Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XXVIII. Effect of the nature of the verdazyl indicator on salt effects in dehydrobromination of 3-bromocyclohexene in nitrobenzene

Article abstract of DOI:10.1023/A:1015349429876

The salt effect on the rate of dehydrobromination of 3-bromocyclohexene in PhNO₂ depends on the nature of the verdazyl indicator. With triphenylverdazyl and its chloro and nitro derivatives in the presence of Et₄NClO₄, a n

Full text of DOI:10.1023/A:1015349429876

Russian Journal of General Chemistry, Vol. 72, No. 1, 2002, pp. 79 85. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 1, 2002,
pp. 86 92.
Original Russian Text Copyright
2002 by Ponomarev, Stambirskii, Dvorko.
Kinetics and Mechanism of Monomolecular Heterolysis
of Commercial Organohalogen Compounds:
XXVIII.1 Effect of the Nature of the Verdazyl Indicator
on Salt Effects in Dehydrobromination of 3-Bromocyclohexene
in Nitrobenzene
N. E. Ponomarev, M. V. Stambirskii, and G. F. Dvorko
Kievskii politekhnicheskii institut National Technical University of Ukraine, Kiev, Ukraine
Received April 12, 2000
Abstract The salt effect on the rate of dehydrobromination of 3-bromocyclohexene in PhNO2 depends
on the nature of the verdazyl indicator. With triphenylverdazyl and its chloro and nitro derivatives in the
presence of Et4NClO4, a normal salt effect is observed, in the presence of bromides, a superposition of normal
and special salt effects, while in the presence of chlorides, a superposition of normal and special negative salt
effects. With the dimethoxy verdazyl derivative, a normal salt effect is always observed.
In one the previous communications of this series
[2] we presented kinetic data for dehydrobromination
of 3-bromocyclohexene (I) in PhNO2 in the presence
of LiOPic, Et4NClO4, Et4NBr, Bu4NBr, Et4NCl, and
Et3(PhCH2)NCl. The kinetic experiments were per-
formed in the presence of 1,3,5-triphenylverdazyl (II)
as internal indicator [3]. The reaction occurs by the
stoichiometric equation (1):
RX
R+X
R+
Reaction products.
X
R+ Solv X
The reaction begins with equilibrium formation of
a contact ion pair which in the rate-limiting stage
reacts with solvent cavity to give a space-separated
ion pair (cavities occupy ca. 10% of the liquid volume
[6, 7]). The space-separated ion pair quickly trans-
forms into a solvent-separated ion pair, and the latter
quickly transforms into reaction products.
Ph
Br
N
N
N
N
+ 2
Ph
In the presence of triphenylverdazyl, the solvent-
separated ion pair quickly and quantitatively reacts
with the indicator, which allows the reaction rate to
be followed by the decrease in the concentration of
Ph
I
II
Ph
Ph
radical II (
720 nm) and/or by the increase in the
max
concentration of verdazylium salt III (
540 nm).
N
NH
N
N
N
max
(1)
+
+
Br
Ph
N+
Apparently, first verzadyl is alkylated and then the
alkylation product readily decomposes into cyclo-
hexadiene and leucoverdazyl (IV).
N
N
Ph
Ph
Ph
III
IV
The reaction rate is strongly dependent both on the
initial concentration and on the nature of the neutral
salt MY [2].
The reaction rate in the presence and in the absence
of salt is described by the first-order equation (2),
mechanism E1.
In the presence of Et4NClO4, the reaction rate in-
creases linearly with salt concentration (Fig. 1, plot 1),
i.e. a normal salt effect takes place [Eq. (3)].
v = k[I].
(2)
According to recent data [3 5], E1 (SN1) reactions
involve consecutive formation of three ion pairs:
contact, space-separated, and solvent-separated.
kS = k0(1 + b[salt]).
(3)
Here kS and k0 are the reaction rate constants in the
presence and in the absebce of salt and b is a para-
1
For communication XXVII, see [1].
1070-3632/02/7201-0079$27.00 2002 MAIK Nauka/Interperiodica

80
PONOMAREV et al.
In the presence of Et4NBr (Bu4NBr), the reaction
k
107,
rate increases with salt concentration, first sharply,
and then a gently sloping linear dependence takes
place (Fig. 1, plot 2). Here we deal with a superposi-
tion of normal and special salt effects [2, 7]. The
special salt effect can be explained by the effect of
the salt on the contact ion pair, which catalyzes ion
separation and formation of a space-separated ion pair
[3, 7]. The kS [salt] dependence for this salt effect
looks like a saturation curve: The reaction rate first
sharply increases with salt concentration and then,
once a certain point has been attained, when the salt
is practically completely binds the contact ion pair,
the reaction rate no longer depends on salt concentra-
tion, and dkS/d[salt] = 0.
