Kinetics and product distribution of 1-adamantyl picrate heterolysis in acetonitrile in the presence of triphenylverdazyls

Article abstract of DOI:10.1134/S1070363212110151

The reaction of triphenylverdazyls with strong acids in acetonitrile in the presence of salts with chloride anion is reversible. The observed rate of the heterolysis of 1-adamantyl picrate in the presence of triphenylverdazyls does not depend on the subst

Full text of DOI:10.1134/S1070363212110151

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 11, pp. 1822–1826. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © I.M. Serebryakov, A.N. Moiseeva, 2012, published in Zhurnal Obshchei Khimii, 2012, Vol. 82, No. 11, pp. 1832–1836.
Kinetics and Product Distribution
of 1-Adamantyl Picrate Heterolysis in Acetonitrile
in the Presence of Triphenylverdazyls
I. M. Serebryakov and A. N. Moiseeva
Donetsk National University, Universitetskaya St. 24, Donetsk, 83055 Ukraine
e-mail: serebryakov@dn.farlep.net
Received August 1, 2011
Abstract—The reaction of triphenylverdazyls with strong acids in acetonitrile in the presence of salts with
chloride anion is reversible. The observed rate of the heterolysis of 1-adamantyl picrate in the presence of
triphenylverdazyls does not depend on the substituent in the latter and its concentration. The contribution of the
verdazyl alkylation pathway is minor, the indicator is consumed mainly in the reaction with the acid liberated
from the solvolysis. Thus, triphenylverdazyls are not indicators for the solvent-separated ion pairs.
DOI: 10.1134/S1070363212110151
In their papers Dvorko and co-workers proposed
and subtantiated the hypothesis [1, 2] that triphenyl-
verdazyls (I) react in aprotic solvents selectively and
quantitatively with the solvent-separated ion pairs
formed during monomolecular heterolysis of the sub-
strates [Eq. (1)].
C6H4-Y
C6H4-Y
C6H4-Y
R
N
N
N
N
N
N
N
N
N
N
N
N
(1)
(2)
RX
HX
X
+ 2
Ph
+
+
Ph
Ph
Ph
Ph
Ph
II
C6Y4-Y
I
III
C6H4-Y
C6H4-Y
H
N
N
N
N
N
N
N
N
X
+ 2
Ph
N
N
N
N
Ph
Ph
Ph
Ph
Ph
IV
On the contrary, our data [3, 4] on the product
distribution of benzhydryl bromide heterolysis in
acetonitrile, nitrobenzene and propylene carbonate,
as well in acetonitrile in the presence of triethyl-
benzylammonium chloride, show that under these
conditions triphenylverdazyl reacts predominantly with
the acid released in the course of the solvolysis of the
substrate [Eq. (2)]. It remained unclear, however, why
in some cases [5–7] the observed reaction rate depended
on the indicator structure and concentration. For
example, in the heterolysis of 1-adamantyl tosylate and
picrate in acetonitrile and propylene carbonate in the
presence of triethylbenzylammonium chloride [8] a
linear increase in the reaction rate with increasing the
indicator concentration was observed, the radicals
bearing electron-donating substituents being more
reactive.
Such a dependence is formally indicative of the
participation of triphenylverdazyls in the rate limiting
or a competing product determining step. Obviously
the reaction (2) cannot be one of these. Thus, it was
necessary to find an alternative explanation for the
observed kinetic and structural effects, if they are not
associated with the verdazyl alkylation in the reac-
tion (1).
1822

