degrees of conversion of
cyclohexane
linear alkanes and cyclohexane, respectively.
Ä
The following similarities between the oxidation of
linear alkanes and cyclohexane are clear:
We found that the relative rate constants of oxida-
tion of linear alkanes and cyclohexane lie in the range
0
.6–1.7; that is, the reactivity of n-alkanes, except for
(
1) The hydrocarbon skeleton remains unaffected by
n-pentane, is commensurable with the reactivity of
oxidation.
4
cyclohexane in the test reaction (Table 3). Because
(
2) All the test substrates are oxidized to the corre-
there is no experimental or theoretical evidence for the
formation of a strong vanadium(V)/hydrocarbon com-
plex [28, 29], all differences in reactivity should be
related to the interaction of an active vanadium com-
plex with a substrate.
sponding alcohols and ketones.
Table 1. Conditions of the GLC analysis of oxidized sub-
strates
Similar values of the rates of oxidation of linear
alkanes and cyclohexane and the similarity between
Substrate
Internal standard
T , °C
inj
(V)
their transformations in the V /H O /AcOH system
2
2
C –C n-alkanes
MeCN
50–70
100
80
5
10
4
The consumption of low-boiling n-pentane can be somewhat
overestimated because of its removal from the reactor with a flow
of oxygen released on the decomposition of hydrogen peroxide
C –C n-alkanes
PhCN
1
1
22
C –C FAMEs
n-decane
n-hexadecane
3
8
(
the reaction temperature is 30°C, and the boiling temperature of
C –C FAMEs
100
9
18
n-pentane is somewhat higher and equal to 35–36°C).
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

4
6
GEKHMAN et al.
Table 2. Oxidation of n-alkanes in the V(V)/H O /AcOH system
2
2
4
3
[
Alcohol] × 10 , [Ketone] × 10 ,
Σ
Σ
No.
Substrate (Sub)
n-pentane
[Sub] , mol/l
Conversion, %
0
mol/l
mol/l
1
2
0.10
0.10
49
47
–
45a
43
–
40b
38
–
n-pentane + ionol
0
.01
3
4
5
6
7
8
9
n-hexane
0.10
0.10
0.10
0.05
0.05
0.05
0.05
0.05
0.05
32
32
30
31
29
21
16
15
11
37c
24e
32g
18
22
33
23
18
21
25d
18f
20h
9i
n-heptane
n-octane
n-nonane
n-decane
8
n-tetradecane
n-pentadecane
n-hexadecane
n-nonadecane
6
5
1
1
0
1
5
3
Note: 30°C; [V(V)] = 4 × 10–3 mol/l; [H O ] = 1 mol/l.
2
2 0
a
b
c
d
e
f
g
h
i
[
2-pentanol]/[3-pentanol] = 1.6/1.
[
2-pentanone]/[3-pentanone] = 1.9/1.
[
2-hexanol]/[3-hexanol] = 0.7/1.
2-hexanone]/[3-hexanone] = 0.8/1.
2-heptanol]/3-heptanol]/[4-heptanol] = 0.8/2/1.
[
[
[
2-heptanone]/[3-heptanone]/[4-heptanone] = 1.4/1.6/1.
[
[
2-octanol]/[3-octanol]/[4-octanol] = 0.3/1/2.
2-octanone]/[3-octanone]/[4-octanone] = 0.7/1.2/1.
[
2-nonanone]/[3-nonanone]/([4-nonanone] + [5-nonanone]) = 1.3/1/1.
provide an opportunity to study the main regularities of addition of 2-propanol (0.01–0.14 mol/l), which is
linear alkane oxidation using cyclohexane as an exam- more active in chain oxidation than cyclohexane, did
ple. The number of cyclohexane oxidation products is not change the shape of the kinetic curves of the
considerably lower, and they are easier to analyze.
buildup of cyclohexanol and cyclohexanone. The inhib-
itors of free-radical chain reactions (para-benzo-
quinone, tetranitromethane, and tri-tert-butylphenol) in
concentrations from 7 × 10 to 2 × 10 mol/l did not
decrease the rates of buildup of cyclohexanol and
cyclohexanone and their final concentrations. All these
facts suggest a nonchain mechanism of cyclohexane
oxidation.
In the oxidation of cyclohexane (1) with hydrogen
peroxide in the presence of vanadium(V) compounds in
an AcOH solution at 20–50°C, only cyclohexanol (2)
and cyclohexanone (3) were detected as the products
–3
–2
(Scheme 4; Table 4).
Neither C–C bond cleavage products nor the prod-
ucts of skeletal isomerization of 1 were detected. The
As in the case of linear alkanes, the curves of
buildup of compounds 2 and 3 in cyclohexane oxida-
tion have no singular points and approach a limit
OH
related to H O depletion. After the addition of a new
+
H O
2
2
2
portion of H O , the oxidation of cyclohexane recom-
2
2
H O
2
2
menced at the previous rate.
2
(a)
Cyclohexanone was absent from the products of
(b)
(V)
cyclohexanol oxidation in the V /H O /AcOH sys-
2
2
2
H O
O
1
2
2
tem. In the simultaneous oxidation of cyclohexane and
cyclohexanol in the test system, the buildup rates of
compounds 2 and 3 were independent of the initial
+
3H O
2
3
cyclohexanol concentration ([2] ). The limiting cyclo-
0
Scheme 4. Reaction paths of cyclohexane oxidation.
hexanone concentration ([3] ), which was reached after
∞
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROPEROXIDE OXIDATION OF DIFFICULT-TO-OXIDIZE SUBSTRATES
47
Table 3. Relative rate constants of alkane and cyclohexane ox- increased with [H O ] (Table 4, nos. 6, 7). In contrast
2
2 0
) in the V( /H O /AcOH system at 30°C
V)
idation (k /k
A
cyclohexane
2 2
to this, the limiting concentration of compound 2
depended only slightly on changes in the concentration
of cyclohexane (Table 4, nos. 1, 2 and 3–5); this fact
suggests that the order of reaction with respect to the
concentration of cyclohexane is close to zero.
This may be due to the following reasons:
(1) a free-radical chain reaction mechanism, which
includes quadratic termination;
Alkane
k /k
A cyclohexane
n-pentane
n-heptane
n-octane
n-nonane
n-decane
1.71 ± 0.05
0.88 ± 0.05
0.64 ± 0.04
0.83 ± 0.05
0.72 ± 0.04
(
(
2) an inhibition of oxidation by reaction products;
3) an inhibition of oxidation by a substrate because
of the formation of its strong complex with the catalyst
at a step that precedes the rate-limiting step;
the complete consumption of hydrogen peroxide, was
(
4) a steady-state concentration of an active oxidiz-
ing agent.