1
s
2
3
2
1
3
1
1
2
3
[MY] 102, M
The superposition of normal and special salt effects
is described by three parameters: k0ext/k0, b1, and b2.
The first parameter measures the maximum net special
salt effect. The k0ext/k0 value is found by extrapolation
of the linear portion of the curve to zero salt concentra-
tion. The parameters b1 and b2 are found by Eq. (3)
for the first and second portions of the kS [salt] plot.
The parameter b2 measures the normal salt effect. The
b1 and b2 values depend on the association constants
of the salt with the contact ion pair and the covalent
substrate, respectively, and on the activity of the
resulting associates [7].
Fig. 1. Effect of the concentration of (1) Et NClO and
4
4
(2, 3) Et NBr on the rate constant of dehydrobromination
of compound I in PhNO in the presence of indicator
(light circles) II, (squares) IX, (triangles) X, and (dark
circles) XI at 25 C.
4
2
k
107,
1
4
s
3
2
In the presence of Et4NCl [Et3(PhCH2)NCl], the kS
[salt] dependence passes through a maximum, and the
reaction rate decreases and attains a value after which
it no longer depends on salt concentration (Fig. 2,
plot 1). It is suggested that here a superposition of
special and negative special salt effects takes place [2].
The negative salt effect is associated with the effect
of the salt on the solvent-separated ion pair [7]. In this
case, the kS [salt] dependence, too, looks like a satura-
tion curve which is a mirror image of the corresponding
curve for the special salt effect: At small salt additives,
the reaction rate sharply decreases to a certain point,
after which it no longer depends on salt concentration,
dkS/d[salt] = 0 [7, 8].
1
1
2
3
0
1
2
3
4
[MY] 102, M
The negative special salt effect is described by two
parameters: kmin/k0 and b. The first parameter mea-
sures the fraction of solvent-separated ion pairs return-
ing to the covalent substrate. The kmin value is kS at
dkS/d[salt] = 0 [8].
Fig. 2. Effect of the concentration of Et (PhCH )NCl on
the rate constant of dehydrobromination of bromide I in
3
2
PhNO in the presence of indicator (light circles, curve
2
1) II, (squares, curve 2) IX, (triangles, curve 3) X, and
(dark circles, curve 4) XI at 25 C. (Crosses) Experiments
with Et NCl and indicator II.
4
Since salt effects are independent on the nature of
the salt cation, a conclusion has been drawn that the
active agent of the salt is its anion [2]. Consequently,
the effect of perchlorate, bromides, and chlorides on
the rate of dehydrobromination of compound I can be
explained by formation of intermediates V VII.
meter measuring the salt effect; for Et4NClO4, b = 16.
The normal salt effect is associated with the effect
of the salt on the covalent substrate, favoring forma-
tion of a contact ion pair.
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XXVIII.
81
The resulting data, together with data for indicator
II, are depicted in Figs. 1 and 2.
+
ClO4 R Br
Br R+Br
VI
II
V
In the presence of Et4NClO4, a normal salt effect
is observed, independent on the nature of the indicator,
b = 16 (Fig. 1, plot 1).
Cl R+ Solv Br
Cl R+ Solv Br
VII
VIII
With Et4NBr and indicators IX and X, like with
indicator II, there is a superposition of normal and
special salt effects, and the strengths of the effects are
independent of the nature of the indicator: k0ext/k0 =
1.4, b1 = 400, and b2 = 50 (Fig. 1, plot 2). Since the
b1 and b2 values are srongly dependent of the forma-
tion constants of intermediates V and VI, it was
reasonable to expect that the constants would be much
higher when the intermediate is formed from a contact
ion pair than from the covalent substrate. This proved
the case.
In terms of the principle of hard and soft acids and
bases, a stronger (hard) nucleophile (base) reacts with
a harder electrophile [9]. It is reasonable to suggest
that the electrophilicity of carbon in RBr increases
with increasing degree of charge separation in the
C Br bond.
The negative special salt effect is associated with
the fact that the charge-transfer complex VIII formed
by verdazyl reaction with three-ion species VII has two
reaction pathways: It gives reaction products with a
second indicator molecule or decomposes to give the
covalent substrate. The second pathway takes place
because the reaction of the carbocation with the three
ligands (Br , Cl , and II) incorporated in complex
VIII reduces the energy of binding of solvent mole-
cule in the interionic space, and this molecule readily
passes into the nearly formed cavity. This results in
ion pair return. This return can be rather high [8].
Thus, in heterolysis of Ph2CHBr in -butyrolactone
(SN1 reaction) in the presence of LiCl and indicator II
the return is ca. 90% (kmin/k0 = 0.1) [10].