KINETICS AND PRODUCT DISTRIBUTION
1823
Table 1. Dependence of the rate constant and contribution of the alkylation pathway in the reaction of heterolysis of 1-
adamantyl picrate in acetonitrile at 25°C in the presence of triphenylverdazyl on the water content in the solvent
cinit
c0
cmin
cmax
Exp. no.
α, %
k×106, s–1
104 M
1а
2b
3c
2.69
2.37
2.36
2.32
2.00
1.91
0.649
0.831
0.670
2.55
2.28
2.32
17
16
6
9.62±0.09
5.14±0.05
14.1±0.02
Acetonitrile distilled over P2O5 and then over K2CO3. b Acetonitrile “Spectroscan,” water content no higher than 6×10–4 M. c Acetonitrile
a
“Spectroscan” with addition of water to 4.5×10–2 M.
The object of the present study was chosen for
from the substrate solvolysis, it is necessary to know
the stoichometry of this process. Triphenylverdazyl is
known to react with strong acids rapidly and
quantitatively in aqueous-organic solvents according to
Eq. (2) [11]. But it was not known whether this holds
true in aprotic solvents with low water content and in
the presence of salts. We studied the effect of addition
of 3×10–2 M (as in [8]) triethylbenzylammonium
chloride (Et3BnNCl), tetraethylammonium chloride
(Et4NCl), and tetraethylammonium bromide (Et4NBr)
on the reactions of triphenylverdazyl Ia and 3-(4-
nitrophenyl)-1,5-diphenylverdazyl Ib with picric, hyd-
rochloric, and p-toluenesulfonic acids in acetonitrile.
several reasons. First, only with 1-adamantyl and benz-
hydryl substrates Dvorko and co-workers obtained the
products of verdazyl alkylation in their preparative
experiments and studied the product distribution of the
heterolysis reactions under kinetic conditions [1, 9].
Second, the kinetic and structural effects in this reac-
tion in the presence of triethylbenzylammonium
chloride are pronounced [8] and clearly exceed the
possible error limits. Third, 1-adamantyl picrate is a
highly reactive substrate and at the same time this
compound is enough stable and easy to handle, in
contrast to, for example, 1-adamantyl tosylate. Fourth,
high purity acetonitrile is commercially available,
allowing to maximally exclude the side reactions and
to obtain the most reliable results.
In the absence of salts and in the presence of
Et4NBr both indicators react quantitatively with acids
(Fig. 1, curve 1). However, in the presence of salts
with the chloride anion the verdazyl consumption is
significantly lower than the stoichiometric, and there-
fore we deal with an equilibrium reaction. Apparently,
in this case the acidity of the medium decreases due to
the complexation of protons with chloride ions, whose
basicity is higher than that of bromide ions.
Effect of water content in the solvent on the rate of
heterolysis of 1-adamantyl picrate in the absence of
salts. In acetonitrile purified along the adopted by the
authors general procedure [10] (Table 1, entry 1), we
obtained the value of the rate constant somewhat lower
than was found in [8], k = 12.3×106 s–1. In acetonitrile
“Spectroscan” with water content less than 6×10–4
M
The equilibrium consumption of the indicator in the
presence of chloride ions does not depend on the
nature of the acid and the structure of the
tetraalkylammonium cation of the added salt (Fig. 1,
curve 2), however, it depends on the initial
concentration of the indicator (Fig. 1, curves 2 and 3).
As expected, the equilibrium in the case of nitro
verdazyl Ib under these conditions is shifted to the left
due to lower basicity of the indicator (Fig. 1, curve 4).
the rate constant is significantly lower (entry 2),
although we did not undertake any special measures
for the protection from the air moisture. On the
contrary, in acetonitrile “Spectroscan” with water added
to the concentration 4.5×10–2 M the rate constant was
slightly higher than in the solvent purified by the usual
procedure (entry 3). In order to standardize the
experimental conditions, we performed all further
experiments in acetonitrile “Spectroscan” with water
added to 4.5×10–2 M, because the water concentration
of (4–5)×10–2 M was suggested in [10] as “normal” for
the kinetic experiments.
We were unable to find an analytical expression for
the equilibrium constant ensuring the independence of
its calculated values of both the concentration of added
acid and the initial concentration of the indicator.
Apparently, the complexity of the observed dependence
is due to the association processes (binding of ions in
the conglomerates, dimerization of acid, etc.).
Influence of substituents and salt additives on the
reaction of triphenylverdazyls with acids in aceto-
nitrile. If the indicator reacts with the acid resulting
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SEREBRYAKOV, MOISEEVA
Δ[I]×104, M
х/а
t, s
Fig. 2. Kinetics of heterolysis of 1-AdOPic in the presence
of 3×10–2 M Et3BnNCl in acetonitrile at 25°C, [H2O] =
4.5×10–2 M: (1) [Ia]init = 1.16×10–4 M (○); [Ia]init
2.32×10–4 M (□); (2) [Ib]init = 2.96×10–4 M; (3) [Ib]init
2.97×10–4 M, [PicOH]0 = 9.05×10–4 M.
=
=
[PicOH]×104, M
Fig. 1. The dependence of the indicator consumption on the
concentration of added acid in acetonitrile at 25°C, [H2O] =
4.5×10–2 M. (1) Ia, PicOH, without salt (○); Ia, PicOH,
Et4NBr (□); Ia, HCl, without salt (Δ); Ib, PicOH, without
salt (●); Ib, PicOH, Et4NBr (■). (2) [Ia]init = 2.31×10–4 M,
PicOH, Et3BnNCl (*); PicOH, Et4NCl (○); TsOH,
Et3BnNCl (□); HCl, Et3BnNCl (Δ). (3) [Ia]init = 1.16×10–4
M, PicOH, Et3BnNCl; (4) [Ib]init = 2.96×10–4 M, PicOH,
Et3BnNCl.
the rate constants of non-catalytic and catalytic pro-
cesses, respectively.
The regression in the appropriate coordinates (Fig. 3)
gives the value of non-catalytic rate constant.
Kinetics and distribution of reaction products. To
calculate the yield of picric acid in the presence of
chlorides, we used the curves 2–4 (Fig. 1) as calibra-
tion plots. The kinetic graphs thus obtained for the
reaction in the presence of Ia are nearly linear (Fig. 2,
curve 1), i.e. the observed rate constant is independent
of the indicator concentration. In the case of Ib the rate
constant tends to increase during the process (Fig. 2,
curve 2). We suppose that this is due to the
autocatalytic effect of free picric acid that accumulates
in the reaction mixture because of low basicity of the
indicator. This is confirmed by the increase in the
reaction rate in the presence of added picric acid (Fig. 2,
curve 3).
Table 2 shows that the rate constant for the
heterolysis of 1-adamantyl picrate does not depend on
the indicator concentration and structure both in the
absence and in the presence of chloride ions.
Triethylbenzylammonium chloride exhibits a positive
salt effect. The contribution of the alkylation pathway
in all cases is minor. Thus, both the kinetics and the
products distribution in the studied reactions confirm
that the verdazyls react with the acid released in the
course of the substrate solvolysis.
It is noteworthy that in most cases mentioned in the
review papers [2, 7] for the substrates RX the
dependences of the rate of indicator consumption on its
concentration and structure were observed specifically
in the presence of salts with the chloride anion, and
these dependences showed a “normal” behavior
(increase in the reaction rate with increasing the
indicator concentration and basicity). Probably, these
results originate from the effect we have found in the
present study. In principle, some other salts also may
produce similar effects, and the effect of a specific salt
may be substantial or not depending on the solvent.
Thus, the mentioned above results need to be revised.
Assuming that the catalytic process is first order
with respect to the acid, the system can be described
by the differential Eq. (3), integration of which gives
Eq. (4):
(dx)/(dt) = [k0 + ka(x + cHX0)]a,
(3)
x/(at) = k0 + ka[(∫t xdt)/t].
(4)
0
where x = сHX – cHX0 is the reaction yield, k0 and ka are
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 11 2012