Assumption (1) can be rejected because of the
also independent of [2] . Thus, the formation of cyclo-
0
hexanone from cyclohexanol, as well as the coupled
oxidation of cyclohexane and cyclohexanol, can be
excluded from consideration. All the data suggest that
cyclohexanol and cyclohexanone are formed in parallel
reactions (a) and (b) (Scheme 4) [24].
absence of an induction period of the reaction. More-
over, the reaction is insensitive to traditional inhibitors
and reactor material.
Assumption (2) is inconsistent with the results of
The initial rate of buildup of cyclohexanol was prac- experiments on the simultaneous oxidation of cyclo-
tically proportional to an increase in the concentration hexane and cyclohexanol or cyclohexanone (see
of vanadium(V) at constant initial concentrations of above). Assumption (3) can be rejected because there
H O and cyclohexane (Fig. 5). The rate of H O
are no experimental data or theoretical considerations
2
2
2
2
decomposition remained unchanged regardless of the in support of the idea that cyclohexane can form strong
presence or absence of compound 1 and its oxidation complexes with vanadium compounds.
products, all other factors being the same [24]. There-
The formation of an active oxidizing agent at the
fore, the limiting concentration of an oxidation product, rate-limiting step of the reaction (assumption (4))
that is, the concentration reached after the complete seems most probable. The rate of deactivation of the
consumption of H O , can characterize the rate of reac- active oxidizing agent, that is, its decomposition with
2
2
tion. Table 4 summarizes the limiting conversions (x∞) the release of O , should be lower than the rate of its
2
of cyclohexane and the limiting concentrations of the reaction with cyclohexane, or these rates should be
products. The data indicate that the rate of cyclohex- close to each other in their order of magnitude. In terms
anol buildup, which is characterized by [2] , noticeably of the mechanism of hydrogen peroxide conversion in
∞
Table 4. Cyclohexane oxidation in the V(V)/H O /AcOH system at 50°C
2
2
[
V(V)] × 103,
2
2
Conversion*
No.
[1] , mol/l
[H O ] , mol/l [2] × 10 , mol/l [3] × 10 , mol/l
2 2 0 ∞ ∞
0
mol/l
of 1, %
1
2
3
4
5
6
7
8
9
0.14
0.14
0.65
0.65
0.65
0.15
0.15
0.07
0.14
1.7
0.85
1.55
0.43
0.82
1.51
0.44
0.44
0.45
1.55
0.39
0.28
0.13
0.10
0.10
0.44
0.44
0.44
0.48
0.19
0.10
0.08
0.71
0.82
0.70
3.0
3.5
5.3
6.3
7.5
2.6
3.2
4.1
4.5
6.1
9.0
5.2
2.3
2.8
0.7
0.4
2.2
1.3
0.6
4.5
2.5
1.0
0.4
6.6
9.6
13
11
5.6
2.4
3.3
1
1
1
0
1
2
14
15
9.6
11
12
7.1
2.4
1.7
Note: 1 is cyclohexane, 2 is cyclohexanol, and 3 is cyclohexanone.
Limiting conversion x∞.
*
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

4
8
GEKHMAN et al.
2.5
2.0
1
2
2
1
1
1
0
.5
.0
.5
3
1
2
3
4
0
5
10
15
20
Time, min
0
20
40
Reaction time, min
Fig. 6. Accumulation of (1, 3) cyclohexanol and (2, 4)
cyclohexanone in the oxidation of cyclohexane (1) (1, 2) in
the presence or (3, 4) in the absence of anthracene. [1] =
Fig. 5. Formation of cyclohexanol (2) in the oxidation of
(
V)
cyclohexane (1) in the V /H O /AcOH system. 50°C;
2
2
1] = 0.5 mol/l; [H O ] = 0.08 mol/l; [V(V)] = (1) 3.8 ×
(V)
–4
[
0.6 mol/l; [anthracene] = 0.1 mol/l; [V ] = 1.8 × 10 mol/l;
0
2 2 0
(V)
–
4
–5
–5
[H O ] = 1.0 mol/l; 30°C; V /H O /AcOH system.
2 2 0 2 2
1
0 , (2) 7 × 10 , or (3) 3 × 10 mol/l.
the test system (Scheme 1), only two intermediate com- (if k [Sub ] ≅ kdeact) or the termination of 2-ethylan-
1
1
plexes of vanadium are formed at two different slow thracene oxidation (if k [Sub ] ӷ kdeact) can occur.
1
1
steps:
We found experimentally that no considerable
changes in the rate of buildup of cyclohexanol were
observed in the presence of 2-ethylanthracene
(1) a vanadium complex with singlet dioxygen,
which is responsible for the oxidation of unsaturated
compounds;
(
0.1 mol/l). Similarly, the addition of cyclohexane
(
0.6 mol/l) had no effect on the rate of consumption of
(
2) a vanadium complex with the trioxo ligand,
–2
2
-ethylanthracene ([Sub ] = 2 × 10 mol/l) (Fig. 6).
2
0
which is the precursor of ozone.
These facts allowed us to exclude the complex of
vanadium with singlet dioxygen from the number of
active intermediates responsible for alkane oxidation.
It is well known that the vanadium complex with
singlet dioxygen is responsible for the oxidation of
2
-ethylanthracene. Let us assume that the same com-
The experimental data suggest that an intermediate
vanadium complex formed at another rate-limiting step
is responsible for the consumption of alkanes in the
plex also reacts with alkanes. Then, for the simulta-
neous oxidation of cyclohexane and 2-ethylanthracene,
we can write
(V)
(V)
2–
V /H O /AcOH system. This is the V /(O ) complex
2
2
3
with the trioxo ligand, which can oxidize an alkane to an
alcohol and the corresponding ketone in parallel reactions.
d[Sub ]
k [Sub ]w
1 1 0
1
–
–
----------------- -- = ---------------------------------------------------------------- -- ,
dt
k
+ k [Sub ] + k [Sub ]
deact 1 1 2 2
Because CCl , which is a scavenger of C-centered
4
free radicals [28], has no effect on the rate of buildup of
an alcohol (Fig. 7) and radical chain oxidation of
alkanes does not occur in the test system (see above),
alkane hydroxylation can be considered as a purely
molecular process of the insertion of one of the oxygen
atoms of the trioxo ligand into a C–H bond of the
hydrocarbon (Scheme 5, reaction path (I)). Note that
the kinetic isotope effect (KIE) of cyclohexane hydrox-
ylation with the complex with the trioxo ligand is close
to unity (1.16 ± 0.07 [24]), whereas a much larger KIE
d[Sub ]
k [Sub ]w
2 2 0
2
----------------- -- = ---------------------------------------------------------------- -- ,
dt
k
+ k [Sub ] + k [Sub ]
deact 1 1 2 2
where w is the rate of formation of the vanadium com-
0
plex with singlet dioxygen; [Sub ] and [Sub ] are the
1
2
concentrations of cyclohexane and 2-ethylanthracene,
respectively; kdeact is the rate constant of deactivation of
an active intermediate; k and k are the rate constants
1
2
of reactions of cyclohexane and 2-ethylanthracene with
the active intermediate, respectively; kdeact/k2
.014 mol/l [23].
