Kinetics and mechanism of the reaction of benzyl bromide with copper in dimethylacetamide in the presence of oxygen

Article abstract of DOI:10.1134/S1070363214090047

The reaction of copper metal with benzyl bromide in dimethylacetamide in the presence of oxygen has been studied. Oxidative dissolution of copper follows the single-electron transfer mechanism with formation of benzaldehyde, benzyl alcohols, and copper(II

Full text of DOI:10.1134/S1070363214090047

ISSN 1070-3632, Russian Journal of General Chemistry, 2014, Vol. 84, No. 9, pp. 1667–1675. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © A.M. Egorov, S.A. Matyukhova, 2014, published in Zhurnal Obshchei Khimii, 2014, Vol. 84, No. 9, pp. 1425–1434.
Kinetics and Mechanism of the Reaction of Benzyl Bromide
with Copper in Dimethylacetamide in the Presence of Oxygen
A. M. Egorov and S. A. Matyukhova
Tula State University, pr. Lenina 92, Tula, 300600 Russia
e-mail: silver_sun@inbox.ru
Received February 24, 2014
Abstract—The reaction of copper metal with benzyl bromide in dimethylacetamide in the presence of oxygen
has been studied. Oxidative dissolution of copper follows the single-electron transfer mechanism with forma-
tion of benzaldehyde, benzyl alcohols, and copper(II) coordination compounds. The kinetic and thermodynamic
parameters of the process have been determined by resistometry. Intermediately formed species have been
identified and quantitated, and the reaction stereochemistry has been studied. The reaction mechanism is discussed.
Keywords: benzyl bromide, oxidative dissolution of copper, kinetics, reaction mechanism, dimethylacetamide
DOI: 10.1134/S1070363214090047
Oxidative dissolution of metals in organic media is
a promising field of chemistry and chemical techno-
logy, which includes problems related to catalysis,
corrosion, metal refining, analysis of metals and alloys,
extraction, and preparation of various catalytic systems
and coordination compounds. The use of organic
solvents offers broad variation of donor–acceptor
properties and allows oxidative dissolution to be
accomplished under mild conditions both in the
absence and in the presence of oxygen [1].
presence of oxygen has been studied relatively poorly,
mechanisms of such processes generally have not been
studied, and kinetic parameters have not been deter-
mined; therefore, these reactions cannot be performed
on a large scale.
In the present work we studied the kinetics and
mechanism of the reaction of benzyl bromide with
copper in N,N-dimethylacetamide (DMA) in the presence
of oxygen. Oxidative dissolution of copper in the system
benzylbromide–DMA–oxygen leads to the formation
of benzaldehyde and benzyl alcohol at a ratio of 2 : 1
and copper(II) coordination compounds (Scheme 1).
Unlike processes occurring in organic media in an
inert atmosphere, oxidative dissolution of metals in the
Scheme 1.
CH Br + DMA + O2
Cu +
+
CH2 CH2
2
5%
O
H
H C
3
CH3
+
C
+
CH OH
2
<0.01%
60%
33%
[CuBr mDMA]
O
O
O
+
C
+
C
+
2
OH
CH2
<0.1%
<0.01%
m = 2, 3.
1
667

1
668
EGOROV, MATYUKHOVA
Table 1. Parameters of the ESR s[ectra of 2,2,6,6-tetramethylpiperidine 1-oxyl
g factor
a
N
, G
a
H
, G
ΔH, G
Solvent
2
.0055±0.0002
16.6±0.1
15.4±0.1
16.2±0.1
16.4±0.1
0.2±0.2
0.2±0.2
0.2±0.2
0.2±0.2
1.5±0.3
1.5±0.2
1.5±0.2
1.5±0.2
Benzyl bromide–dimethylacetamidea
Benzene [4]
Diethylene glycol [4]
Formamide [4]
2
2
2
.0055
.0055
.0055
a
Molar ratio 5 : 8. Data of this work.
Table 2. Reaction of benzyl bromide with copper in dimethylacetamide in the absence and in the presence of dicyclohexyl-
deuterophosphine (DCPD)
a
Yield, mol %
DCPD /Cu,
mol
PhCH
0
2
Db
PhCH
2
CH
2
Ph
PhCHO
PhCH OH
2
c
0
5±1
2±1
0
60±1
19±1
2±1
33±1
11±1
1±1
3
68±1
5
97±1
a
Dicyclohexyldeuterophosphine contained 1% of dicyclohexylphosphine as impurity. Toluene, 1%. c Benzoic acid (<0.1%) and benzyl
b
benzoate (<0.01%) were also detected.
The observed benzaldehyde-to-benzyl alcohol ratio,
which is capable of reacting with radical intermediates.
