Distribution of the products of benzhydryl bromide heterolysis in the presence of triphenylverdazyl in aprotic solvents

Article abstract of DOI:10.1134/S1070363206050239

The distribution of products of benzhydryl bromide heterolysis in the presence of triphenylverdazyl in anhydrous nitrobenzene and propylene carbonate, as well as in anhydrous acetonitrile in the presence of benzyltriethylammonium chloride was studied. In kinetic experiments the contribution of verdazyl alkylation was always minor, and verdazyl was mostly consumed in the reaction with HBr evolved during solvolysis. Thus, triphenylverdazyl is not an indicator of the solvent-separated ion pair of benzhydryl bromide. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S1070363206050239

ISSN 1070-3632, Russian Journal of General Chemistry, 2006, Vol. 76, No. 5, pp. 798 800.
Pleiades Publishing, Inc., 2006.
Original Russian Text I.M. Serebryakov, E.B. Kryzhanovskaya, S.S. Dzhurinskaya, 2006, published in Zhurnal Obshchei Khimii, 2006, Vol. 76, No. 5,
pp. 836 838.
Distribution of the Products of Benzhydryl Bromide Heterolysis
in the Presence of Triphenylverdazyl in Aprotic Solvents
I. M. Serebryakov, E. B. Kryzhanovskaya, and S. S. Dzhurinskaya
National University, ul. Universitetskaya 24, Donetsk, 83055 Ukraine
e-mail: serebryakov@dn.farlep.net
Received March 17, 2005
Abstract The distribution of products of benzhydryl bromide heterolysis in the presence of triphenylver-
dazyl in anhydrous nitrobenzene and propylene carbonate, as well as in anhydrous acetonitrile in the presence
of benzyltriethylammonium chloride was studied. In kinetic experiments the contribution of verdazyl alkyla-
tion was always minor, and verdazyl was mostly consumed in the reaction with HBr evolved during solvolysis.
Thus, triphenylverdazyl is not an indicator of the solvent-separated ion pair of benzhydryl bromide.
DOI: 10.1134/S1070363206050239
Dvorko and co-workers [1, 2] have put forward and
substantiated the hypothesis that triphenylverdazyl (I)
in aprotic solvents selectively and quantitatively reacts
with solvent-separated ion pairs formed on mono-
molecular heterolysis of substrates [reaction (1)]. By
contrast, our data [3] on the product distribution in
benzhydryl bromide heterolysis in anhydrous aceto-
nitrile in the absence of salts show that in these
conditions verdazyl prefers to react with the acid
formed by solvolysis of the substrate [reaction (2)].
Ph
Ph
Ph
R
N
N
N
N
N
N
N
N
N
X
+
RX + 2
N+
(1)
(2)
N
N
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
I
II
III
Ph
Ph
Ph
H
N
N+
N
N
N
N
N
N
N
N
N
X
+
HX + 2
N
Ph
Ph
Ph
I
IV
III
As an argument in favor of their theory
[1] Dvorko and Ponomareva have adduced the correla-
tion of the rate of consumption of verdazyls with their
concentration and structure, observed in heterolysis
of a series of substrates in the presence of certain
salts. The referees explained this effect in terms of
verdazyl salt competition the cationoid intermediate,
but did not find out product distribution under those
conditions. To our knowledge, the benzhydryl system
exhibited such behavior in the heterolysis of benz-
hydryl bromide in acetonitrile in the presence of
benzyltriethylammonium chloride or tetraethyl-
ammonium iodide [4].
Proceeding with this discussion, we have studied
the distribution of products in the above reaction
system. As seen from Table 1 (exp. nos. 1 and 2),
substrate solvolysis is still always prevailing, while
increased concentration of verdazyl in the presence of
salt actually slightly increases the initial reaction rate.
Thus, the observed dependence of reaction rate on
verdazyl concentration is not associated with verdazyl
salt competition for the cationoid intermediate.
It is known that preparative reactions of certain
compounds with triphenylverdazyl in acetonitrile give
products comprising a solvent molecule. Even though
798