=
(
2.77 [13]) was obtained for cyclohexane hydroxyla-
0
tion with peroxo complexes. The small KIE is indica-
A near-zero order of reaction with respect to the tive of a minor elongation of the C–H bond in the tran-
concentration of cyclohexane is observed if k [Sub ] ≥ sition state; that is, it is believed that in the reaction of
1
1
kdeact. Then, in the simultaneous oxidation of cyclohex- the trioxo complex the electrophilicity of the oxygen
–
2
ane and 2-ethylanthracene at [Sub ] ≥ 10 mol/l, either atom inserted into the C–H bond is higher than that in
2
a decrease in the rate of cyclohexane oxidation the dipicolinate peroxo complex of vanadium [13].
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROPEROXIDE OXIDATION OF DIFFICULT-TO-OXIDIZE SUBSTRATES
49
1
1
0
0
0
0
.2
.0
.8
.6
.4
.2
4
2
0
1
2
4
2
1
1
0
.0
.5
.0
.5
2
3
4
3
1
2
1
0
10
20
30
40
50
Time, min
0
50
100
Time, min
150
200
Fig. 7. Formation of cyclohexanol in the oxidation of
cyclohexane (1) in the V /H O /AcOH system (1) in
the absence or (2) in the presence of CCl ; 50°C; [V(V)] =
× 10–4 mol/l; [1]0 = 1.0 mol/l; [H O ] = 0.3 mol/l;
(
V)
2
2
Fig. 8. Accumulation of cyclohexanone (1) in the absence
of CCl or (2) in the presence of CCl (1.2 mol/l) and the
accumulation of chlorocyclohexane in the presence of (3)
CCl4 (1.2 mol/l) or (4) CCl4 (1.2 mol/l) and ionol
4
4
4
2
[
2 2 0
CCl ] = 1.2 mol/l.
4 0
−2
(
V
10 mol/l) in the oxidation of cyclohexane (1) in the
(
V)
(V)
–4
/H O /AcOH system; 50°C; [V ] = 2 × 10 mol/l;
2 2
The additives of CCl insignificantly affected the initial
[1]0 = 1.0 mol/l; [H2O2]0 = 0.3 mol/l.
4
rate of buildup of cyclohexanone; however, the following
effects were observed in the presence of CCl (Fig. 8):
4
system is much higher than that for cyclohexanol and
equal to 2.45 ± 0.09 [24]. This is consistent with the pro-
posed mechanism, which includes the insertion of the oxy-
gen radical cation into a C–H bond and the elimination of
a proton at the same carbon atom (Scheme 5). It is likely
that these processes occur synchronously. The reaction
scheme proposed is consistent with all the experimental
(
1) Cyclohexanone continued to accumulate after
the exhaustion of hydrogen peroxide.
2) The accumulation of chlorocyclohexane and
hexachloroethane was observed.
3) The addition of ionol inhibited the buildup of
these products.
(
(
The above data suggest that, in the presence of CCl4, facts. The hypothesis of the participation of the oxygen rad-
along with the molecular mechanism of cyclohexane
oxidation, the hydrocarbon can undergo chain oxida-
tion. Carbon tetrachloride itself cannot initiate chain
oxidation. It is likely that a chain initiator or chain car-
+
.
ical cation (O ) in oxidation reactions catalyzed by cop-
per complexes was first proposed by Shilov [29].
rier is formed as a result of CCl reactions in the course
of cyclohexane oxidation.
4
Oxidation of Branched Alkanes
Oxidation of 2,2-dimethylbutane. In the oxidation
Thus, it is believed that a C-centered free radical is
formed in the course of cyclohexane oxidation to cyclo-
hexanone. This radical cannot carry out chain oxida-
(V)
of 2,2-dimethylbutane in the V /H O /AcOH system,
2
2
in addition to the products of substrate oxidation at the
methylene group (3,3-dimethyl-2-butanone (4) and
tion; however, it reacts with CCl (a scavenger of
4
3
,3-dimethyl-2-butanol (5)), primary alcohols (3,3-
C-centered free radicals) to form an effective chain car-
rier. The 1-hydroxy-1-cyclohexyl radical (Scheme 5,
radical C), which is the product of the insertion of the
dimethyl-1-butanol (6) and 2,2-dimethyl-1-butanol (7))
were also detected, which are the products of hydroxy-
lation at methyl groups. The appearance of the follow-
ing alcohols among the reaction products seems
unusual: 2-methyl-2-propanol (8), 2,2-dimethyl-1-pro-
panol (9), and 2-methyl-2-butanol (10). The numbers of
+
.
O
species into a substrate C–H bond (Scheme 5,
reaction path (II)), possesses these properties. Subse-
quently, this radical can react with oxygen to give the
.
HO radical, which is inactive in cyclohexane oxida- carbon atoms in these alcohols are lower than that in the
2
5
tion, and cyclohexanone. The reaction of the parent substrate (Scheme 6).
1
-hydroxy-1-cyclohexyl radical with CCl also results
in the formation of cyclohexanone and, in addition,
4
Oxidation of 2-methylpentane. In the oxidation of
-methylpentane, we expected that the process would
2
.
hydrogen chloride and the CCl radical, which can affect only the C–H bonds of methyl, methylene, and
3
methyne groups. Indeed, the following products of this
oxidation were detected in the reaction solutions:
dimerize to hexachloroethane (hexachloroethane was
detected in the oxidation products of 1). The CCl O
.
3
2
.
2
5
Oxidation products containing lower numbers of carbon atoms than
radical reacts with molecular O to give CCl O , which
can carry out the chain oxidation of a hydrocarbon [28].
KIE (k /k ) for the buildup of cyclohexanone in the test
2
3
that in the parent substrate were detected as alcohols and esters in the
reaction mixture. However, for simplicity, henceforth, only the corre-
sponding alcohols are shown in reaction schemes.
H
D
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

5
0
GEKHMAN et al.
#
–
H
C
C
H
O
H
–
O
O
H
C
C
H
O
O O
C OH
O O
–
(I)
V
O
O
+
C
+
V O
O O
O
V O
O O
A
(II)
B1
C+ C H
OH
B2
+C H
.
