Is triphenylverdazyl an indicator for solvent-separated ion pair?

Article abstract of DOI:10.1007/s11176-005-0421-9

The hypothesis implying complete capture of solvent-separated ion pairs by triphenylverdazyls used as internal indicators for unimolecular heterolysis of organic compounds was critically analyzed. Products of heterolysis of diphenylmethyl bromide in anhyd

Full text of DOI:10.1007/s11176-005-0421-9

Russian Journal of General Chemistry, Vol. 75, No. 8, 2005, pp. 1331 1335. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 8, 2005,
pp. 1402 1406.
Original Russian Text Copyright
2005 by Serebryakov, Kryzhanovskaya, Dzhurinskaya, Kladova, Azarova.
DISCUSSIONS
Is Triphenylverdazyl an Indicator
for Solvent-Separated Ion Pair?
I. M. Serebryakov, E. B. Kryzhanovskaya, S. S. Dzhurinskaya,
A. V. Kladova, and E. A. Azarova
Donetsk National University, Donetsk, Ukraine
Received May 27, 2004
Abstract The hypothesis implying complete capture of solvent-separated ion pairs by triphenylverdazyls
used as internal indicators for unimolecular heterolysis of organic compounds was critically analyzed. Prod-
ucts of heterolysis of diphenylmethyl bromide in anhydrous acetonitrile in the presence of triphenylverdazyl
were identified, and their distribution was determined. Alkylation of verdazyl under kinetic conditions almost
does not occur, and verdazyl is consumed as a result of its reaction with hydrogen bromide liberated during
the solvolysis.
G.F. Dvorko and co-workers [1, 2] advanced a
hypothesis according to which triphenylverdazyl (I)
in aprotic solvents completely traps solvent-separated
ion pairs formed by heterolytic dissociation of organic
substrates; therefore, it can be used as internal indi-
cator for the rate of formation of ion-pair inter-
mediates. This statement was put as the basis for
detailed interpretation [3, 4] of extensive experimental
data on salt effects in heterolytic reactions; in parti-
cular, the authors elucidated the mechanism of ne-
gative special salt effect and substantiated the occur-
rence on the reaction coordinate of an additional
intermediate, space-separated ion pair.
In the general case, triphenylverdazyl is capable of
concurrently reacting with the substrate and acid
formed by hydrolysis of the substrate with traces of
water present in the solvent [reactions (1) and (2)] [5].
Therefore, the proposed concept requires at least that
the contribution of alkylation in kinetic experiments
be predominating and that external solvent-separated
ion pair return be absent.
Ph
Ph
Ph
R
N
N
N
N
N
N
N+
N
N
(1)
(2)
+
RX + 2
Br
N
N
N
Ph
Ph
Ph
Ph
Ph
Ph
Ph
I
II
III
NH
N
N
N
HX + 2I
+ III
Ph
Ph
IV
These conditions imply an extremely high reactivity
of verdazyl toward solvent-separated ion pair. To
successfully compete with traces of water, the activity
of triphenylverdazyl in unimolecular reaction with
diphenylmethyl chloride should exceed the activity of
azide ion in the same reaction by almost two orders
of magnitude [2], whereas chloride ion (and hence
azide ion) under these conditions combines with car-
bocationic intermediates derived from diphenylmethyl
chloride at a diffusion rate [6]. Furthermore, according
to the data of [7, 8], the rate constant for external
return of the solvent-separated ion pair formed from
1070-3632/05/7508-1331 2005 Pleiades Publishing, Inc.

