Kinetics of methoxy-NNO-Azoxymethane hydrolysis in strong acids

Article abstract of DOI:10.1134/S0023158411010228

The kinetics of methoxy-NNO-azoxymethane (I) hydrolysis in concentrated solutions of strong acids (HBr, HCl, HClO₄, and H₂SO ₄) has been investigated by a manometric method. The gas evolution rate is described by the equation corresponding to two consecutive first-order reactions, with the rate constant of the second reaction considerably exceeding the rate constant of the first reaction, i.e., k ₂ {ie17-1} k ₁. The temperature dependences of k ₁ (s^-1) in 47.59% HBr in the temperature range from 60 to 90°C and in 64.16% H ₂SO₄ between 80 and 130°C are described by Arrhenius equations with IogA= 12.7 ± 1.5 and 13.6 ± 1.4 and E _a = 115 ± 10 and 137 ± 10 kJ/mol, respectively. The parameters of the Arrhenius equation for the rate constant k ₂ for the reaction in 64.16% H₂SO₄ between 80 and 130°C are IogA= 9.1 ± 2.5 and E _a = 91 ± 18 kJ/mol. An analysis of the UV spectra of compound I in concentrated H₂SO₄ shows that I is a weak base (pK_BH⁺ ≈ - 6). The rate-determining step of the hydrolysis of I is the attack of the nucleophile on the carbon atom of the MeO group of the protonated molecule of I. The resulting methyldiazene dioxide decomposes via a complicated mechanism to evolve N₂, NO, and N₂O. The pseudo-first-order rate constant k ₁ of the reaction at 80°C depends strongly on the acid concentration and on the type of nucleophile (Br^-, Cl^-, or H₂O). The relationship between k ₁ and the rate constant k of the bimolecular nucleophilic substitution reaction (S_N2) is given by the linear equation log [k₁/(C_H + C_Nu)] = m ≠ mX ₀ + log (k/K^BH+), where C_H ⁺ and C _Nu are the concentrations of H+ and nucleophile, respectively; X ₀ is the excess acidity; and m and m are coefficients. The Swain-Scott equation log (k_Nu/k_H2 _O) = ns, where n is the nucleophilicity factor and s is the substrate constant (s = 0.72), is applicable to the rate constants k of the S _N2 reactions of the protonated molecule of I with Br^-, Cl^-, and H₂O.

Full text of DOI:10.1134/S0023158411010228

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2011, Vol. 52, No. 1, pp. 17–25. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © I.N. Zyuzin, D.B. Lempert, 2011, published in Kinetika i Kataliz, 2011, Vol. 52, No. 1, pp. 19–27.
Kinetics of MethoxyꢀNNOꢀAzoxymethane Hydrolysis
in Strong Acids
I. N. Zyuzin and D. B. Lempert
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia
eꢀmail: zyuzin@icp.ac.ru
Received January 12, 2010
Abstract—The kinetics of methoxyꢀNNOꢀazoxymethane (I) hydrolysis in concentrated solutions of strong
acids (HBr, HCl, HClO4, and H SO4) has been investigated by a manometric method. The gas evolution rate
2
is described by the equation corresponding to two consecutive firstꢀorder reactions, with the rate constant of
the second reaction considerably exceeding the rate constant of the first reaction, i.e., k2
ӷ
k1. The temperꢀ
–1
ature dependences of k1 (s ) in 47.59% HBr in the temperature range from 60 to 90°C and in 64.16% H SO
2
4
between 80 and 130°C are described by Arrhenius equations with log
15 10 and 137 10 kJ/mol, respectively. The parameters of the Arrhenius equation for the rate constant k2
for the reaction in 64.16% H SO between 80 and 130°C are log = 9.1 2.5 and Ea = 91 18 kJ/mol. An
KBH
in concentrated H SO4 shows that is a weak base ( 6).
is the attack of the nucleophile on the carbon atom of the
. The resulting methyldiazene dioxide decomposes via a compliꢀ
A
= 12.7
±
1.5 and 13.6
±
1.4 and Ea =
1
±
±
A
±
±
2
4
analysis of the UV spectra of compound
The rateꢀdetermining step of the hydrolysis of
MeO group of the protonated molecule of
I
I
p
+
≈
⎯
2
I
I
cated mechanism to evolve N2, NO, and N O. The pseudoꢀfirstꢀorder rate constant k1 of the reaction at 80°C
2
depends strongly on the acid concentration and on the type of nucleophile (Br , Cl–, or H O). The relationꢀ
–
2
ship between k1 and the rate constant
by the linear equation log[k1 /(CH CNu)] = m m*X0
trations of H+ and nucleophile, respectively; X0 is the excess acidity; and
Swain–Scott equation log(kNu /kH2O ns, where is the nucleophilicity factor and
= 0.72), is applicable to the rate constants of the S 2 reactions of the protonated molecule of I
k of the bimolecular nucleophilic substitution reaction (SN2) is given
≠
+
+
log(k/KBH
+
)
, where CH
+
and CNu are the concenꢀ
and m* are coefficients. The
is the substrate constant
with Br–,
≠
m
)
=
n
s
(
s
k
N
–
Cl , and H O.