No TEMPO signal appeared after complete oxidation
of copper and reduction of the reaction mixture with
hydrogen peroxide under alkaline conditions [2, 3].
as well as the presence of 1,2-diphenylethane and
traces of 4,4'-dimethylbiphenyl, suggest radical me-
chanism of the reaction.
In order to analyze the reaction stereochemistry we
synthesized (–)-(R)-1-bromo-1-phenylethane which
was brought into reaction with copper in DMA.
Organic products were isolated by preparative liquid
chromatography and were optically inactive; their
molar ratios were as follows: (RS)-1-phenylethanol,
Radical intermediates were detected and identified
using dicyclohexyldeuterophosphine (DCPD) which is
known as an efficient trap for benzyl radical; also, the
concentration of the latter in solution can be deter-
mined quantitatively in this way [2, 5]. The results are
presented in Table 2.
2
1
: 1; (RS)- and (RR)/(SS)-2,3-diphenylbutanes,
The product composition in the dehalogenation of
benzyl bromide with copper in DMA on exposure to
air in the presence of DCPD depended on the Cu–
DCPD molar ratio (Table 1). At a Cu–DCPD molar
ratio of 1 : 5, the major product was α-deuterotoluene
.03 : 1; styrene–ethylbenzene, 1 : 1. These ratios are
very consistent with published data on the oxidation
(
with oxygen), recombination, and disproportionation
of 1-phenylethyl radical in solution [2].
The radical mechanism of the oxidative dissolution
(
97%), and small amounts of benzaldehyde (2%) and
of copper in PhCH Br–DMA was confirmed by the
2
benzyl alcohol (1%) were detected (Scheme 2).
ESR study performed according to the known
procedure [2, 3] at 293 K in the presence of 2,2,6,6-
tetramethylpiperidine 1-oxyl (TEMPO) as stable
Analysis of the products formed in the reactions of
copper with benzyl bromide in dipolar aprotic solvents
in the presence and in the absence of DCPD indicated
intermediate formation of benzyl radical (Table 2). In
all cases, a green finely crystalline solid was isolated;
it was a mixture of two coordination compounds of
copper(II), which were difficult to separate. The
copper compounds were identified as the complex with
radical species; the molar ratio Cu–PhCH Br–DMA–
2
TEMPO was 1 : 5 : 8 : 0.2. The ESR spectrum of the
reaction mixture was a triplet with a width of 85 G,
which corresponded to coupling of the unpaired
electron with the nitroxyl nitrogen nucleus. The ESR
parameters in combination with published data [4] are
given in Table 1. These findings allowed us to attribute
the observed ESR spectrum to nitroxyl radical.
triphenylphosphine oxide (TPPO) [CuBr (TPPO) ],
2
2
which was obtained by ligand exchange according to
the following scheme:
The formation of radical species during the process
follows from the disappearance of signals of TEMPO
CuBr
2
·mDMA + 2TPPO → [CuBr
2
(TPPO) ] + mDMA.
2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 9 2014

KINETICS AND MECHANISM OF THE REACTION OF BENZYL BROMIDE
1669
Scheme 2.
DCPD
Cu +
CH Br + DMA + O
CH D
2
2
2
O
+
C
+
CH OH + [CuBr mDMA]
2
2
H
m = 2, 3.
Analysis of the reaction mixture by ion chromato-
graphy showed the presence of copper(II) complexes
and small amounts of copper(I) complexes, while no
copper(III) compounds were detected. Copper(I)
complexes could be formed via dehalogenation of
benzyl halide with copper or as a result of reduction of
copper(II) compounds with copper metal [3] or benzyl
radical in dipolar aprotic solvents [6]. However, the
process under anoxic conditions was very slow; there-
fore, the formation of large amounts of copper(I)
coordination compounds in this way is hardly
probable. When the reaction of copper with benzyl
bromide was carried out in the presence of oxygen and
anhydrous CuCl (Cu–CuCl molar ratio 1 : 1), the
Scheme 3.
O
O
O
O
O
+
+2
Cu
Cu
n
n
O
Ia
Ib
As shown in [10–12], oxygen adsorption on
metallic copper is so rapid that its rate cannot be
measured accurately. This means that the energy of
activation of this process is very low. Presumably,
adsorption of oxygen on the copper surface and
formation of surface oxygen complexes like Ia and Ib
2
2
[
13] are the initial steps of the reaction. Complex Ib is
more stable due to stabilization of copper(II) (Scheme 3).
mixture contained 2% of benzyl chloride, indicating
that benzyl radical partially reduces copper(II)
compounds in solution to copper(I).