–
O
O
C
+
V
OH
O
O
O
B
C C
O
–H+
HX, O2
H
or
OH
O2
C
C O2
.
–
HO
O
2
C C
C
–HCl
H
CCl4
or
C X
C
O
Cl C–CCl
.
X=OH, AcO
3
3
CCl + C C Cl
.
3
O2
Chain oxidation
O CCl
OH
2
3
Scheme 5. Hypothetical mechanism of alkane oxidation in the presence of V(V)/H O /AcOH.
2
2
4
2
4
4
-methyl-1-pentanol (11), 2-methyl-1-pentanol (12), lower numbers of carbon atoms in the molecular skele-
-methyl-3-pentanol (13), 2-methyl-2-pentanol (14), ton than that in the parent substrate were also detected
-methyl-2-pentanol (15), 2-methyl-3-pentanone (16), among the oxidation products of 2,3-dimethylbutane:
-methyl-2-pentanone (17), and 2-methylpentanal (18). 2-propanol (36), 3-methyl-2-butanol (37), and
In addition to these products, 2-pentanol (19) and 3-methyl-2-butanone (38) (Scheme 9).
-pentanone (20) were also detected; the carbon skele-
2
Oxidation of isooctane. The fullest information on
the reactivity of various C–C bonds can be obtained
from the oxidation of isooctane, in the molecule of
which primary, secondary, tertiary, and quaternary car-
bon atoms occur simultaneously. Therefore, its oxida-
tion products were studied in more detail than the oxi-
dation products of other branched alkanes.
ton of these compounds is shorter than that in the parent
substrate (Scheme 7).
Oxidation of 3-methylpentane. The predominant
products of 3-methylpentane oxidation in the test sys-
tem are 3-methyl-2-pentanone (21) and 3-methyl-3-
pentanol (22). Other hydrocarbon oxidation products
were also detected, in which the carbon skeleton of the
substrate was retained: 3-methyl-2-pentanol (23),
2
,2,4-Trimethyl-3-pentanone (39, the main prod-
uct), 2,2,4-trimethyl-3-pentanol (40), 2,2,4-trimethyl-
3
3
-methyl-1-pentanol (24), 2-ethyl-1-butanol (25), and
-methylpentanal (26). In addition to the oxidation
1
-pentanol (41), 2,4,4-trimethyl-1-pentanol (42), and
,4,4-trimethyl-1-pentene (43), as well as compounds
2
products expected, the following alcohols, their esters, and
ketones, whose carbon skeletons contain lower numbers
of carbon atoms than that in the initial substrate, were
identified by GC–MS: 3-pentanol (27), 3-pentanone (28),
with shortened hydrocarbon chains (4,4-dimethyl-2-
pentanol (44), 2,4-dimethyl-2-pentanol (45), 4,4-dime-
thyl-2-pentanone (46), 1,1-dimethylethanol (47),
2
2
-methyl-1-propanol (48), 2-methyl-1-propyl acetate,
,2-dimethyl-1-propanol (49), 2,2-dimethyl-1-propyl
2
-butanone (29), 2-butanol (30), 2-butyl acetate and etha-
nol (31), and ethyl acetate (Scheme 8).
acetate, 4-methyl-2-pentanone (50), and 4-methylpen-
tan-4-ol-2-one (51)) were detected by GC–MS in the
reaction mixture of isooctane oxidation. A trace of
Oxidation of 2,3-dimethylbutane. Among the oxi-
dation products of 2,3-dimethylbutanol, 2,3-dimethyl-
2
-butanol (32) was predominant. Other oxidation prod-
2
-propanol was also detected (Scheme 10).
ucts, in which the carbon skeleton of the substrate was
retained, were also detected in the reaction solutions:
Free-radical chain reaction inhibitors, such as ionol
,3-dimethyl-1-butanol (33), 2,3-dimethylbutanal (34), and para-benzoquinone, have no effect on the product
2
and 2,3-dimethyl-2-butene (35). In addition to the composition of isooctane oxidation in the
above compounds, the following compounds with V /H O /AcOH system. This makes it possible to
(V)
2
2
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROPEROXIDE OXIDATION OF DIFFICULT-TO-OXIDIZE SUBSTRATES
51
CH3
CH3
H C C CH
CH3
CH3
H C C C
3
3
O
OH
CH
CH3
3
4
5
CH3
CH3
CH3
CH2 OH
CH3
CH3
H C C CH2
H C C CH
H C C CH2
2
3
3
2
CH3
6
CH3
HO
CH3
7
CH3
CH3
CH3
H C C OH
H C C CH2
3
3
CH3
CH
OH
OH
10
3
8
H C C CH2
3
CH3
9
Scheme 6. Identified products of 2,2-dimethylbutane oxidation in the V(V)/H O /AcOH system.
2
2
CH3
CH3
CH
OH
CH
C
CH2
H C
3
CH2
CH3
H C
3
CH2
CH3
1
5
OH
14
CH3
CH
CH3
CH
CH2
CH2
H C
3
C
CH3
H C
CH
OH
CH3
3
1
6 O
1
3
CH3
CH
CH
CH3
CH
O
C
CH2
CH
CH2
H C
3
CH2
CH3
H C
CH2
CH3
H C
3
CH2
CH3
2
1
7
OH
12
CH3
CH
CH3
CH
CH2
CH2
H C
3
CH2
CH2
OH
OH
CH
HC
CH2
CH3
1
1
O
1
8
O
CH2
C
CH2
H C
CH2
CH3
H C
3
CH2
20
CH3
3
1
9
Scheme 7. Identified products of 2-methylpentane oxidation in the V(V)/H O /AcOH system.
2
2
exclude the chain oxidation of this substrate and the group of isooctane. However, along with these com-
above hydrocarbons.
pounds, the following products of methyl group
hydroxylation were also observed: 2,2,4-trimethyl-1-
pentanol (41) and 2,4,4-trimethyl-1-pentanol (42). Sur-
prisingly, 2,2,4-trimethyl-2-pentanol (a product of
hydroxylation at the tertiary carbon atom of isooctane),
By analogy with the oxidation of cyclohexane and
n-alkanes, which was described above, the expected
products
of
isooctane
conversion
in
the
(V)
V /H O /AcOH system would be 2,2,4-trimethyl-3-
2
2
pentanone (39) and 2,2,4-trimethyl-3-pentanone (40), which is expected based on free-radical reaction
which result from the oxidation of the sole methylene schemes and schemes with the abstraction of the hydride
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

5
2
GEKHMAN et al.