With indicator XI in the presence of Et4NBr, only
a normal salt effect is observed, b = 50 (Fig. 1, plot 3).
Consequently, the use of the most electron-donor
verdazyl fully eliminates the special salt effect.
In the presence of Et3(PhCH2)NCl, each indicator
has its own kS [salt] dependence (Fig. 2, plots 1 4).
With indicators II, IX, and X, these dependences pass
through a maximum, and then, with indicators IX and
X, the kS values continuously decrease with increasing
salt concentration, or, with indicator II, dkS/d[salt] = 0
at high salt concentrations. The latter result led us to
conclude that in the presence of chlorides no salt
effect takes place [2]. However, the dependences for
indicators IX and X may imply that the independence
of kS on salt concentration (plot 1) results from a
compensation of positive and negative salt effects.
Unlike the other salts, LiOPic produces no effect
on the rate of dehydrobromination of bromide I in
PhNO2. This fact is consistent with the conclusion
that the salt cation and ion pair exert virtually no rate
effect. The same is true of the weakly nucleophilic
picrate anion.
Proceeding with studies on salt effects in mono-
molecular heterolysis reactions (SN1, E1, solvolysis),
we turned to the effect of the nature of the verdazyl
indicator on the rate of dehydrobromination of bro-
mide I in nitrobenzene in the presence of Et4NClO4,
Et4NBr, and Et3(PhCH2)NCl. It is known [7, 8, 10]
that the nature of the verdazyl indicator can affect the
negative salt effect. As indicators we used 3-(4-chloro-
phenyl)-1,5-diphenylverdazyl (IX), 3-(4-nitrophenyl)-
1,5-diphenylverdazyl (X), and 3,5-bis(4-methoxy-
phenyl)-1-phenylverdazyl (XI).
The strength of the salt effect in the presence of
chlorides depends both on salt concentration and
on the nature of the indicator. The strength of the salt
effect at all salt concentrations decreases as the elec-
tron-donor capacity of verdazyl increases in the series
II < IX < X (Fig. 2). The estimates for b1 from
the first portions of curves 1 3 are 800, 600, and 400,
respectively. This points to decreasing contribution of
the negative salt effect in the mentioned series.
However, it is known that the negative salt effect may
differently vary with electron-donor capacity of
verdazyl: It may increase, decrease, or be independent
of this parameter [7, 8, 10]. This fact can be ac-
counted for by a multistage reaction of verdazyl with
intermediate VII; therewith, in the first stage the reac-
tion rate increases with increasing electron-donor
capacity of the indicator, and in the other stage it
decreases. Therefore, any dependence of kmin on the
nature of verdazyl is possible [10].
The reactions occur according to a stoichiometric
equation similar to Eq. (1). The concentration of
bromide I in the kinetic experiments was ca. 1
2
10 M, and the concentrations of the indicators were
4
ca. 1.3 10 M. In certain experiments the concentra-
5
tion of the indicator was varied from 2 10 to
4
2.5 10 M. The conversion of the substrate was ca.
0.01%. The reaction rate is in all cases fairly des-
cribed by Eq. (2).
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82
PONOMAREV et al.
With indicator XI in the presence of Et3
separated ion pair to form charge-transfer complex
VIII; and (3) the negative salt effect is associated with
the ability of complex VIII to decompose to the
starting reagents.
(PhCH2)NCl, a normal salt effect is observed exclu-
sively, b = 60 (Figs. 2, 4). Special experiments
showed that the salt effect here is independent of the
salt cation. It is also independent of the indicator
concentration, like with indicator II [2]. Thus, in the
experiments with 0.0011 M Et3(PhCH2)NCl at
We found that the salt effects of Et4NClO4 in the
presence of all the verdazyls and of Et4NBr in the
presence of indicators II, IX, and X are independent
of the nature of the indicator. In the first case, a
normal salt effect takes place, and in the second, a
superposition of normal and special salt effects. In the
presence of Et3(PhCH2)NCl with indicators II, IX,
and X, there is a superposition of normal, special,
and negative special salt effects. These salt effects are
associated with intermediate V VIII formation, and
they can be explained in terms of pathways a and b of
scheme (4).
indicator concentrations of (0.2, 0.4, 0.7, 1.3, 2.0,
4
and 2.5) 10 M, the mean kS was (1.6
10 s .