Thus, assuming that the reaction follows the
Langmuir–Hinshelwood pattern including adsorption
of oxygen, benzyl bromide, and dimethylacetamide on
different active sites of the surface, the process can be
represented by the following scheme:
PhCH
2
+ CuCl
2
→ PhCH Cl + CuCl.
2
We previously showed [7] that benzyl halides react
with copper in dipolar aprotic solvents in an inert
atmosphere under mild conditions and that the reaction
occurs on the metal surface according to the radical me-
chanism. The products were 1,2-diphenylethane, traces
of 4,4'-dimethylbiphenyl, and copper(I) coordination
compounds. The reaction rate increases on exposure to
atmospheric oxygen, and the product composition
changes, indicating change of the reaction mechanism.
K
2
1
Cu
1
+ O
ҙ Cu
1
O
2
,
(1.1)
K
2
Cu
1
O
2
+ O
2
ҙ O
2
Cu
1
O
2
,
(1.2)
(1.3)
(1.4)
(1.5)
K
3
Cu
2
+ Solv ҙ Cu
2
(Solv),
K
4
The kinetics of the oxidative dissolution of copper
in the system PhCH –DMA–O were studied by the
Cu
Cu
Solv
3
+ PhCH
+ Cu
2
Hlg ҙ Cu
3
(PhCH
2
Hlg),
2
2
K
5
Solv
resistometric method [8], which previously ensured
high accuracy and reproducibility in kinetic studies of
fast heterofeneous reactions. The reaction was carried
O
2
1
O
2
2
(Solv) ҙ –————– + Cu
2
,
+
O–O–
O
2
Cu
1
K
6
2Solv
out in benzene [donor number DN(SbCl ) = 0.42 kJ/mol]
5
—
————– + Cu
2
(Solv) ҙ –——–———– + Cu
2
, (1.6)
(1.7)
as inert solvent. Figure shows the dependences of the
reaction rate on the concentrations of benzyl bromide
and DMA. These dependences are typical of reactions
following Langmuir–Hinshelwood or Eley–Rideal
mechanism [2, 9].
+
O–O–
–
2+
–
O
2
Cu
1
O–OCu O–O
1
2Solv
—
——————– + Cu
3
(PhCH Hlg)
2
–
2+
O–O–
O–OCu
1
k
7
→
Reaction products.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 9 2014

1
670
EGOROV, MATYUKHOVA
Here, Cu , Cu , and Cu are active sites on the copper
K and K are the equilibrium constants of electron
1
2
3
5
6
surface, where oxygen is adsorbed; Solv stands for the
transfer to oxygen; k is the rate constant of the rate-
7
solvent molecule (dimethylacetamide); K , K , K , and
determining step. Assuming that equilibria (1.1)–(1.6)
are stationary, we obtain expression (1.8) for the
overall reaction rate.
1
2
3
K are the adsorption constants of oxygen, dimethyl-
4
acetamide, and benzyl bromide on the copper surface;
2
2 2
k
7
S
1
S
3
K
1
K
2
K
3
K
4
K
5
K
6
[PhCH
2
Br][Solv] P (O )
2
w = ——————————————————————————————–——————————— .
(1.8)
2
2
2
2
3
2
2
4
(1 + K [PhCH
2
Br]){1 + K
1
P (O
2
) + K
1
K
2
P (O
2
) + K1ꢀ
K
2
K
3
K
5
P (O
2
)[Solv] + K
1 2
K K
K
5
K
6
P (O
2
)[Solv] }
K
3
Here, S and S are the surface areas of active sites Cu
1
3
1
Cu
+ PhCH
+ Cu
2
+ Solv ҙ Cu
2
2
(Solv),
(2.3)
(2.4)
(2.5)
and Cu , P(O ) is the oxygen pressure, and w is the
3
2
4
K '
reaction rate.
Cu
2
2
Hlg ҙ Cu
(PhCH
Solv
2
Hlg),
K
5
Taking into account that the constants K , K , and
1
2
O
2
Cu
1
O
2
2
(Solv) ҙ –————– + Cu
2
,
+
O–O–
K are very high (K > K , K >> 1, K >> 1) [10–12],
O
5
1
2
2
5
2
Cu
1
Eq. (1.8) is transformed into
[PhCH
Solv
K
6
Solv
k
7
S
1
S
3
K
3
K
4
K
6
2
Br][Solv]
—————– + Cu (Solv) ҙ –——–———– + Cu , (2.6)
2
2
+
O–O–
–
O–OCu
O–O–
2+
O
2
Cu
1
1
w = ——————————————— . (1.9)
(
1 + K
4
[PhCH
2
Br])(1 + K
3
6
K [Solv])
2Solv
k
o7
—
——————– + Cu
2
(PhCH
2
Hlg) → Products. (2.7)
–
2+
O–O–
O–OCu
1
If benzyl bromide and dipolar aprotic solvent
molecules are adsorbed on the same sites, the reaction
scheme is given by Eqs. (2.1)–(2.7).