CH3
CH
CH3
CH
HO
CH3
CH3
CH3
CH3
HO
CH2
CH
CH2
CH2
CH2
CH3
2
3
2
4
OH
CH3
C
CH2
CH
CH3
CH3
CH2
CH2
CH3
OH
CH2
2
2
25
CH3
CH
CH3
CH
CH3
CH
O
H3
CH3
CH3
CH3
CH3
C
C
CH2
CH2
CH2
CH2
CH2
2
6
O
21
OH
O
CH
C
CH3
CH3 CH3
CH3
CH2
CH2
CH2
CH2
2
7
O
OH
28
C
CH
CH3
OH
CH3
CH3
CH2
CH3
CH2
CH3
CH2
3
1
2
9
3
0
Scheme 8. Identified products of 3-methylpentane oxidation in the V(V)/H O /AcOH system.
2
2
CH CH
CH CH
3 3
3
3
H C CH CH CH OH H C CH CH CH
3
2
3
3
3
3
4
O
CH3 CH3
CH CH
CH3 CH3
3
3
H C C
3
CH CH3
H C CH CH CH
3
H C C
3
C CH3
3
3
5
OH
32
OH
H C CH CH
CH3
CH3
H C CH CH CH
3
3
3
3
3
6
H C C CH CH
OH 38
3
3
3
7
O
Scheme 9. Identified products of 2,3-dimethylbutane oxidation in the V(V)/H O /AcOH system.
2
2
ion from the alkane, was not found in this reaction:
explained by the oxidative dehydrogenation of isooc-
tane (more precisely, alkane hydroxylation followed by
alcohol dehydration) and the oxidative cleavage of
2,4,4-trimethyl-1-pentene (43). It is well known that
H C
3
CH3
H C
3
CH3
H C
3
H C
3
OH
CH3 CH3
CH3 CH3
(V)
oxidative cleavage can occur in the V /H O /AcOH
2
2
system [20, 21].
It is likely that this alcohol or the corresponding ace-
tate gives 2,4,4-trimethyl-1-pentene (43), which was
detected in the reaction mixture, as a result of dehydra-
tion or AcOH elimination.
Evidently, the formation of 4,4-dimethyl-2-pentanol
44) and 2,4-dimethyl-2-pentanol (45) in isooctane oxi-
(
dation is indicative of C–C bond cleavage. This is the
least expected route in the test reaction. The presence of
C –C hydrocarbon derivatives (44–51, Scheme 10) in
4 7
Oxidative dehydrogenation was observed in the
reaction of cyclooctane in the GIF system (Fe(III)
salt/TBHP) [30]. In this context, the formation of 4,4- the reaction products cannot be explained by the occur-
dimethyl-2-pentanone (46) (Scheme 10) can be rence of corresponding impurities in the parent isooc-
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROPEROXIDE OXIDATION OF DIFFICULT-TO-OXIDIZE SUBSTRATES
53
OH
H C
CH3
2
OH
OH
H C
3
H C
3
CH3
H C
3
CH2
CH3 CH3
H C
3
41
H C
3
CH3 CH3
CH3 CH3
4
2
4
0
O
H C
CH3
CH CH
3
H C
CH3
3
H C
CH3
H C
3
3
H C
3
3
3
H C
3
CH3 CH3
CH3 CH2
3
9
4
3
H C
CH3
H3C
H3C
CH3
3
H C
CH3
H C
3
3
CH3 OH
CH3 O
H C
3
4
4
OH CH
3
46
4
5
H C
H C
OH H C
CH3
3
3
3
H C
3
OH
H C
OH
H C
3
3
O
CH3
H C 47
H C
4
9
3
48
3
50
H C
3
H C
3
CH3
H C
3
OH O
5
1
Scheme 10. Identified products of isooctane oxidation in the V(V)/H O /AcOH system.
2
2
tane. The GLC analysis of the parent hydrocarbon dem- of the reaction products was higher than the concentra-
onstrated that the isooctane purity was higher than tion of corresponding alcohols.
9
2
9.8%, the concentration of 2,2-dimethylpentane and
,4-dimethylpentane was no higher than 0.02%, and the tion of alkanes with dioxygen catalyzed by
Similar transformations were observed in the oxida-
concentration of C –C hydrocarbons was lower than Co(II)/Co(III) in a solution of CF COOH [31, 32].
4
6
3
0
.01%. Thus, the formation of products 44–51 cannot
Some distinctions between the oxidation of satu-
be explained by oxidation of C –C alkane admixtures rated branched-chain hydrocarbons and the reactions of
4
7
in initial isooctane. We can conclude that alcohols and linear alkanes should be noted:
ketones with shortened carbon chains (1,1-dimethyle-
(1) In the oxidation of linear alkanes, alcohols and
thanol (47), 2-methyl-1-propanol (48), 2,2-dimethyl-1- ketones with carbon chains shortened compared to the
propanol (49), 4-methyl-2-pentanone (50), 4-methyl- chain of the parent substrate are not formed, whereas the
pentan-4-ol-2-one (51), and 2-propanol) are formed concentration of shorter chain alcohols and ketones in the
from isooctane in processes that result in C–C bond oxidation products of branched hydrocarbons is compara-
cleavage.6
ble with the amount of “normal” hydroxylation and keton-
ization products. In this case, oxidative degradation prima-
In the test system, the oxidation of both linear and rily occurs at the Ctert–C bond.
branched alkanes primarily occurs at the C–H bonds of
(2) Esters were detected only for alcohols with
the substrate with retention of the molecular carbon chains shortened compared to the carbon skeleton of
skeleton. In this case, primary C–H bonds are much less the initial substrate, whereas esters were not detected
reactive than secondary and tertiary bonds, although for substrate hydroxylation products (the molecular
difference between the activities of primary and sec- carbon skeleton was retained).
ondary C–H bonds is less pronounced in the oxidation
of branched alkanes. In both cases, the ketone content oxidation products of branched alkanes, whereas the termi-
(3) Primary alcohols and aldehydes were detected in the
nal methyl groups of linear alkanes remained unaffected.
6
2
-Propanol is fairly soluble in water. For this reason, in the course
All these facts, as well as the regularities of alkane
hydroxylation, can be explained in terms of the mecha-
nism that involves a common transition state for all of
of sample preparation by extraction with diethyl ether, a consid-
erable portion of 2-propanol remained in the water causing an
underestimate of the apparent yield of this compound.
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

5
4
GEKHMAN et al.
the alkane conversion routes (hydroxylation, ketoniza- of structure B1 is greater than that of B2, an alcohol or
tion, and oxidative degradation) (Scheme 5). As in the its ester and a carbonyl compound with a chain short-
oxidation of cyclohexane, the vanadium(V) complex ened compared to the substrate (an aldehyde or ketone
with the trioxo ligand (Scheme 5) is taken as the active at R = H or alkyl, respectively). (2) If the contribution
oxidizing agent in these reactions. In activated complex A, of structure B2 is greater than that of B1, a nucleophilic
+
a C–H bond of the alkane molecule is subjected to an attack on the C(OH)R group results in the same carbonyl
.