DISTRIBUTION OF THE PRODUCTS OF BENZHYDRYL BROMIDE HETEROLYSIS
799
Table 1. Product distribution in heterolysis of benzhydryl bromide in the presence of triphenylverdazyl in acetonitrile,a
propylene carbonate, and nitrobenzeneb
Exp.
no.
[RX],
M
cinit 104,
c0 104,
cmin 104 cmax 104
k 106,
s
k 106,
s
1
1
M
M
M
M
Acetonitrile, 25 C
1
2
0.053
0.056
2.05
0.822
2.00
0.797
0.23
0.115
1.87
0.720
0.90d
0.86d
0.98 0.03
0.85 0.05
0.86c
0.76c
Propylene carbonate, 25 C
3
4
0.022
0.020
1.27
1.25
1.22
1.14
0.26
0.26
1.22
1.19
0.97d
0.92d
4.25 0.10
4.39 0.06
4.61 0.07 [5]
Nitrobenzene, 40 C
5
6
0.050
0.050
1.26
1.25
1.11
1.21
0.22
0.44
1.06
1.14
0.65e
0.82e
0.185 0.011
0.21 0.04
0.16f
a
4
b
c
In the presence of Et PhCH NCl (c 3.11 10 M). For denotations of concentrations, see the legend for formula (3). Inter-
3
2
d
e
f
polated over data in [4].
By formula (3).
By formula (4). Interpolated over data in [6].
this circumstance has no effect on reaction kinetics
and product distribution [2, 7], we also studied the
product distribution in benzhydryl bromide heterolysis
in the presence of triphenylverdazyl in chemically
inert solvents [8], viz. anhydrous propylene carbonate
and nitrobenzene. For reducer in propylene carbonate
we used the same reagent as in acetonitrile [3]
(Table 2, exp. nos. 1 4). The contribution of pathway
(2) in the apparent rate constant in this solvent was
calculated by formula (3).
As seen from Table 1 (exp. nos. 3 6), our
measured rate constants of benzhydryl bromide he-
terolysis in both the solvents fairly fit those reported
in [5, 6]. However, in our case, too, the substrate
hydrolysis still remains the prevailing pathway. The
slightly larger contribution of verdazyl alkylation in
nitrobenzene is probably explained by the fact that
this solvent contains slightly less water.
Table 2. Regeneration of triphenylverdazyl for its reaction
products with HCl and from triphenylverdazylium bromide
in propylene carbonate at 25 C and in nitrobenzene at
40 C
2( c
cr + c0)
(3)
=
.
c
Here c0 = cinit c0 is the jumpwise fall of the
verdazyl concentration on addition of benzhydryl
bromide; c = c0 cmin, verdazyl consumption during
the reaction; and cr = cmax cmin, increase of the
radical concentration on addition of reducer.
Exp. Starting cinit 104, cmin 104, cmax 104,
ra
no. compound
M
M
M
Propylene carbonate (Reducer A)b
1
2
3
4
I + HCl
1.35
1.35
1.39
1.32
0.329
0.395
1.33
1.33
1.44
1.31
0.98
0.98
1.00
0.99
In nitrobenezene, satisfactory results were obtained
with N,N-dimethyl-p-anisidine as a reducer. This
reducer provides quantitative regeneration of verdazyl
from salt (Table 2, exp. nos. 8 and 9), but the degree
of regeneration of the radical from its reaction pro-
ducts with HCl is slightly lower in this case (r 0.94,
Table 2, exp. nos. 5 7).1 With this in mind, the
contribution of pathway (2) can be calculated by
formula (4).
IIIc
Nitrobenzene (Reducer B)b
5
6
7
8
9
I + HCl
3.16
1.04
0.97
1.36
0.53
0.96
0.56
0.23
3.01
1.02
0.92
1.33
0.52
0.93
0.95
0.93
0.98
0.98
IIIc
2 cr 2r c0
c(2r 1)
c
=
.
(4)
a
(r) Degree of regeneration of compound I, r = c / c.
r
b
(A) One drop of 33% p-anisidine and 10% morpholine in
propylene carbonate per 1 ml of the reaction mixture; (B)
2.5 mg of N,N-dimethyl-p-anisidine per 1 ml of the reaction
mixture.
Triphenylverdazylium bromide, initial concentration by the
absorption at 540 nm.
1
Reduction with a mixture of N,N-dimethyl-p-anisidine and
N-methylmorpholine in nitrobenzene was not studied. With
pure N,N-dimethyl-p-anisidine in propylene carbonate, an
unsatisfactory result was obtained (r 0.59).
c
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 5 2006

800
SEREBRYAKOV et al.
Thus, under conditions of conventional kinetic
REFERENCES
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cator on the solvent-separated ion pair of benzhydryl
bromide. In our opinion, there are no reasons why in
all other cases in the absence of special protection
from moisture one would expect complete capture of
cationoids with verdazyl. Since with most substrates
the above method is unsuitable for studying the pro-
duct distribution [2], we consider the theory in ques-
tion experimentally unsubstantiated and problematic
for wide verification.
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EXPERIMENTAL
All experiments were performed as described in
[3]. Propylene carbonate was purified as described in
[5]. Nitrobenzene was twice distilled in a vacuum
over P2O5 and once over BaO. Benzyltriethylam-
monium chloride was prepared from triethylamine and
benzyl chloride in acetonitrile, recrystallized from the
same solvent, and dried in a vacuum at 100 C. The
measurement errors were calculated at a 95% con-
fidence level.
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p. 598.
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