2
DOI: 10.1134/S0023158411010228
AlkoxyꢀNNOꢀazoxy compounds (AlkAz) have ylamines) or their anions (HAzꢀA) with electrophilic
been known since the end of the 19th century [1, 2]. agents [4, 7–10]. The main drawback of this reaction
Some representatives of this poorly studied class of is its insufficient selectivity. Along with target AlkAz,
compounds are now considered as components of gasꢀ byꢀproducts (alkoxynitrosoamines AlkNA) are always
generating compositions [3] and biologically active formed due to the electrophilic attack of an HAzꢀA
compounds (NO donors, antineoplastic drugs) [4–6]. anion on the O(1) atom (Scheme 1).
The AlkAz synthesis methods were systematizes in
Usually, it is unnecessary to synthesize AlkNA via
a number of reviews [4, 7, 8]. Usually, AlkAz comꢀ Scheme 1 because these compounds can be obtained
pounds are synthesized by reacting hydroxylꢀNNOꢀ by the simple and fully predictable alkylalkoxyamine
azoxy compounds (HAz, also called
Nꢀnitrosohydroxꢀ nitrosylation reaction. As a consequence, only a few
Н
H
ᮎ
ᮎ
O ᮍ
ᮎ
O ᮍ
ᮎ
O
ᮎ
O
O
O
O
O
R
O
O ᮍ
N N
ᮎ
B
–
R'X
R'
'R
N N
N N
N N
+
N N
BH
R
R
R
R
HAz
HAzꢀA
AlkAz
AlkNA
Scheme 1
.
17

18
ZYUZIN, LEMPERT
examples of the isolation of AlkNA along with AlkAz distillation of the acid (analytical grade). The solution
from the reaction according to Scheme 1 are known concentrations were determined by titration and were
[
1, 2, 11–13]. It is substantially more important to checked by density measurements. The molarity of
remove the AlkNA traces from the target compound acids was derived from their density by extrapolating
AlkAz and from the reaction mixture and synthesis the data available for HCl [28], HBr [28], H SO [29],
2
4
waste, because the AlkNA compounds are potentially and HClO4 [30] to the experimental temperature. The
–
–
+
dangerous carcinogens [14, 15]. A number of AlkAz’s molarities of the Cl , Br , and
2, 11–13] was purified by making use of the high therꢀ and HClO4 solutions, respectively, were taken to be
mal stability of AlkAz [16] as compared to AlkNA [13, equal to the molarities of the acids. The molarity of the
H ions in HCl, HBr,
[
+
1
7]. However, this approach requires rather severe
conditions in some cases. In particular, allyloxyꢀ lated taking into account the density of the solution at
NNOꢀazoxymethane was purified from an ꢀallyꢀ the experimental temperature, assuming that the
loxyꢀ ꢀmethylnitrosoamine by heat treatment at molarity is proportional to the density.
40°C for 1.5 h [11], based on the thermal decompoꢀ
H ions in H SO4 solutions at 25°C [31] were calcuꢀ
2
N
N
1
–
6
sition constant data for this AlNA (2.8
1
×
10 and 4.9
×
–3
–1
Kinetic Measurements
0
s
at 80 and 130°C, respectively [11]). In our
opinion, acidic treatment is more appropriate for the
The kinetics of acidic hydrolysis was studied in
removal of AlkNA’s, since these compounds are sealed evacuated glass vessels 140–170 ml in volume
readily hydrolyzable in solutions of strong acids at fitted with a crescentꢀshaped null manometer. The
room temperature [15, 18–21]. For example, after sample weight was 100–200 mg, and the acid volume
treatment with 40% H SO for 3 h at 25°C, ethoxyꢀ was 10 ml. In the hydrolysis of compound
I in H SO
2
4
2 4
NNOꢀazoxymethane becomes purer than the initial and HClO4 at 80 and 90°C, the acid volume was 2 ml
sample [19].
and the free volume was 3.3–8.2 ml. The hydrolysis
kinetics of hydroxylꢀNNOꢀazoxymethane (IV, also
The kinetics and mechanisms of the acidic hydrolꢀ
called
NꢀmethylꢀNꢀnitrosohydroxylamine) was studꢀ
ysis of selected AlkNA’s were studied in [15, 21].