.
5
electrophilic attack by an oxygen atom of the trioxo
group. Depending on the mode of C–H bond vibrations
in the attacked hydrocarbon fragment of the alkane
molecule, the activated complex can decompose to
form different products. In the case of antisymmetric
valence vibrations of the H–C–H group (Scheme 5,
reaction path (I)), the oxygen atom is inserted into a
C−H bond, and an alcohol molecule and a diperoxo com-
compound. The free-radical fragment CH or C H gives
3
2
a peroxy radical, which could in principle carry out chain
oxidation; however, it is not an active initiator [28]. Indeed,
we experimentally found that the addition of ionol, which
is an inhibitor of free-radical chain reactions, did not
change the composition and ratio of oxidation products.
The proposed reaction scheme explains almost com-
plex of vanadium are formed. In the case of symmetrical pletely the kinetics of the oxidative degradation of
valence vibrations, it is likely that the oxygen radical cation branched alkanes. The only disadvantage of this
is inserted into a C–H bond (reaction path (II)). In this case, scheme is that we failed to detect low-molecular-
a radical–cation pair (B) and a complex of the superoxide weight aldehydes (formaldehyde, acetaldehyde, and
anion with V( are formed. As evidenced by EPR spectra, propionaldehyde) in the oxidation products of 2-meth-
the latter species always occurred in the reaction solutions ylpentane and 3-methylpentane. However, the proce-
V)
[
6, 7]. Radical–cation pair B, in which a positive charge dure used for the identification of mixture components
and an unpaired electron are mainly localized at different cannot reliably identify low-molecular-weight alde-
carbon atoms, can participate in the following reactions:
1) Conversion into a radical C with the elimination of
hydes in such complex mixtures. At the same time, the
appearance of all of the other oxidative degradation
products, including difficult-to-predict compounds, is
adequately explained in terms of the proposed mecha-
nism. Thus, it is well known that alcohols in acetic acid
solutions do not undergo esterification under the condi-
tions of our experiments. However, alkyl acetates with
shortened carbon chains, such as 2-butyl acetate and
ethyl acetate, were found in the reaction mixture
(
a proton and the subsequent oxidation to a carbonyl com-
pound (it is not improbable that the radical is directly
formed from transition state A as a result of a synchronous
+
.
process: the insertion of the oxygen radical cation O into
a C–H bond and the elimination of a proton bound to the
same carbon atom through another bond).
(2) Reaction with a nucleophilic species and molec- (Scheme 8). According to Scheme 5, these esters result
ular oxygen. In this case, the direction of the nucleo- from an attack of a nucleophile HX on the electrophilic
philic attack and, probably, the rate of the reaction center of a radical–cation pair at the site of positive
depend on the predominant localization of a positive charge localization (resonance structure B1). The prod-
charge, whereas molecular oxygen abstracts a hydro- ucts of C–C bond cleavage with the formation of a
gen atom from the carbon atom at which an unpaired ketone appear unusual (Scheme 7, product 20; Scheme 8,
electron is mainly localized. In this case, C–C bond product 28; Scheme 9, product 38; and Scheme 10,
cleavage takes place. Radical cation B can undergo the product 46). The mechanism of their formation can be
following two transformations: (1) If the contribution represented as follows:
H C
CH3
H
V(V), H O
H C
CH3
H C
CH3
3
3
+
O2
.
3 2
3
2
2
CH O +
OH +H+
H C
3
H C
3
H C
3
CH3 CH3
CH .
H2O2
CH3
O
3
CH3
.
–HO
2
H CO + H O
2
2
Oxidation of Linear Fatty Acid Methyl Esters
Other evidence of oxidative C–C bond cleavage cat-
position of H O was complete after ~1 h at 30°C. As in
2 2
the oxidation of alkanes, the FAME amount consumed
monotonically increased with initial H O concentra-
(V)
alyzed by the V /H O /AcOH system was found in a
2
2
2
2
study of the oxidation of unbranched FAMEs.
tion ([H O ] ) (Fig. 9). A more than tenfold molar
2 2 0
As in the case of alkanes, in the absence of a catalyst excess of H
from a solution of H O /AcOH, FAME consumption
O was required for reaching a limiting
2
2
7
2
2
FAME conversion (x = 90%).
∞
was not observed for several hours. The reaction began
only after the addition of a catalyst (~10 mol/l vana-
dium compound) to the reaction mixture. The decom-
–
2
7
The limiting substrate conversion was determined after the com-
plete consumption of H O .
2
2
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROPEROXIDE OXIDATION OF DIFFICULT-TO-OXIDIZE SUBSTRATES
55
According to our data, the relative reactivity of
FAMEs increases with the number of carbon atoms in
the hydrocarbon skeleton (Fig. 9). The longer the car-
bon chain of the molecule, the smaller the excess of
H O required for reaching a fixed degree of substrate
1
0
8
6
4
2
2
2
8
conversion.
1
2
3
4
5
We found that not only the hydrocarbon chains of
aliphatic FAMES are shortened in the course of reac-
tion but also hydroxyl and keto groups are actively
formed in this case (Table 5). For example, three iso-
mers of hydroxyheptanoic acid, three isomers of
hydroxyhexanoic acid, three isomers of hydroxypen-
tanoic acid, two isomers of hydroxybutanoic acid, and
glycolic acid were formed in the oxidation of methyl
octanoate along with products with the same hydrocar-
bon-chain length as that in the parent FAME.
0
5
10
H O ] /[Sub]
0
15
20
[
2
2 0
Fig. 9. Limiting FAME conversion as a function of
H O ] /[Sub] in the oxidation of the following compounds
[
2
2 0
0
The good water solubility of reaction products and
their trend to condensation reactions present problems
for the quantitative analysis of the reaction mixture.
However, based on the chromatographic peak intensi-
ties, we can conclude that glycolic acid or its ester pre-
dominates over the oxidation products.
in the V(V)/H O /AcOH system: (1) methyl stearate,
2 2
(
2) methyl stearate with the addition of 2-ethylanthracene
−2
(~10 mol/l), (3) methyl caprylate, (4) methyl butyrate,
and (5) methyl propionate.
oxidation products of methyl isobutanoate because the
The oxidation of acetic acid (i.e., the solvent) could
be a source of glycolic acid in the test system. To reveal oxidative degradation of methyl isobutanoate in princi-
the routes of glycolic acid formation, we studied the ple cannot result in the formation of glycolic acid. We
oxidation of methyl stearate in CD COOD. In the chro- can conclude that the formation of glycolic acid is
3
matogram of the oxidation products treated with indicative of the oxidative degradation of FAMEs in the
BSTFA, a signal was detected with the mass spectrum test system.
that corresponds to the unlabeled trimethylsilyl ester of
The presence of other identified compounds with
glycolic acid (CH ) SiOCH COOSi(CH ) (Fig. 10).