KINETICS AND PRODUCT DISTRIBUTION
The origin of the “anomalous” dependences, when
1825
(x/at)×106
the reaction rate depends on the indicator structure in a
different order or it depends either on the indicator
concentration or on its structure only, remain unclear.
Such dependences were usually observed in the
relatively slow reactions and/or in the solvents that are
difficult to purify or to monitor their purity (e.g.,
heterolysis of 1-adamantyl iodide in γ-butyrolactone).
In these cases there exists an enhanced probability of
obtaining the incorrect results due to side reactions.
For example, we were not able to get any reproducible
results when studying the kinetics of the heterolysis of
1-adamantyl iodide in γ-butyrolactone in the presence
of Ib, despite repeated purification of the solvent.
We wish to emphasize again that in order to obtain
the reliable kinetic results, one needs to carefully check
the stoichiometry of the indicator interaction with the
acid and the stability of the optical density of the
resulting mixture during the appropriate time. In the
experiments on the product distribution, the degree of
the indicator regeneration after its reaction with acid
must be checked. Our observations show that if the
mixture is kept for longer times (above 15–20 min)
before the addition of the reducing reagent, the degree
of the triphenylverdazyl regeneration tends to fall. This
is equivalent to false “increase” in the contribution of
the alkylation pathway (2). We did not evaluate the
significance of this effect, since the calculated con-
tribution of the alkylation pathway in our case was
small even without taking it into account. In fact, the
alkylation reaction in our kinetic experiments
apparently did not practically occur.
t
(∫xdt/t)×106
0
Fig. 3. Plot of dependence according to Eq. (5) for the
heterolysis reaction of 1-AdOPic in acetonitrile at 25ºC in
the presence of compound Ib at [Et3BnCl] = 3×10–2 M and
[H2O] = 4.5×10–2 M.
vacuo. Photometric measurements were carried out on
a KFK-3 spectrocolorimeter in the temperature-
controlled cells with the optical length of 1 cm.
Study of the reactions equilibria and kinetics.
Standard solutions of picric and p-toluenesulfonic
acids in acetonitrile were prepared from precisely
weighed samples. Hydrochloric acid solution was
prepared in propylene carbonate by dilution of 1 M
aqueous solution (the solution of hydrochloric acid in
acetonitrile is not enough stable). The amounts of the
solutions added to the photometric cell were monitored
by weighing to ensure the accuracy of dozing. Optical
density was measured at 720 nm for Ia, 730 nm for Ib,
Anyway, the “abnormal” results for the substrates
RX are rare and they cannot serve as a basis for the
approval of the hypothesis of the applicability of
triphenylverdazyls as indicators for the solvent-
separated ion pairs.
EXPERIMENTAL
Table 2. Rate constants and product distribution of 1-AdOPic
heterolysis in the presence of 3×10–2 M of Et3BnNCl in
acetonitrile at 25ºC, [H2O] = 4.5×10–2 M
Adamantyl picrate [12], triphenylverdazyl Ia and 3-
(4-nitrophenyl)-1,5-diphenylverdazyl Ib [13] were
synthesized according to published procedures. The
Contribution of
Indicator
с
init×104, М
k×105, s–1
flow alkylation
%
1
purity of the substrate was monitored by H NMR
spectroscopy (CDCl3). For kinetic experiments aceto-
nitrile “Spectroscan” (Labscan) was used without
further purification, and some experiments were
carried out in analytical grade acetonitrile purified
according to [10]. Propylene carbonate (Acros, 99.5%)
was kept for several months over BaO and distilled in
2.31
1.16
2.97
2.05±0.01
1.99±0.02
2.0±0.2a
n = 10, r = 0.966
14
12
1
Iа
Iа
Ib
a Calculated by Eq. (5).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 11 2012