AlkAz was found to have very high chemical resistance
toward strong acids [2, 19, 22]. For instance,
bis(methoxyꢀNNOꢀazoxy)methane was recrystallized
from 70% HNO3, and methoxyꢀNNOꢀazoxybenzene
was isolated unchanged after heat treatment at 100°C
with 85% H SO [22]. However, we found no examples
ied by placing an acid solution in a “finger” sealed to
the reaction vessel and pouring this solution to salt III
after heating the vessel. The temperature was mainꢀ
tained with an accuracy of
mostat, and an oil thermostat (
used above 100°C
±
0.1°C with a water therꢀ
±
0.2°C accuracy) was
.
2
4
of studying the kinetics of AlkAz acidic hydrolysis.
Chemical modification of the substituents in AlkAz
obtained via Scheme 1 constitutes an important group
of methods for the synthesis of functionalized AlkAz.
Many of these reactions, for instance nitration, are
carried out in strong acids [22]. Strong acids can also
act as reactants [23–25]. To optimize the conditions
for the preparation of these and similar reactions, it
would be pertinent to estimate the hydrolytic stability
of the AlkAz group at different temperatures and for
various types and concentrations of acids.
Product Analysis
UV spectra were recorded on a Specord UV VIS
spectrophotometer. Gaseous hydrolysis products were
analyzed on an LKhMꢀ8MD chromatograph (therꢀ
malꢀconductivity detector, helium as the carrier gas,
flow rate of 100 ml/min, copper columns 6 m in length
and 5 mm in diameter, Polisorbꢀ1 with a particle size
of 0.2–0.3 mm as the stationary phase, column temꢀ
peratures of ⎯78 and 90°C).
The kinetics of the acidic hydrolysis of methoxyꢀ
NNOꢀazoxymethane (I), the simplest AlkAz, was
RESULTS AND DISCUSSION
studied in this work to elucidate the mechanism of the
reaction.
The kinetic curves describing the pressure rise in
the system are similar to firstꢀorder curves, except for
their initial portions (up to a conversion of 0.1), where
most of them are Sꢀshaped. A typical curve is shown in
Fig. 1. The kinetic data were processed by least squares
using the steepest descent method and the kinetic
equation for consecutive firstꢀorder reactions:
EXPERIMENTAL
Chemicals
The synthesis and characteristics of compounds
26], 2,2ꢀdimethylꢀ1ꢀ(methoxyꢀNNOꢀazoxy)proꢀ
I
[
P = P + P {1 – [k exp(–k t)
0
∞
2
1
pane (II) [26], and hydroxyꢀNNOꢀazoxymethane
sodium salt (III) [27] were studied earlier. Solutions of
HCl, HClO4, and H SO were obtained by dilution of
(1)
–
k exp(–k t)]/(k – k )},
1 2 2 1
2
4
where k1 and k2 are the apparent pseudoꢀfirstꢀorder
rate constants is time, P0 is the vapor presꢀ
= (
the corresponding acids (reagent grade) with distilled
water. HBr solutions were obtained from the azeotrope
k2
( > k1), t
(
47.59% HBr). The HBr azeotrope was obtained by sure over the acid after heating (2–3 min),
Р
P +
0
KINETICS AND CATALYSIS Vol. 52
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2011

KINETICS OF METHOXYꢀNNOꢀAZOXYMETHANE HYDROLYSIS IN STRONG ACIDS
19
Ppr
) is the observed pressure, Ppr is the pressure of the
P, Torr
gaseous hydrolysis products, and
P
=
Ppr at
t
,
∞
.
∞
(a)
Four parameters were optimized, namely, P0
P , k1,
∞
600
4
and k2. At k2
/
k1 > 10 , the curves were fitted to a firstꢀ
order kinetic equation and P0
,
P
, and k1 were optiꢀ
∞
500
400
300
mized. Next, we fixed the P0 and P∞ values thus found
and refined k1 and the error of its determination by the
least squares method. The refined errors are presented
for the 95% confidence level. The points obtained
below a conversion of 0.8 were taken into account.
ranged from 300 to 500 Torr. The rootꢀmeanꢀsquare
deviation of the calculated pressure from the experiꢀ
P
∞
2
00
00
1
mental pressure,
mental error of pressure measurement. In the slow
hydrolysis reactions of compound in H SO4 and
HClO4 at 80, 90, and 108.5°C, the conversion was
.006–0.30. In these cases, was calculated from the
s(
Δ
P) (Table 1), is close to the instruꢀ
0
500
b)
1000 1500 2000 2500
I
2
(
0
P
∞
1
10
00
yield of the gaseous products (α∞
H SO at 119.8°C and from the temperature depenꢀ
= P /P) in 64.16%
∞
2
4
dence of
Next, having fixed
The results are listed in Table 1.