Here, K ' is the adsorption constant of benzyl bromide
on Cu , and k is the rate constant of the rate-deter-
4
2
07
mining step provided that benzyl bromide and
dimethylacetamide are adsorbed on the same active
sites. Application of the steady-state approximation
yields Eq. (2.8) for the reaction rate.
K
1
Cu
1
+ O
2
ҙ Cu
1
O
2
,
(2.1)
(2.2)
K
2
Cu
1
O
2
+ O
2
ҙ O
2
Cu
1
O ,
2
2
3
2 2
k
o7
S
1
S
3
K
1
K
2
K
K
4
'K
5
K
6
[PhCH
2
Br][Solv] P (O )
2
w = –——————————————————————————————————–—————————— . (2.8)
2
2
2
2
3
2
2
(
1 + K
3
[Solv] + K
4
'[PhCH
2
Br]){1 + K
1
POꢀ + K
1
K
2
P (O
2
) + K
K
1 2
K
3
K
5
P (O
2
)[Solv] + K
1 2
K K
K
5
K
6
P (O
2
)[Solv] }
w, g cm–2 min–1
Here, S and S are the surface areas of active sites Cu
1
3
1
and Cu , respectively.
3
Using the same approximations as in the derivation
of Eq. (1.9), we obtain Eq. (2.9).
3
k
о7
S
1
S
3
K
3
K
4
'K
w = ——————————————————— . (2.9)
1 + K [Solv] + K '[PhCH Br])(1 + K [Solv])
6
[PhCH
2
Br][Solv]
(
3
4
2
3
K
6
If dioxygen anion reacts with benzyl bromide
supplied from the bulk solution, the following reaction
scheme can be proposed:
1
2
K
1
Cu
1
+ O
2
ҙ Cu
1
O
2
,
(3.1)
(3.2)
(3.3)
K
2
с, mol/L
Cu
Cu
1
O
2
+ O
2
ҙ O
2
Cu
1
O ,
2
Plots of the rate of oxidative dissolution of copper in the
K
3
system benzyl bromide–dimethylacetamide–oxygen versus
2
+ Solv ҙ Cu
2
(Solv),
Solv
initial reactant concentrations at 353 K in benzene: (1) cRBr
=
K
5
0
.5 M, cDMA = 0–7 M; (2) cDMA = 0.5 M, cRBr = 0–7 M;
O Cu O + Cu (Solv) ҙ –————– + Cu , (3.4)
2
1
2
2
2
+
(
3) cDMA = 2 M, cRBr 0–7 M.
O
2
Cu
O–O–
1
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 9 2014

KINETICS AND MECHANISM OF THE REACTION OF BENZYL BROMIDE
1671
Solv
K
6
2Solv
Here, k' is the rate constant of the rate-determining
7
—
————– + Cu
2
(Solv) ҙ –——–———– + Cu
2
, (3.5)
+
O–O–
–
2+
–
O
2
Cu
2
1
O–OCu
1
O–O
step assuming approach of benzyl bromide from the
bulk solution. Using the steady-state approximation we
arrive at Eq. (3.7).
Solv
7
k '
—
——————– + PhCH
2
Hlg → Reaction products. (3.6)
–
2+
O–O–
O–OCu
1
2
2 2
k
7
'S
1
K
1
K
2
K
3
K
4
K
5
K
6
[PhCH
2
Br][Solv] P (O )
2
w = ————————————————————————————————— . (3.7)
2
2
2
2
3
2
)[Solv]2
1
+ K
1
P (O
2
) + K
1
K
2
P (O
2
) + K
1
K
2
K
3
K
5
P (O
2
)[Solv] + K
1
K
2
K
K
5
K
6
P (O
2
This equation can be simplified to (3.8) by applying
the same approximations as for Eq. (1.9):
Here, K' and K' are the equilibrium constants for
5
6
electron transfer to oxygen from solvent molecules in
the bulk solution. In this case, the rate of the process is
given by Eq. (4.7) or (4.8) in terms of the steady-state
approximation.
k
7
'S [PhCH Br][Solv]
1
K
3
K
6
2
w = –——————————— .
+ K [Solv]
(3.8)
1
K
3 6
If the dioxygen molecular complex reacts with
dimethylacetamide coming from the bulk solution,
while the resulting dioxygen anion reacts with benzyl
bromide adsorbed on the copper surface, the reaction
scheme may be represented as follows:
According to the Eley–Rideal mechanism, in the re-
actions of dioxygen molecular complex with dimethyl-
acetamide and of dioxygen anion thus formed with
benzyl bromide, the solvent and reagent molecules are
supplied from the bulk solution.