1332
SEREBRYAKOV et al.
1
1
diphenylmethyl chloride in acetonitrile exceeds 108 s .
Complete capture of that intermediate by triphenyl-
tion of verdazyl and substituents in the benzene rings
of the latter (see, e.g., [12, 13]). This means that
verdazyl is involved in the process at the rate-deter-
mining stage, i.e., it reacts with the substrate rather
than with acid. Despite its persuasiveness, the above
argument is nevertheless indirect; and it may be
proved or disproved by direct experiments. Had the
authors proved that alkylation products either would
not have formed or would have formed in minor
amounts (assuming that they are stable under the
given conditions), an alternative interpretation of the
observed kinetic effects should be sought for.
4
verdazyl (at a typical concentration of 10 M) re-
quires the corresponding rate constant to be about
1013 mol 1 s . However, the diffusion limit for re-
1
actions of neutral amines with cationic species derived
diphenylmethyl chloride in acetonitrile is
5
109 mol 1 s [10]. Such values of rate constants are
unprecedented for reactions involving outer-sphere
electron transfer in nonstructurized systems. Taking
into account that reaction of triphenylverdazyl with
solvent-separated ion pair is a multistep trimolecular
process [4], the above value seems to be especially
surprising.
1
We believe that serious counterarguments force the
proposed theory to be substantiated more rigorously.
Here, the key problem is product distribution in reac-
tions (1) and (2) under kinetic conditions. We per-
formed experiments which allowed us to directly
identify products of heterolysis of diphenylmethyl
bromide in the presence of triphenylverdazyl and
The arguments given by the authors in support to
the proposed theory for substrates like RX may be
grouped into three categories.
(1) Alkylverdazyls II were isolated in preparative
experiments performed with diphenylmethyl and 1-
adamantyl systems [5, 9, 11]. However, this fact does
not prove their quantitative (or even predominant)
formation in kinetic experiments where the concentra-
tion of triphenylverdazyl was lower by about three
orders of magnitude.
elucidate their distribution in anhydrous acetonitrile
2
([H2O]
5 10 M) where (in keeping with the
conclusions drawn in [2]) hydrolysis of the substrate
almost does not occur.
Identification of the products. Treatment of com-
pounds II (R = Ph2CH) and IV with nitric acid leads
to formation of intensely colored salt III, and they are
readily determined by thin-layer chromatography at a
(2) The relative contributions of reactions (1) and
(2) were estimated [2, 9] by kinetic experiments in
moist acetonitrile ([H2O] = 0.13 0.3 M). The calcula-
tion procedure was based on the fact that addition of
an alkaline reducing agent to reaction mixture pro-
motes quantitative transformation of all products,
except for alkylverdazyl, into the initial radical. In-
complete regeneration of triphenylverdazyl indicates
formation of alkylation product, and the ratio of the
yields (or concentrations) of compounds II and IV is
proportional to the concentration ratio of the corres-
ponding reagents ([I]/[H2O]) [9]. Unfortunately, the
authors did not perform experiments to estimate
product distribution in anhydrous solvents used in
kinetic measurements. Instead, they automatically
concluded that in all aprotic solvents (as well as in the
presence of any salt), the hydrolysis process is ruled
out regardless of the verdazyl structure and concentra-
tion.
4
concentration of 10 M (Rf = 0.62, 0.38, and 0,
respectively: silica gel/benzene). The reaction of HBr
4
with triphenylverdazyl (2 10 M) afforded com-
pounds III and IV [according to scheme (2)]. In the
reaction of triphenylverdazyl with diphenylmethyl
bromide, performed under the same conditions, no
product II was detected in the reaction mixture even
after fivefold application to a chromatographic plate.
Instead of II, we identified leucoverdazyl IV. Some
difficulties in the analysis of the reaction mixture
arose from the fact that compound IV applied to the
start line underwent fast oxidation to the correspond-
ing salt on drying. A freshly prepared solution of a
mixture of leucoverdazyl with diphenylmethyl
bromide behaved similarly, whereas pure leucover-
dazyl remained unchanged upon drying on silica gel.
(3) In some reactions carried out in the presence
of salts, the reaction rate depended on the concentra-
Distribution of the products. As stated in [2],
verdazyl is completely regenerated from a mixture of
its salt and leuco base by the action of an alkaline
formaldehyde solution. This statement was based on
the data of [14], where the regeneration was effected
in a 3:1 acetonitrile water mixture. Our results are
approximately similar (Table 1, run nos. 1, 2).
However, complete recovery of triphenylverdazyl after
its reaction with HCl cannot be achieved using the
same reagent in acetonitrile with a lower concentra-
1
There are also other methods for estimation of the same
quantity; however, discussion on this topic goes beyond the
scope of the present article. Even though the rate constant for
external solvent-separated ion-pair return amounts to ca.
6
1
10 s [9], the rate constant for its capture by triphenylverda-
zyl should exceed the diffusion limit by an order of magnitude.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 8 2005