The temperature dependences of the rate constant
а
for the hydrolysis of
I
in 47.59% HBr.
∞
1
P
, we optimized k1 k2, and P0
,
.
∞
9
0
0
–1
k1 (s ) for compound
I hydrolysis in 47.59% HBr at
6
0–90°C and in 64.16% H SO at 80–130°C are satꢀ
2
4
8
isfactorily described by the Arrhenius equation with
log = 12.7 1.5 and 13.6 1.4 and Eа = 115 10 and
37 10 kJ/mol, respectively. The constant k2 obeys
the Arrhenius law to a much lesser extent: log
.1 ± ±2.5 and Eа = 91 18 kJ/mol for 64.16% H SO in
the temperature range from 80 to 120°C
The strong dependence of the hydrolysis rate of
compound on the acid concentration (Table 1) indiꢀ
cates that the first reaction step is the protonation of
molecule . To estimate the basicity of compound , we
recorded its UV spectra in water and in H SO4 soluꢀ
0
20
40
60
t, min
A
±
±
±
±
1
Fig. 1. (a) Typical kinetic curve for the acidic hydrolysis of
compound in 47.59% HBr at 70°C and (b) its initial porꢀ
tion. The points represent experimental data, and the
curves represent the data calculated using Eq. (1). P0
10–5 s–1, and
I
A
=
9
±
2
4
=
.
8
0.5 Torr, = 609.6 Torr, k1 = 2.162
P
×
∞
10–4 s–1
k2 = 6.87
×
.
I
I
I
The concept of excess acidity was reviewed in [32, 33].
Knowing the product m m* derived from kinetic data
≠
see below; Table 3) and assuming that
2
≠
tions (Table 2). The
tonated molecule of
band of the free molecule
π
–
π
*
transition band of the proꢀ
is shifted relative to the same
to the short wavelengths
(
m
m
= 1 and that
I
*
is temperatureꢀindependent, we can estimate m* at
I
(
Table 2). In 94.3% H SO4, the protonated and neutral 0.58. Using the known values of logCH+ [31] and H0
2
forms of compound
amounts (the band is asymmetric, broadened, and less
I are observed in comparable
[
34], we found that
p
KBH
+
= –6.26 ≈ –6. For comparꢀ
intense at the maximum point). In 64.16% H SO4 (the ison, Et O is one order of magnitude stronger base
2
2
most acidic medium used in the study of the hydrolysis
kinetics), the proportion of the protonated form is
small. Accepting that the concentrations of protoꢀ
(
p
KBH
+
= –5.2 [35]) and MeNO is a much weaker
2
base (
p
KBH
+
= –11.9 [35]).
nated (CBH ) and free (CB) compound I in 94.3%
+
The
π
ꢀelectron system of molecule I can be viewed
H SO are the same, one can approximately estimate
2
4
as a set of mesomeric forms (Scheme 2) [36], and proꢀ
tonation at the N atom ( ) seems most probable
Scheme 2).
p
KBH
+
for compound
I
using the equation [31]
V
(
p
KBH log(CBH
+
=
+
/
CB) – logCH X0
+
–
m
*
,
(2)
However, protonation at the Nꢀoxide O atoms (VI)
with stabilization by an intramolecular hydrogen bond
similar to that in hydroxyꢀNNOꢀazoxy compounds
(Nꢀnitrosohydroxylamines) cannot be excluded [37,
where X0
=
–H0
–
logCH
+
is the excess acidity (H0 is
is a coefficient.