K
1
K
1
Cu
1
+ O
2
ҙ Cu
1
O
2
,
(4.1)
(4.2)
(4.3)
(4.4)
Cu
1
+ O
2
ҙ Cu
1
O
2
,
(5.1)
(5.2)
K
2
K
2
Cu
1
O
2
+ O
2
ҙ O
2
Cu
1
O
2
,
Cu
Cu
Solv
1
O
2
+ O
2
ҙ O
2
Cu
1
O ,
2
K
4
Cu
Cu
Solv
3
+ PhCH
2
Hlg ҙ Cu
3
(PhCH
Solv
2
Hlg),
K
5
Solv
O
2
1
O
2
+ Solv ҙ –————– ,
(5.3)
+
O–O–
5
K '
O
2
Cu
1
O
2
1
O
2
+ Solv ҙ –————– + Cu
2
,
+
–
O
2
Cu O–O
1
6
K '
Solv
—
————– + Solv ҙ –——–———– , (5.4)
+
O–O–
–
O–OCu
O–O–
2+
K
6
'
2Solv
O
2
Cu
1
1
—
————– + Solv ҙ –——–———– ,
(4.5)
+
–
–
2+
–
O
2
Cu
1
O–O
O–OCu
1
O–O
2
Solv
2+
7
k '
—
——————– + PhCH Hlg → Reaction products. (5.5)
2
–
O–O–
2
Solv
O–OCu
1
—
——————– + Cu
3
(PhCH
2
Hlg)
–
2+
–
O–OCu O–O
1
Steady-state approximation gives Eq. (5.6) for the
reaction rate:
k
7
→
Reaction products.
(4.6)
2
2
k
7
'S
1
K
1
K
2
K
5
'K
6
'[PhCH
2
Br][Solv] P (O )
2
w = ————————————————————————————————– .
(5.6)
2
2
2
2
)[Solv]2
1
+ K
1
P (O
2
) + K
1
K
2
P (O
2
) + K
1
K
2
K
5
'PP (O
2
)[Solv] + K
1
K
K
2 5
'K
6
'P (O
2
Assuming that K >> K , K >> 1, and K ' >> 1 [9–
described by Eq. (2.9) passes through a maximum.
Equations (3.8) and (5.7) are linear dependences of the
reaction rate on the concentration of benzyl bromide.
Therefore, reaction schemes (2.1)–(2.7), (3.1)–(3.6),
and (5.1)–(5.5) can be ruled out.
1
2
2
5
1
2], this equation is simplified to Eq. (5.7).
k
7
'S '[PhCH Br][Solv]
1
K
6
2
w = –—————————— .
(5.7)
1
+ K '[Solv]
6
All dependences of the reaction rate on the concen-
tration of dimethylacetamide are smooth; the curve
Thus, the experimental kinetic curves can be
described by Eqs. (1.9) and (4.8). Insofar as it was
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 9 2014

1
672
EGOROV, MATYUKHOVA
Table 3. Kinetic and thermodynamic parameters of the oxidative dissolution of copper in the system benzyl bromide–
dimethylacetamide according to schemes (1.1)–(1.7) and (4.1)–(4.6)a
k × 10 , g cm min–1
4
–2
K
4
, L/mol
K
'
K
3
b, L/mol
K
6
c
Т, K
6
3
3
3
3
3
3
13
23
33
38
43
53
8.1±0.1
15.7±0.2
27.7±0.3
34.6±0.4
46.1±0.5
74.2±0.6
0.95±0.03
0.69±0.02
0.49±0.02
0.44±0.01
0.39±0.01
0.29±0.01
7.74±0.05
3.65±0.04
1.55±0.03
1.11±0.03
0.85±0.03
0.40±0.02
1.49±0.02
1.04±0.02
0.70±0.02
58±0.01
0.51±0.01
0.36±0.01
5.19±0.9
3.51±0.8
2.21±0.7
1.91±0.6
1.67±0.5
1.11±0.5
a
0
0
0
6
= –35.2±2.6; ΔH 06 ' = –68.0±3.6 kJ/mol; 298 K: ΔS 0R Br = –86.8±0.8; ΔS D0 MA
E
a
= 50.6±2.2, ΔHRBr = –27.0±1.5; ΔHDMA = –32.8±1.4; ΔH
=
0
6
0
–1
–1
–
102±1; ΔS
= –200±7, ΔS6' = –98.1±0.9 J mol K ; sample correlation coefficients for the dependence ln Y = f(1/T): R = 0.999 (Y = k),
b
c
R = 0.995 (Y = K
4
), R = 0.999 (Y = K'
6
). Data of [16]. Calculated by the formula K
6
= K'
6
3
/K .
impossible to distinguish reaction mechanisms
corresponding to these equations, we calculated the
kinetic and thermodynamic parameters of the process
according to both schemes.