1826
SEREBRYAKOV, MOISEEVA
and 540 nm for triphenylverdazylium salt [14]. In
experiments with Ib the concentration of its salt was
not measured because of the strong absorption of Ib in
this area.
REFERENCES
1. Dvorko, G.F., Ponomareva, E.A., and Pervishko, T.L.,
Reakts. Sposobn. Org. Soed., 1980, vol. 17, no. 4, p. 434.
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vol. 60, no. 10, p. 2089.
The kinetic experiments were carried out as
described in [3]. The first order rate constant in the
presence of chlorides was calculated by formula (5)
3. Serebryakov, I.M., Kryzhanovskaya, E.B., Dzhu-
rinskaya, S.S., Kladova, A.V., and Azarova, E.A., Zh.
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Ponomareva E.A., Zh. Obshch. Khim., 1994, vol. 64,
no. 8, p. 1281.
k = x/(at) = (cHX – cHX0)/(at),
(5)
where x – yield of the reaction, cHX and cHX0 – current
and initial concentrations of acid, respectively,
determined from the calibration graphs, a – substrate
concentration, t – reaction time.
6. Dvorko, G.F., Golovko, N.N., and Ponomareva, E.A.,
Zh. Obshch. Khim., 1996, vol.66, no. 8, p. 1334.
In other cases x = Δ[I]/2 was taken, in accordance
with the stoichiometry of reaction (2). Errors are given
at 95% confidence level.
7. Dvorko, G.F. and Ponomareva, E.A., Zh. Org. Khim.,
1998, vol. 34, no. 4, p. 487.
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Zh. Obshch. Khim., 1990, vol. 25, no. 3, p. 578.
The contribution of the alkylation pathway was
calculated by formula (6):
9. Ponomareva, E.A., Tarasenko, P.V., Yurchenko, A.G.,
and Dvorko, G.F., Zh. Org. Khim., 1983, vol. 19, no. 3,
p. 548.
2(Δс – Δсr + Δc0)
10. Ponomareva, E.A., Pervishko, T.L., and Dvorko, G.F.,
α = ———————— ×100%,
Δc
(6)
Reakts. Sposobn. Org. Soed., 1979, vol. 16, no. 2, p. 113.
11. Kuhn, R. and Trischmann, H., Angew. Chem., 1963,
vol. 75, p. 294.
where Δc0 = cinit – c0 is an abrupt decrease in the
verdazyl concentration after the addition of the
substrate due to the free acid impurity in it, Δc = c0 –
min is verdazyl consumption during the reaction, Δcr =
cmax – cmin is increase in the radical concentration after
12. Sinnott, M.L. and Whiting, M. C., J. Chem. Soc., B,
1971, p. 965.
13. Kuhn, R. and Trischmann, H., Monatsh. Chem., 1964,
vol. 95, no. 2, p. 457.
c
14. Ponomareva, E.A., Dvorko, G.F., and Kulik, N.I.,
Reakts. Sposobn. Org. Soed., 1974, no. 9, p. 331.
the addition of the reducing reagent.
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