the Hammett acidity function) and
m*
KINETICS AND CATALYSIS Vol. 52
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2011

20
ZYUZIN, LEMPERT
Table 1. Apparent rate constants of the pseudoꢀfirstꢀorder hydrolysis reactions of compounds I, II, and IV in solutions of strong
acids
Acid concentraꢀ
k1, s–1
k2, s–1
*
*
Compound
Acid
HBr
T
,
°
C
s(
Δ
P
), Torr
*
α
∞
tion, %
I
24.25
80
80
80
80
80
60
70
80
90
80
80
80
80
80
80
80
80
90
90
4.678
0.007)
3.775
9.49
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
10–7
10–6
10–6
10–6
10–5
10–6
10–5
10–5
10–4
10–7
10–7
10–6
10–6
10–6
10–8
10–8
10–7
10–7
10–6
10–6
10–5
10–5
10–8
10–4
10–5
10–4
10–3
4.12
×
10–4
0.5
2.4
1.0
1.2
2.8
0.9
1.7
2.2
2.2
0.7
1.3
0.8
0.6
1.3
1.2
2.0
1.4
2.6
0.4
0.5
1.1
1.1
2.1
1.6
1.2
0.8
0.6
1.74
2.00
2.27
2.39
2.15
2.10
2.07
2.14
2.22
2.37
2.49
2.63
2.66
2.54
1.20
1.20
1.20
1.25
1.25
1.33
1.39
1.69
1.20
1.42
1.73
1.01
1.01
30.03
34.84
39.30
45.10
47.59
47.59
47.59
47.59
(1.485
±
(***)
6.70
×
×
×
×
×
×
×
10–4
1.2
5.40
4.70
6.87
1.60
8.67
10–2
10–4
10–4
10–4
10–3
10–3
4.335
6.022
2.162
6.75
1.824
0.008)
5.517
1.107
2.252
4.407
2.28
HCl
16.36
(2.487
±
(***)
19.34
22.34
25.26
28.25
3.31
×
×
×
×
×
×
×
×
×
×
×
10–3
1.08
4.37
3.90
1.95
2.33
4.42
1.13
8.44
3.90
8.87
10–3
10–4
10–4
10–5
10–5
10–5
10–4
10–5
10–4
10–4
H SO
2
35.30
4
51.52
64.16
64.16
64.16
64.16
64.16
64.16
7.92
2.42
7.08
1.00
105.8
119.8
130
80
5.78
2.77
(7.250
±
0.008)
3.08
(***)
7.4
10–5
1.19
10–3
HClO4
HBr
42.93
47.01
64.16
47.59
×
II
80
1.153
×
H SO
2
119.8
50
(6.87
(6.41
±
±
±
0.04)
0.05)
0.06)
(***)
(***)
(***)
4
IV
HBr
47.59
60
(1.522
2 0.5
*
Standard deviation
s
(
Δ
P
) = [(
n
– 1)–1
t
Σ
(Pexp – Pcalc) ]
= (P∞ P0)/P1 mol, where P1 mol is the calculated pressure of an equimolar amount of an
.
*
*The yield of gaseous products at
ideal gas.
∞
:
α
–
∞
*
** k2/ .
k1 > 104
3
8]. This protonation is suggested by the fact that it is
At a comparable acidity, the hydrolysis rate of comꢀ
depends strongly on the nature of the acid:
decreases in the order HBr > HCl > H SO4
this O atom that serves as the donor for the hydrogen pound
bond [39].
I
k
1
HClO4
≈
2
(
Table 1). This order coincides with the nucleophilicꢀ
ity order Br > Cl > H O [40]. This suggests that the
–
–
2
Table 2. UV spectra of compound I at different H2SO4 conꢀ rateꢀdetermining step of the reaction is the attack of a
centrations
nucleophile on the C atom of the CH O group of the
3
protonated molecule of I. The leaving group is Nꢀ
methyldiazene dioxide (VII) or hydroxyꢀNNOꢀ
azoxymethane (IV), depending on the type of moleꢀ
H SO
2
concentration,
Band width
at halfꢀε, nm
4
λ
max, nm
ε
%
cule I protonation (Scheme 3).
0
233.7
230.1
224.7
9130
7950
5110
28.9
28.2
36.2
The Sꢀshaped initial portion of the kinetic curves
Fig. 1) explains the accumulation of intermediates IV
6
4.16
4.3
(
9
or VII, and the constant k2 (Table 1) characterizes the
KINETICS AND CATALYSIS Vol. 52
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KINETICS OF METHOXYꢀNNOꢀAZOXYMETHANE HYDROLYSIS IN STRONG ACIDS
21
ᮎ
ᮎ
ᮍ
ᮍ
O
O
O ᮍ
O
O
O
O
O
ᮍ
ᮎ
ᮎ
ᮍ
CH3
CH3
CH3
CH3
H
N N
N N
N N
N N
H C
3
I
H C
3
H C
3
H C
3
H
H
O
ᮍ
ᮍ
O
O ᮍ
N N
O
H
O
O ᮍ
O
O
CH3
CH3
CH3
CH3
N N
N N
N N
+
H C
H C
H
H C
H C
3
3
3
3
V
VI
Scheme 2.
H
ᮎ
ᮎ
O ᮍ ᮍ
ᮎ
O
O ᮍ
O
ᮎ
k
2
k
CH Nu
V
or VI + Nu
N N
N N
N O, NO, N , CH OH, …
2 2 3
–
3
H C
H
3
H C
3
VII
IV
Scheme 3.