(2) Electron transfer from the metal surface to
oxygen and simultaneous reaction of adsorbed species
IIa with dimethylacetamide according to Langmuir–
Hinshelwood or Elley–Rideal. In the first case,
adsorbate IIa reacts with dimethylacetamide adsorbed
on a different site of the metal surface.
By linearization of the experimental curves in the
1
7
/w—1/c coordinates, we calculated the values of k =
K
3
k S S , K , and K' using Eqs. (1.9) and (4.8) and
1
3
4
6
Cu
2
+ DMAc ҙ Cu
2
(DMA),
determined the heat of adsorption of benzyl bromide
(
ΔH0 ), change in the enthalpy of steps (1.6) (ΔH )
0
RBr
6
DMA
0
and (4.6) (ΔH ), and the energy of activation (E ) for
O
2
Cu
1
O
2
+ Cu
2
(DMA) ҙ –————– + Cu
2
,
6
'
a
O
2
Cu
O–O–
1
+
the rate-determining step.
As follows from Eqs. (1.9) and (4.8), K' = K K .
6
3
6
DMA
K
6
DMA
—
————– + Cu
2
(DMA) ҙ –——–———– + Cu
2
.
We showed in [14] that the oxidative dissolution of
copper in the system carbon tetrachloride–dimethyl-
acetamide follows the Langmuir–Hinshelwood me-
chanism involving independent adsorption of the
components on different active sites on the metal
surface. Comparison of the mechanism given by
reactions (1.1)–(1.7) with the mechanism proposed in
+
O–O–
–
2+
O–O–
O
2
Cu
1
O–OCu
1
IIb
In keeping with the Eley–Rideal mechanism,
adsorbate IIa reacts with dimethylacetamide arriving
from the bulk solution.
K
5
DMA
[
14] indicated that steps (2) [14] and (1.4) refer to the
O
2
Cu
1
O
2
+ DMA ҙ –————– + Cu
2
,
+
O–O–
IIa
O
2
Cu
1
same process; therefore, these steps should be
characterized by similar thermodynamic parameters;
i.e., the adsorption constant of dimethylacetamide on
DMA
K
6
'
DMA
—
————– + DMA ҙ –——–———– .
+
O–O–
–
O–OCu
O–O–
2+
copper surface can be used to calculate K . The results
are presented in Table 3.
O
2
Cu
1
1
6
IIb
Thus, the results of our present study and published
data [7, 10–14] allowed us to propose the following
steps of the oxidative dissolution of copper in the system
benzyl bromide–dimethylacetamide under oxic conditions:
These mechanisms are impossible to distinguish.
Positive charge generated as a result of electron
transfer is delocalized over a group of copper atoms,
thus stabilizing superoxide anions. Coordination of
(
1) Adsorption of oxygen on the copper surface.
2+
dimethylacetamide to Cu also favors stabilization of
1
K
1
superoxide complexes IIb.
Cu
Cu
1
2
+ O
2
2
ҙ Cu
1
O
2
,
(
3) Electron transfer from dioxygen anion IIb to
K
ҙ O
2
1
O
+ O
2
Cu
IIa
1
O
2
.
benzyl bromide adsorbed on a different active site of
the metal surface.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 9 2014

KINETICS AND MECHANISM OF THE REACTION OF BENZYL BROMIDE
1673
K
4
(
7) Reaction of PhCH OO· with copper(I) coordi-
2
Cu
3
+ PhCH
2
Hlg ҙ Cu
3
(PhCH
2
Hlg),
nation compounds [16, 17].
2
DMAc
PhĊH
2
OO· + [CuBr(DMAc) ]
m
—
——————– + Cu (PhCH
3
2
Hlg)
–
2+
–
O–OCu
1
O–O
→ PhCHO + [CuBr(OH)(DMAc)
m =1, 2.
m
,
IIb
k
7
2DMAc
+ —–———– + Cu
(
8) Formation of copper(II) coordination compounds.
→
PhĊH
2
3
+ O .
2
+
–
BrCu
1
O–O
CuBr
2
(DMAc)
2
] + m-2DMAc → [CuBr
m = 2–4.
2
(DMAc) ],
m
IIc
Transfer of one electron leads to decomposition of
dioxygen anion with liberation of molecular oxygen
and formation of adsorbed benzyl radical and
superoxide anion IIc.