3
3
2
3 3
The absence of D atoms from the CH group excludes chains shorter than that in the parent FAME in the prod-
2
the possibility of glycolic acid formation from the sol- ucts also provides support for the conclusion on the oxi-
vent. Moreover, glycolic acid was not detected in the dative degradation of FAMEs:
(V)
H O , AcOH, V
2
2
CH (CH ) COOMe
HO–CH (CH ) COOH
2 2
n
3
2
n
+
HO–CH (CH ) COOH + … + HOCH COOH.
n – 1
2 2 2
We found experimentally that the composition and reactivity of the aliphatic hydrocarbon chain. However,
distribution of the reaction products and the conversion the actual rate of FAME oxidation insignificantly dif-
of FAMEs remained unchanged in the presence of fered from the rates of oxidation of cyclohexane and
ionol, which is an effective scavenger of O-centered aliphatic alkanes. As well as alkanes, the FAME mole-
radicals. Analogously, the addition of 2-ethylan- cule oxidized with the formation of the corresponding
hydroxy and keto acids and their esters.
thracene had no effect, in particular, on the oxidation of
methyl stearate. These facts demonstrate that a free-
radical chain reaction mechanism does not take place in
the case under consideration.
The above facts suggest a similarity between the
mechanism of the oxidative degradation of alkanes,
which is presented in Scheme 5, and the mechanism of
FAME oxidation. In the case of alkanes, which cannot
coordinate to the V( atom, an attack occurs at the most
reactive C–H bond of the substrate. However, in con-
It would be expected that the electronegative group
V)
–
CH OOC in the FAME molecule would decrease the
3
(
V)
trast to alkanes, FAMEs can coordinate to the V atom
through the ester group, and the insertion of the oxygen
8
The test reaction includes the oxidative cleavage of the hydrocar-
bon chain of a methyl ester (see below). Therefore, the number of radical cation into a C–H bond occurs within the inner
moles of H O required for the oxidation of 1 mol of FAMEs
2
2
sphere. It is likely that the ability of the FAME mole-
cule to form a complex with vanadium(V) through the
coordination of the carboxyl O atom affects both the
rate and the selectivity of the reaction. This should be
should increase with the length of the hydrocarbon chain. This
would result in underestimated rather than overestimated reactiv-
ities of long-chain FAMEs; that is, the experimental data obtained
are undoubtedly reliable.
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

5
6
GEKHMAN et al.
Table 5. Products of FAME oxidation in the V(V)/H O /AcOH system at 30°C
2
2
FAME
Oxidation products and their concentrations on a glycolic acid basis*
cyclo-C H COOMe
Hydroxy acids:
HOCH COOH 1.00,
6
11
2
HOC H COOH 0.35,
3
6
HOC H COOH 9.30
6
10
EtCOOMe
Hydroxy acids:
Hydroxy acids:
HOCH COOH 1.0,
2
HOC H COOH (2) 0.23
2
4
n-PrCOOMe
HOCH COOH 1.00,
2
HOC H COOH (2) 0.15,
2
4
HOC H COOH (2) 0.24
3
6
**
HOC H COOH (1) 1.00
2 4
,
iso-PrCOOMe
Hydroxy acids:
HOC H COOH (1) 2.30
3
6
iso-PrCH COOMe
Hydroxy acids:
Hydroxy acids:
HOCH COOH 1.00,
2
2
HOC H COOH (1) 0.29
4
8
Me(CH ) COOMe
HOCH COOH 1.00,
2
2
5
HOC H COOH (3) 4.75,
3
6
HOC H COOH (1) 6.65,
4
8
HOC H COOH (1) 4.67,
5
10
HOC H COOH (4) 12.90
6
12
Dicarboxylic acid:
Hydroxy acids:
HOOCC H COOH (1) 2.00
5 10
Me(CH ) COOMe
HOCH COOH 1.00,
2
6
2
HOC H COOH (3) 1.13,
3
6
HOC H COOH (1) 0.65,
4
8
HOC H COOH (1) 1.40,
5
10
HOC H COOH (1) 0.85,
6
12
HOC H COOH (5) 6.65,
7
14
Keto acid:
CH (CH ) C(O)(CH2)5 – nCOOH (2) 2.77
3 2 n
Me(CH ) COOMe
Hydroxy acids:
HOCH COOH 1.00,
2
7
2
HOC H COOH (1) 0.24,
3
6
HOC H COOH (1) 0.94,
4
8
HOC H COOH (1) 1.08,
5
10
HOC H COOH (3) 6.78,
8
16
Keto acid:
CH (CH ) C(O)(CH2)6 – nCOOH (3) 5.21,
3 2 n
Dicarboxylic acid:
Hydroxy acids:
(COOH) 0.12
2
Me(CH ) COOMe
HOCH COOH 1.00,
2
14
2
HOC H COOH (2) 0.38,
2
4
HOC H COOH (1) 0.22,
3
6
HOC H COOH (1) 0.08,
5
10
HOC H COOH (1) 0.09,
6
12
HOC H COOH (1) 0.26,
7
14
HOC H COOH (1) 0.11,
8
16
Dicarboxylic acids:
Hydroxy acid:
HOOCC H COOH (1) 0.18,
5 10
HOOCC H COOH (1) 0.17
7
14
Me(CH ) COOMe
HOCH COOH 1.00
2
2
16
*
After extraction with chloroform, the reaction mixture was treated with BSTFA, and all of the products were analyzed and identified as
trimethylsilyl derivatives; for example, HOCH COOH was identified as (CH ) SiOCH COOSi(CH ) . The number of isomers found
2
3 3
2
3 3
is given in parentheses.
* HOCH COOH was not detected.
*
2
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROPEROXIDE OXIDATION OF DIFFICULT-TO-OXIDIZE SUBSTRATES
57
(a)
1
66
2
29
1
77
1
20
1
60
63
2
16
1
43
2
52
1
2
38
0
120
140
160
180
161
200
220
240
Retention time, s
(
b)
7
7
3
3
1
47
177
4
5
5
9
205
1
33
(c)
(d)
1
00
200
300
m/z
Fig. 10. Oxidation of methyl pelargonate in a CD COOD solution (30°C; [V(V)] = 4 × 10–3 mol/l; [Sub] = 0.1 mol/l; [H O ] =
3
0
2 2 0
1
mol/l): (a) chromatogram of the reaction mixture after treatment with BSTFA; (b) mass spectrum corresponding to the peak with
a retention time of 166 s; (c) library spectrum of the trimethylsilyl ester of (trimethylsilyl)hydroxyacetic acid; and (d) difference
between mass spectra (a) and (b).