ᮎ
O
O
HO
O
ᮍ
ᮎ
CH3
CH3
V
or VI + Nu
N O, NO, N , CH OH, …
2 2 3
N N
N N
–CH Nu
3
H
Scheme 4.
stability of intermediates IV and VII under the hydrolꢀ instead of compound I. Due to steric hindrance, the
ysis conditions.
SN2 reaction rate for compound II at the CH2 atom of
the neopentyl group might be expected to be lower by
several orders of magnitude [40]. In fact, the experiꢀ
mentally measured hydrolysis rate of compound II is
The possibility of a nucleophilic attack on another
C atom of the protonated molecule of
excluded a priori (Scheme 4).
I cannot be
even higher than the hydrolysis rate of compound
I: in
To check this possibility, we used 2,2ꢀdimethylꢀ1ꢀ
methoxyꢀNNOꢀazoxy)propane (II
4
7.01% H SO4 at 80°C it is higher by a factor of 1.8,
2
(
)
and in 64.16% H SO4 at 119.8°C it is 2.5 times higher
2
ᮎ
O
O
(cf. k , Table 2). Thus, the mechanism shown in
1
ᮍ
CH3
Scheme 4 is ruled out.
H C
N N
3
The influence of the acid concentration on the
C CH2
H C
hydrolysis rate of compound
The relationship between the constant
lecular nucleophilic substitution reaction (SN2) and
I
was studied at 80°C
.
3
H C
k
of the bimoꢀ
3
II
≠
Table 3. Parameters m m
*
and log(
k
/
KBH
+
)
at 80°C in Eq. (4)
), l2 mol–2 s–1
KBH
Estimate of k,
≠
m m
log(k
/
+
Acid
Nucleophile
*
–1 –1
l mol
s
HBr
Br–
Cl–
0.77 ± ±0.06
0.63 ± ±0.04
0.58 ± ±0.05
–7.96 ± ±0.11
–8.57 ± ±0.07
2
5
3
×
×
×
10–2
10–3
10–5
HCl
H SO и HClO
2
H O
2
–10.74 ± ±0.13
4
4
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2011

22
ZYUZIN, LEMPERT
log(kNu
2
−2 −1
/k
)
H O
2
log[k1/(C C )] [l mol s ]
H Nu
3
2
1
0
−6
−7
−8
−9
HBr
HCl
H SO4
2
HClO4
0
0.5
1.0
1.5 n
Fig. 3. Applicability of the Swain–Scott equation (5) to the
SN2 reactions of protonated compound
and H O.
–
–
I with Br , Cl ,
2
strongest nucleophile in the H SO4 and HClO4 soluꢀ
2
tions. The water concentration in solutions of strong
acids is unknown, so CNu for H SO4 and HClO4 was
2
obtained by multiplying the activity of water by its
concentration in neat water at 80°C (53.94 mol/l).
−
10
1
2
3
4
X0
The water activity at 80°C in the H SO4 and HClO4
2
solutions was obtained by extrapolation [42] of data
available from the literature [30, 43]. Figure 2 demonꢀ
strates the validity of Eq. (4). In Fig. 2, the point corꢀ
Fig. 2. Applicability of Eq. (4) to the description of the
effect of the medium on the acidic hydrolysis rate constant
k1 of compound
culated via a polynomial formula [44] for HBr and HClO4
and via the formula X0 –H0 logCH for HCl and
responding to the hydrolysis of compound I in HClO4
I at 80°C. The X0 values at 25°С were calꢀ
lies on the straight line for H SO4. This confirms that
2
water is correctly chosen as the nucleophile for the
description of the hydrolysis mechanism in these
=
–
+
H SO4. The acidity functions H0 were taken from the litꢀ
acids, in which the protonated molecule of
I is
2
erature (HCl [44], H SO [34]).
involved in the rateꢀdetermining step. The values of
≠
2
4
m m
the experimental pseudoꢀfirstꢀorder reaction rate using the least squares method with 95% confidence
constant k1 (Table 1) is as follows [41]:
probability are listed in Table 3.
KBH
Accepting –6, one can estimate the true
values of the rate constants of the S 2 reactions
Table 3). The Swain–Scott correlation can be applied
*
and log(k/KBH
+
)
at 80°C calculated via Eq. (4)
p
+
≈
logk1 – log[C /(C
+
+ C )] – logCH
+
B
B
BH
k
N
(3)
≠
(
=
m m*X + logCNu
+
log(k/KBH ).
+
0
to these reactions in aqueous media [40]:
Because there are no published X0 data for HBr, we
used the parameter X0 = –H0 logCH , which is simꢀ
log(k /kH2O) = ns,
(5)
Nu
–
+
where
n
is the nucleophilicity factor and
s
is the subꢀ
ilar in meaning, for all acids. For the same reason, the
X0 values referring to 25°C were used. The second term
of the leftꢀhand side of Eq. (3) can be neglected
strate constant. Figure 3 indicates that the Swain–
Scott equation is applicable to the S 2 reactions of
N
protonated compound
that KBH is the same in all acids. The numerical values
for the Cl– and Br– anions were taken from [40].