Side reactions:
Cu + [CuBr
2
(DMAc)
m
] + n-mDMAc
],
→
2[CuBr(DMAc)
n
PhĊH
2
2
+ [CuBr (DMAc)
2
m
]
(
4) Transfer of the second electron from superoxide
→
PhCH
Br + [CuBr(DMAc)
n
] + n-mDMAc,
IIc to benzyl bromide.
n = 2–4, m = 2–4.
2
DMA
—
————– + Cu
3
(PhCH
2
Hlg)
+
–
EXPERIMENTAL
BrCu
1
O–O
IIc
The ESR spectra of reaction mixtures were
recorded on a Radiopan SE/X-2543 spectrometer
→
PhĊH
2
+ CuBr
2
(DMA)
2
+ Cu
3
+ Cu
1
' + O .
2
(
9400 MHz) at 293 K according to the procedure
In this case, the formation of radical ion pair is ener-
getically less favorable than the formation of benzyl
radical.
described in [2, 3] using 2,2,6,6-tetramethylpiperidine
-oxyl [2, 14] as stable radical. The reaction mixtures
were analyzed by GC/MS on a Hewlett Packard HP-
890 chromatograph coupled with an HP 5972 mass-
1
(
5) Recombination and isomerization of benzyl
5
1
radicals.
selective detector (for conditions, see [14]). The H
NMR spectra were recorded on a Jeol LTD FX-90Q
spectrometer with Fourier transform from 25–30%
solutions in carbon tetrachloride or chloroform-d; the
chemical shifts were determined with an accuracy of
PhĊH
2
+ PhĊH
2
Қ
PhH
2
C:CH
2
Ph → PhCH
2
CH Ph
2
Қ
±
0.01 ppm relative to hexamethyldisiloxane as internal
reference. The IR spectra were measured in the range
H C
CH3
3
–
1
from 400 to 4000 cm on a Nicolet IMPACT 400d
instrument from samples prepared as KBr pellets.
H C
CH3
The purity of the initial reactants was checked, and
the organic reaction products were quantitated, by
GLC under the conditions described in [18].
3
The recombination and isomerization of two benzyl
radicals in solution involves intermediate formation of
the radical pair Ph–H C···CH –Ph, which is directly
confirmed by the detection of traces of 4,4'-
dimethylbiphenyl.
Inorganic products (copper cations) were analyzed
by ion chromatography on a Tsvet-3006 chromatograph
as described in [14]. The elemental analyses were
obtained on a Carlo Erba 1100 analyzer according to
standard procedures. The composition of deuterium-
containing compounds was determined according to
the procedure reported in [19].
2
2
(
6) Oxidation of PhCH radicals with atmospheric
2
oxygen follows a radical chain mechanism [15, 16],
which implies generation of PhCH OO· radicals and
2
their disproportionation.
Copper powder with a purity of no less than 99.99%
Ph–CH
2
+ O
2
→ PhCH
2
OO·,
was prepared by reduction of CuSO (analytical grade)
4
2
PhCH OO· → PhCH
2
2
OH + PhCHO + O
2
.
with magnesium powder (MP-1) in water as described
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 9 2014

1
674
EGOROV, MATYUKHOVA
in [20]; the product was washed with acetone and dried
under reduced pressure (10 mm). Yield 95%.
The stereochemistry of the reaction was studied
following the procedure described above for the
synthesis of copper coordination compounds. (+)-(R)-
Copper wire (GOST 7262-54, 99.99%) with a diameter
of 0.05 mm was kept for 24 h in dimethylformamide.
Just before use, swelled insulation film was removed
mechanically from a sample of copper wire, and the
wire was kept for 5–10 s in concentrated nitric acid and
washed with water, acetone, and dimethylacetamide.
1
-Bromo-1-phenylethane was used as benzyl halide.
After precipitation of solid product, the ether solution
contained (according to the GC/MS data) aceto-
phenone, (RS)-1-phenylethanol, (RS,RS)-2,3-diphenyl-
butane, (RR/SS)-2,3-diphenylbutane, ethylbenzene,
styrene, and unreacted (+)-(R)-1-bromo-1-phenyl-
ethane and dimethylacetamide. All organic products
were isolated by preparative liquid chromatography,
and the physical constants of the isolated (RS,RS)-2,3-
diphenylbutane, (RR/SS)-2,3-diphenylbutane,
ethylbenzene, and styrene were consistent with
published data [2, 25].
(
+)-R-1-Bromo-1-phenylethane was synthesized by
reaction of (–)-(S)-1-phenylethanol with POBr in the
3
presence of pyridine in pentane [21]. Yield 74%, bp
2
5
8
[
6–87°C (11 mm), α = +73.3° (neat); published data
D
2
5
21]: α = +96.3° (neat, ee = 100%) [21].