IS TRIPHENYLVERDAZYL AN INDICATOR
1333
Table 1. Regeneration of triphenylverdazyl from products
of its reaction with HCl and from verdazylium bromide
(III) in acetonitrile at 25 Ca
acetonitrile containing much less than 0.05 mol/l of
water. After addition of an alkaline formaldehyde
solution to the reaction mixture (Table 2; run nos. 1
3), the degree of regeneration of verdazyl was ap-
proximately the same as in the reaction of I with HCl,
which indicated the absence or insignificant contribu-
tion of the alkylation process. However, poor repro-
ducibility of the results under these conditions did not
allow us to estimate the relative contributions of reac-
tions (1) and (2) with an acceptable accuracy.
Run Reducing cinit 104, cmin 104, cmax 104,
c
no.
agentb
M
M
M
Triphenylverdazyl (I) + HCl
1d
A
1.13
1.07
1.32
1.18
1.00
1.28
1.13
1.54
1.39
0.26
0.26
0.23
0.32
0.23
0.45
0.15
0.02
0.05
1.07
1.02
1.13
0.85
0.74
0.95
0.72
1.48
1.36
0.93
0.93
0.83
0.63
0.67
0.58
0.58
0.96
0.99
2d
3
4
5
6
7
8
9
In order to raise reliability of the results, it would
be desirable to carry out analogous experiments with
a reducing agent ensuring complete regeneration of
verdazyl from both its salt and products of its reaction
with HCl. Satisfactory data were obtained with the
use of a solution of 33% of N,N-dimethyl-p-methoxy-
aniline and 10% of N-methylmorpholine in aceto-
nitrile (the solution was added in an amount of 50 l
per 2 ml of the reaction mixture; Table 1, run nos. 8,
9, 12). Special photometric experiments showed that
compound II was not converted into the radical
species under these conditions. The results obtained
with the above reducing agent (Table 2; run nos. 4, 5)
unambiguously indicated that verdazyl did not capture
all solvent-separated ion pairs derived from diphenyl-
methyl bromide and, moreover, that its alkylation is
not the major process.
B
Triphenylverdazylium bromide (III)
10
11
12
A
0.85
1.27
1.19
0.85
1.27
1.19
1.00
1.00
1.00
B
a
b
For denotations, see Experimental. Reducing agents: a mix-
ture of equal volumes of 2 N aqueous NaOH and 37% form-
aldehyde solution (A) and a solution of 33% of N,N-dimethyl-
p-methoxyaniline and 10% of N-methylmorpholine in aceto-
c
nitrile.
is the degree of regeneration of compound I; in
d
experiments with HCl, c /(c
c ).
min
In acetonitrile
r
init
water (3:1, by volume).
Thus, under the conditions of kinetic experiments
with diphenylmethyl bromide in anhydrous aceto-
nitrile in the absence of salts, almost no alkylation
products are formed from triphenylverdazyl. There-
fore, the hypothesis implying complete capture of
solvent-separated ion pairs under these conditions is
invalid. This hypothesis should be verified by experi-
ments performed in other solvents, as well as under
the conditions under which the reaction rate depends
on the radical concentration and structure.
tion of water (run nos. 3 7), while pure verdazilium
salt is quantitatively reduced to the radical (run nos.
10, 11). Thus the degree of regeneration of triphenyl-
verdazyl from products of reaction (2) by an alkaline
solution of formaldehyde tends to decrease as the
concentration of water in the solvent decreases.
Neglect of this factor is likely to be responsible for
erroneous conclusions drawn in [2, 9].
EXPERIMENTAL
We measured the rate constant for heterolysis of
diphenylmethyl bromide. The obtained value was very
Diphenylmethyl bromide [15], triphenylverdazyl,
triphenylverdazylium bromide [16], leucoverdazyl
7
1
consistent with the rate constant 1.86 10
s
[5] in
Table 2. Distribution of heterolysis products of diphenylmethyl bromide (c = 0.02 M) in the presence of triphenylverdazyl
in acetonitrile at 25 Ca
b
c
1
Run no. Reducing agent cinit 104, M c0 104, M cmin 104, M cmax 104, M
k 106, s
1
2
3
4
5
A
1.07
1.15
1.20
1.15
1.68
1.05
1.09
1.14
1.09
1.53
0.23
0.23
0.23
0.23
0.23
0.91
0.97
0.88
1.07
1.54
0.81
0.79
0.65
0.91
0.89
2.15 0.03
2.14 0.04
2.31 0.04
1.96 0.02
2.08 0.04
B
0.13
0.16
a
a
b
b
c
For denotations of reducing agents and concentrations, see notes , to Table 1.
= ( c c )/ c. Calculated by formula (3).
r 0
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 8 2005

1334
SEREBRYAKOV et al.
[17], and alkylverdazyl (II, R = Ph2CH) [5] were
prepared by known procedures. Diphenylmethyl bro-
mide was purified by vacuum distillation. The melting
points and spectral parameters of the compounds were
in agreement with those reported in the literature.
Acetonitrile was distilled over P2O5 and, just before
use, over CaH2.
probability of 95%. The contribution of the alkylation
process was calculated by formula (3).3
= [2( c
cr + c0)]/ c.
(3)
Here, c0 = cinit
verdazyl concentration upon addition of substrate due
to the presence of acid in the latter;, c = c0 cmin
c0 is jumpwise decrease in the
,
1
The H NMR spectrum of compound II was re-
consumption of verdazyl during the reaction, and
cr = cmax cmin, increase in the verdazyl concentra-
tion upon addition of reducing agent.
corded on a Gemini-200 spectrometer (200 MHz) in
CDCl3, , ppm: 2.98 d (1H, CH2, J = 11.8 Hz), 5.03 d
(1H, CH2, J = 11.8 Hz), 6.09 s (1H, CH), 7.75
6.60 m (23H, Harom), 7.9 m (2H, Harom).
REFERENCES
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KFK-3 photometer using 1-cm cells: I, = 720 nm,
= 4330; II, = 540 nm, = 12 170 [1]. Thin-layer
chromatography was performed on Silicagel-60 F254
(Riedel-de Haen); layer thickness 0.2 mm; eluent
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constant temperature. The mixture was kept for a
required time, the reducing agent was added, and the
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c = cmes(2 + Vred)/2.
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constant was calculated by the equation dc/2dt =
k[RX]. The confidence intervals were given at a
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Dvorko et al. [2] used a more complex but essentially equi-
2
We failed to detect compound IV in the reaction mixture by
valent formula which also implied measurement of the con-
centration of verdazylium salt.
TLC on Sorbfil STKh-1VE silica gel (8 12 m).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 8 2005

IS TRIPHENYLVERDAZYL AN INDICATOR
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