The substrate constant is 0.72. As a rule, is close to
I at 80°C under the condition
+
Ӷ
because it was shown above that CBH CB in the soluꢀ
+
tions of all of the acids. The transformation of Eq. (3) of
into linear form gives the expression
n
s
s
unity for typical S 2 reactions at 25°C [40]. It is likely
N
≠
log[k /(C + C )] = m m*X + log(k/KBH ). (4)
+
1
H
Nu
0
that the deviation from this rule observed for the reacꢀ
tion examined here is due to the fact that the measureꢀ
In concentrated HBr and HCl solutions, Br– and Cl– ments were taken at 80°C rather than 25°C. This is
act as the strongest nucleophiles, whereas water is the confirmed by the substantial difference between the
KINETICS AND CATALYSIS Vol. 52
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2011

KINETICS OF METHOXYꢀNNOꢀAZOXYMETHANE HYDROLYSIS IN STRONG ACIDS
23
Table 4. Yields of the gaseous hydrolysis products of compounds I, II, and IV in strong acids based on the fraction of the reacted
substance
Concentration
of acid, %
Compound
Acid
HBr
T
,
°
C
Yield, mole fractions
I
24.25
80
80
80
80
70
80
90
80
80
80
80
80
(N + NO)
2
—
0.62, N O—0.45, CH Br
2 3
—
0.64
30.03
39.30
45.10
47.59
47.59
47.59
N2—0.44, NO
—
0.05, N O—0.43, CH Br—0.95
2
3
(N + NO)
2
—
0.63, N O*, CH Br
2
—1.12
3
N2—0.34, NO
—0.05
(N + NO)
2
—
0.55, N O—
2
0.42, CH Br
3
—
0.93
N2
—0.19, NO—0.10
(N + NO)
2
—
0.55, N O—
2
0.46, CH Br
3
—
1.05
1.21
HCl
16.36
(N + NO)
2
—
0.16, N O—0.71, CH Cl
2 3
—
19.34
22.34
25.26
28.25
N2—0.005, NO—0.05, N O—0.78, CH Cl—1.34
2 3
(N + NO)
2
—
—
—
—
—
—
0.06, N O
—
—
—
—
—
—
0.87, CH Cl
3
—
—
—
1.75
2
(N + NO)
2
0.05, N O
0.91, CH Cl
3
1.53
0.99
2
(N + NO)
2
0.04, N O
0.95, CH Cl
2
3
H SO
2
64.16
47.01
47.59
119.8
80
(N + NO)
2
0.02, N O
0.98, CO2—0.01
4
2
II
IV
HBr
HBr
(N + NO)
2
0.19, N O
2
0.45, CH Br
—
0.63
0.15
3
50
(N + NO)
2
0.53, N O
0.31, CH Br
3
—
2
*
No data.
activation energies of the first steps of compound
hydrolysis in 47.59% HBr and in 64.16% H SO4
I
The NO content of the hydrolysis products of comꢀ
pound indicates the intermediate formation of
hydroxyꢀNNOꢀazoxymethane (IV). However, the
measured rate constant k1 of IV decomposition is
.
I
2
An analysis of the reaction products (Table 4) conꢀ
firms the proposed scheme. For the hydrolysis of comꢀ 3.2 times higher than the hydrolysis rate constant k2 of
pound
in HBr and HCl, the yield of MeBr and MeCl compound under the same conditions; i.e., comꢀ
pound IV is unlikely to be the primary intermediate in
the acidic hydrolysis of compound . We found a single
I
I
expressed in mole fractions is higher than unity in sevꢀ
eral cases. This is due to the formation of methanol
upon the decomposition of the intermediate. The rapꢀ
idly reachable equilibrium between methanol and
methyl halide depends on the nature of the acid (the
MeCl yield is higher than the MeBr yield) and on the
acid concentration. The methyl halide yield and,
I
example of kinetic study of hydroxyꢀNNOꢀazoxy
compound acidic hydrolysis—the hydrolysis of
2
ꢀ(hydroxylꢀNNOꢀazoxy)adamantane (VIII) [45],
H
ᮎ
O ᮍ
O
N N
accordingly, the total yield of gaseous products ( ∞,
α
Table 4) decrease with a decreasing acid concentraꢀ
tion. The temperature effect on this equilibrium is
determined to a considerable extent by the fact that
VIII
the solubility of methyl halide in the acid is temperaꢀ However, the measurements in that work were carried
out at 25°C in 0.5–2.5 M HClO4 solutions (k = 1.5 ×
tureꢀdependent. Probably, it is the shift of this equilibꢀ
rium upon cooling the reaction vessel to 20°C that is
responsible for the gradual decrease in the pressure of
the hydrolysis products over 1–2 h before stabilizaꢀ
tion. The contribution from the first hydrolysis step of
⎯
6
–1 –1
1
0
l mol s ).