D
Triphenylphosphine oxide was synthesized by oxida-
tion of triphenylphosphine (Aldrich) with potassium
permanganate in acetone according to [22]. Yield 80%,
mp 156–158°C [22].
The reaction in the presence of radical scavenger
was carried out by analogy with the above described
procedures using dicyclohexyldeuterophosphine. The
ether solution contained unreacted benzyl bromide and
dimethylacetamide, as well as benzyl alcohol, benz-
aldehyde, and α-deuterotoluene. The yields are given
in Table 2.
All other organic reagents used were commercial
products. N,N-Dimethylacetamide [2] and other organic
solvents were purified according to standard proce-
dures [23]. All solvents were degassed by repeated
freezing and thawing under reduced pressure and were
stored in air-tight ampules.
1
α-Deuterotoluene. H NMR spectrum, δ, ppm: 2.32 m
(
2H, CH ), 7.15 m (5H, H ). Mass spectrum, m/z (I ,
2
arom
rel
Copper coordination compounds. A mixture of 1 g
of copper powder, 10 mL of dimethylacetamide, and
+
+
+
%
): 93 (100) [M] , 92 (93) [M – H] , 91 (46) [M – D] , 66
+
+
(
9
9) [M – C H ] , 65 (11) [M – C H D] . Found, %: C
2
3
2
2
1
0 mL of benzyl bromide was heated at 70°C on
0.35; H 7.52; D 2.13. C H D. Calculated, %: C 90.27;
7
7
exposure to air. After 2 h, the mixture was treated with
0 mL of diethyl ether. The diethyl ether layer was
H 7.56; D 2.17.
2
Copper coordination compounds were synthesized
in the presence of anhydrous copper(II) chloride
according to the procedure described above at a Cu-to-
separated, and the residue was dissolved in 20 mL of
acetone–hexane (1 : 1). After 24 h, green crystals
separated, which were an intractable mixture of two cop-
per(II) coordination compounds. The crystals were dis-
CuCl molar ratio of 1 : 1. The ether solution contained
2
unreacted benzyl bromide and dimethylacetamide, as
well as 1,2-diphenylethane, benzyl alcohol, benz-
aldehyde, and benzyl chloride (2%).
solved in 20 mL of dimethylacetamide, 11 g of Ph PO
3
was added, and the mixture was heated for 10 min at
7
0°C. After cooling, dark brown [CuBr (TPPO) ]
2
2
complex separated and was isolated by conventinal
methods. Yield 8.7 g (70%), mp 299–299.5°C [24]. IR
Kinetic study. The kinetics of the reaction of cop-
per with benzyl bromide in dimethylacetamide in the
presence of oxygen were studied by resistometry in
benzene on exposure to air [9]. Three series of
experiments were carried out. In the first series, the
–1
spectrum, ν, cm : 3030 (C–H), 1600 (C–C), 1580 (C–C),
1
1
500 (C–C), 1449 (C–C), 1150 (P=O), 1115 (C–H),
080 (C–H), 730 (C–H), 755 (C–P), 695 (δ C–H), 539
(
P=O). Found, %: C 55.45; H 3.80; Br 20.51; Cu8.17;
concentration of dimethylacetamide was c
= 0.5 M,
DMA
P 7.96. C H Br CuO P . Calculated, %: C 55.46; H
3
36
30
2
2 2
and the initial concentration of benzyl bromide c
RBr
.85; Br 20.50; Cu 8.15; P 7.94.
was varied from 0 to 7 M; in the second series, the
initial concentration of benzyl bromide was c = 0.5 M,
The diethyl ether solution contained unreacted
RBr
benzyl bromide and dimethylacetamide, 1,2-diphenyl-
ethane, benzyl alcohol, benzaldehyde, and traces of
benzoic acid and benzyl benzoate, which were iden-
and c
was varied from 0 to 7 M; in the third series,
DMA
the initial concentration of DMA was c
= 2 M, and
DMA
c
RBr
was varied from 0 to 7 M. The results are
1
tified by H NMR and GC/MS; their yields are given
presented in figure and Table 3. As followed from the
dependence of the rate of dissolution of copper on the
in Table 2.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 9 2014

KINETICS AND MECHANISM OF THE REACTION OF BENZYL BROMIDE
1675
stirring speed, the reaction course conformed to the
kinetic mode.
Vecher, A.A., and Raspopov, S.A., Phys. Chem. Chem.
Phys., 1999, vol. 1, p. 5311.
1
1
3. Lavrent’ev, I.P. and Khidekel’, M.L., Russ. Chem. Rev.,
1
983, vol. 52, no. 4, p. 337.
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 9 2014