+
.
taken into account in explaining the unexpectedly easy or the oxygen radical cation (O ) to the substrate
cleavage of the C–C bond in FAME oxidation. In this (three-electron oxidation) [24, 29]. The entire variety of
case, the direction of attack depends on the geometry of oxidation processes with the participation of the other
the intermediate complex of an active oxidizing agent test substrates (FAMEs and both branched and linear
and a substrate rather than the reactivity of a bond.
alkanes) can also be explained in terms of this hypoth-
+
.
1
esis. Both of the species (O and the O( P) atom)
transferred to the substrate in the activated complex are
strong acids (in terms of the Usanovich theory [33, 34]).
We expected that these species could react even with weak
bases such as molecular nitrogen. Indeed, we found exper-
Oxidation of Molecular Nitrogen
A detailed analysis of the oxidation of cyclohexane
led us to the conclusion that both of the products
imentally that in the test system N was converted into N O
(
cyclohexanol and cyclohexanone) result from the
2
2
under very mild conditions (1 atm; 0–20°C):
interaction of the alkane with the same slowly formed
vanadium(V) intermediate, which is capable of trans-
ferring either the oxygen atom (two-electron oxidation)
N + H O
N O + H O.
2 2
2
2
2
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

5
8
GEKHMAN et al.
(a)
5
7.13
6
3
5
0
60
70
Retention time, s
6
7
(
(
b)
c)
6
3
6
0
70
Retention time, s
Fig. 11. Chromatograms of the gaseous products of N oxidation in the V( /H O /CF COOH system: (a) total ion current chro-
V)
2
2
2
3
matogram; (b) fragmentogram for m/z = 30; and (c) a portion of the total ion current chromatogram.
Note that the oxidation of molecular nitrogen to
The oxidation of molecular nitrogen with hydrogen
N O or NO with molecular dioxygen in the ground peroxide is also not forbidden thermodynamically:
2
3
3
1
1
O ( Σ ) and excited O ( ∆ ) states under near-stan-
2
g
2
g
2
H O + N2(g) = 2NO(g) + 2H O ,
2 2(l) 2 (l)
dard conditions is thermodynamically impossible.
∆
G° = –58.3 kJ/mol;
298
The reaction of molecular nitrogen with ozone is
allowed [35]:
H O + N2(g) = N O + H O ,
2
2(l)
2
(g)
2
(l)
3
∆G2°98 = –13.1 kJ/mol;
N2(g) + O3(g) = N O + O2(g),
2
(g)
3
∆
G° = –58.6 kJ/mol.
3H O2(l) + N2(g) = N O(g) + 3H O(l) + O2(g),
2 2 2
2
98
∆
G° = –185.5 kJ/mol.
298
However, this reaction is unknown, probably,
because of the absence of an effective mechanism for
oxygen-atom transfer from ozone to the substrate.
The chromatogram of gaseous reaction products of
N oxidation with hydrogen peroxide in the presence of
2
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROPEROXIDE OXIDATION OF DIFFICULT-TO-OXIDIZE SUBSTRATES
59
peaks similar in the retention time and in the shape of
the spectrum to the chromatographic signal detected in
nitrogen oxidation products.
(
a)
44
45
3
0
2
7
The experimental data indicate that molecular nitro-
4
4
(
V)
(
(
b)
c)
gen underwent oxidation in the V H O /CF COOH
2 2 3
3
0
9
2
8
catalytic system with the formation of N O. Because
2
nitrous oxide was not detected in the absence of a cata-
lyst and hydrogen peroxide is stable, it is believed that
a vanadium complex that oxidizes molecular nitrogen
is formed by the interaction of H O with the catalyst.
2
2
The reaction found is the first example of the
involvement of molecular nitrogen in catalytic hydrop-
eroxide oxidation reactions.
(
d)
CONCLUSIONS
In this work, an extended range of substrates
allowed us to find a number of novel phenomena in the
chemistry of peroxide oxidation. Thus, we observed for
the first time the reactions of branched alkanes and the
methyl esters of saturated linear long-chain carboxylic
acids with carbon–carbon bond cleavage. The involve-
ment of terminal methyl groups in oxidation is also
unusual for this area of chemistry. Before this study,
phenomena of this kind were observed in neither polar
nor free-radical oxidation. The appearance of esters as
hydroperoxide oxidation products seems to be an
equally novel and unusual phenomenon.
The study demonstrated that, in the reaction of
hydrogen peroxide with vanadium complexes, active
intermediates are formed that can react with substrates
that are difficult to oxidize with oxygen for thermody-
namic reasons or because of a high activation barrier.
Of course, the reaction schemes proposed are hypothet-
ical. However, these mechanisms are consistent with
the experimental facts observed, and they can serve as
a basis for the design of further experiments.
3
0
40
50
m/z
Fig. 12. Oxidation of molecular nitrogen in the
(V)
V
/H O /CF COOH system: (a) mass spectrum of the
2 2 3
component with a retention time of 67 s in the chromato-
gram of the gaseous products of N oxidation (Fig. 11);
2
(
b) library electron-impact (70 eV) mass spectrum of nitro-
gen(I) oxide; (c) difference between mass spectra (a) and
(
b); and (d) baseline mass spectrum in the neighborhood of
the chromatographic peak of N O.
2
V(V) in a CF COOH solution (Fig. 11a) exhibits, along
3
with the unresolved chromatographic peaks of N and
2
O (retention time of 57 s) and a peak due to CO (reten-
2
2
tion time of 63 s), a low-intensity peak (Fig. 11c),
which was detected in a fragmentogram recorded at
m/z = 30 (Fig. 11b).
ACKNOWLEDGMENTS
The mass spectrum of the component that corre-
sponds to this chromatographic signal obtained by the
subtraction of the averaged background before and
after the chromatographic peak contains two signals of
ions with m/z = 44 and 30, whose intensities are much
higher than the other intensities (Fig. 12a). The experi-
mental mass spectrum of the unknown component
practically coincides with the published spectrum of
This work was supported by the Russian Foundation
for Basic Research (project no. 99-03-33248), the Fed-
eral Program of the Support for Leading Scientific
Schools (grant no. 00-15-97429) and the Program
Fundamentals of Energochemical Technologies” of
the Russian Academy of Sciences.
“
N O (Fig. 12b), as evidenced by the difference between
2
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