It is likely that the molecule of
N atom. The rateꢀdetermining step yields intermediꢀ
ate , which partly decomposes to yield N O and
partly isomerizes via transprotonation to turn into IV
I
is protonated at the
V
2
.
compound
from the ratio of the MeBr yields in the hydrolysis of
compounds
I to the total MeBr yield can be estimated
The decomposition of the latter yields NO along with
N O. The constant k2 refers to the totality of possible
2
I
and II under similar conditions, and this parallel and consecutive reactions of the intermediꢀ
contribution is 60%.
ates. The total fraction of Nꢀcontaining gases resulting
KINETICS AND CATALYSIS Vol. 52
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2011

24
ZYUZIN, LEMPERT
from all reactions is close to unity. However, the proꢀ 13. Luk’yanov, O.A., Smirnov, G.A., and Nikitin, S.V., Izv.
Akad. Nauk, Ser. Khim., 1998, no. 10, p. 1996.
portions of these gases depend strongly on the nature
and concentration of the acid, which is likely due to
the strong effect of these factors on the relative contriꢀ
butions from the different parallel decomposition
routes of intermediate VII (Scheme 2).
1
4. Andrews, A.W., Thibault, L.H., and Lijinsky, W.,
Mutat. Res., 1978, vol. 51, no. 3, p. 319.
1
5. Bhat, J.I., Clegg, W., Maskill, H., Elsegood, M.R.J.,
Menneer, I.D., and Miatt, P.C., J. Chem. Soc., Perkin
Trans. 2, 2000, no. 7, p. 1435.
1
6. Zyuzin, I.N., Lempert, D.B., and Nechiporenko, G.N.,
CONCLUSIONS
Izv. Akad. Nauk SSSR, Ser. Khim., 1988, no. 7, p. 1506.
The kinetics of methoxyꢀNNOꢀazoxymethane
hydrolysis in strong acids was studied by a manometric
method. The attack of a nucleophile on the carbon
atom of the methoxy group of the protonated moleꢀ
1
7. Kano, K. and Pierre, J., J. Org. Chem., 1993, vol. 58,
no. 6, p. 1564.
1
8. Boese, A.B., Jones, L.W., and Major, R.T., J. Am.
Chem. Soc., 1931, vol. 53, no. 9, p. 3530.
cule of I is the rateꢀdetermining step of the reaction.
The resulting methyldiazene dioxide decomposes via a 19. Lamberton, A.H. and Yusuf, H.M., J. Chem. Soc. C
complicated mechanism to yield N2, NO, and N O
1969, no. 3, p. 397.
,
.
2
The rate constants of the S 2 reactions of protoꢀ 20. Kano, K. and Anselme, J.ꢀP., Tetrahedron Lett., 1992,
N
–
–
nated methoxyꢀNNOꢀazoxymethane with Br , Cl
,
vol. 48, no. 46, p. 10075.
and H O are described well by the Swain–Scott equaꢀ
2
21. Maskill, H., Menneer, I.D., and Smith, D.I., Chem.
Commun., 1995, no. 18, p. 1855.
tion at a substrate constant of
The basicity of methoxyꢀNNOꢀazoxymethane is
KBH
low ( –6).
s = 0.72.
2
2. George, M.V., Kierstead, R.W., and Wright, G.F., Can.
p
+
≈
J. Chem., 1959, vol. 37, no. 4, p. 679.
2
3. Luk’yanov, O.A., Smirnov, G.A., and Sevost’yanova, V.V.,
When treating alkoxyꢀNNOꢀazoxy compounds
with strong acids at elevated temperatures, it is desirꢀ
Izv. Akad. Nauk, Ser. Khim., 1995, no. 8, p. 1534.
able to avoid the presence of the strong nucleophiles 24. Zyuzin, I.N., Golovina, N.I., Lempert, D.B., Nechiꢀ
–
–
–
Br , Cl , and
I
in the reaction mixture.
porenko, G.N., and Shilov, G.V., Izv. Akad. Nauk, Ser.
Khim., 2008, no. 3, p. 619.
2
5. Zyuzin, I.N. and Lempert, D.B., Izv. Akad. Nauk, Ser.
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KINETICS AND CATALYSIS Vol. 52
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