0.06)
7
1
The fact that the reaction rate is independent of
indicator concentration and depends on the nature of
the indicator in the presence of a salt that produces a
negative salt effect suggests that: (1) verdazyl reacts
with the cationoid intermediate after the rate-limiting
stage; (2) verdazyl completely intercepts the solvent-
k15
c
k16
XI, k17
(Y R+ Br )
(Y R+Br )
(Y R+Br )
k
15
XIV
XV
Y
II
k14
k
14
a
k3
k1
k
k2
II, k4
II
R+Br
R+Br
R+ Br
R+ Solv Br
R+ Solv Br
Reaction products
k5
k
k
4
k
3
1
2
Y
Y
k
k6
k
k12
II
6
12
b
(4)
k8
II, k10
k7
k
k9
II
Y R+ Solv Br
Y R+ Solv Br
Y R+Br
Y R+ Br
Y R+Br
VI
k11
k
k
8
7
9
V
VII
VIII
k13
In the absence of salt, the reaction takes pathway
a (k1 k2 k3 k4 k5). For the limiting formation of a
solvent-separated ion pair we have, under steady-state
assumptions, Eq. (5):
Consequently, b = k8K6K7.
The overall rate constant is given by Eq. (8):
k1k2
k0 = k1k2/(k + k2).
(5)
1
kS
=
(1 + k8K6K7[MY]).
(8)
In the presence of Et4NClO4, the reaction takes
pathways a and b (k6 k7 k8 k9 k10 k11) without
reaction k13. The reaction rate by pathway (b) is v =
k8[VI]. Under steady-state assumptions for pathway
b, we have Eq. (6):
k
+ k2
1
The absence of special salt effect with Et4NClO4
may imply either that reaction k12 is insignificant or
that k2 k8, i.e. that the catalytic effect of ion separa-
tion in the contact ion pair is absent [7].
k6k7k8[MY]
k =
.
(6)
In the presence of bromides and indicators II,
IX, and X, a superposition of normal and special salt
effects is observed. In this case, too, the reaction
follows pathways a and b (at k13 = 0). Here,
however, reaction k12 is significant, and k2 < k8.
k8(k + k7) + k k
7
6
6
At k >> k8, we have Eq. (7):
7
k6k7k8[MY]
k =
= k8K6K7[MY].
(7)
The rate constant in the presence of bromides is
given by Eq. (9).
k k
6
7
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XXVIII.
83
k8[MY][k1k12(k + k7) + k6k7(k + k2 + k12[MY])]
6
1
kS
=
.
(9)
(k + k2 + k12[MY])[(k + k7)(k + k8 + k
)
k7k ]
(k + k7)k12k 12[MY]
6
1
6
7
12
7
When the special salt effect is a maximum, the kS
[salt] dependence gets linear, since increase in the
reaction rate depends on the normal salt effect. This
At the limiting formation of a space-separated ion
pair, when k << << k >> k8, we have Eq. (7)
which describes the normal salt effect.
12
7
is possible when k6k7(k + k2 + k12[MY]) >> k1k12
1
(k + k7) and (k + k2 + k12[MY])[(k + k7)(k
+
6
1
7
k7k ] >> k12k 12(k + k67)[MY].
In the presence of chlorides and indicators II, IX,
and X, we have a superposition of normal, special,
and negative special salt effects. In this case, the
reaction takes pathways a and b, including reaction
k13 [2, 8]. The rate constant, on condition that reac-
tions k9 and k10 has no rate effect, is given by Eq. (11).
k8 + k12)
7
6
In this case, Eq. (10) is valid.
k6k7k8[MY]
k =
.
(10)
(k + k7)(k + k8 + k
)
k7k
6
7
12
7
k8[MY][k1k12(k + k ) + k k (k + k2 + k12[MY])]
1
6
7
6 7
kS
=
.
(11)
(k + k2 + k12[MY])[(k + k7)(k + k8 + k
+ k13)
k7k ]
(k + k7)k12k 12[MY]
6
1
6
7
12
7
When the negative special salt effect is a maximum,
further increase in salt concentration no longer
affects the reaction rate, and dkS/d[salt] = 0. This is
only (Fig. 1, plots 1 and 3, and Fig. 2, plot 4), and its
strength increases in the following order:
PicO
<
ClO4
16
<
Br
50
<
Cl
60
possible, when k1k12(k 6 + k7) >> >> (k6k7)(k + k2 +
1
b
0
k12)[MY] and k12[MY][(k + k7)(k + k8 + k
+
6
k13) k7k ] k12k 12(k + k7)[MY7] >> (k + 1k27)
7
6
6
This result does not contradict the conclusion that
ionization of bromide I produces nucleophilic cata-
lysis. Since the normal salt effect with Et4NClO4 is
independent of the nature of the indicator (Fig. 1, plot
1), we can contend that in the presence of indicator XI
it is described by pathway b of scheme (4). However,
the salt effects of bromides and chlorides in the
presence of indicator XI differ considerably from
those with other indicators. With bromides, the use of
indicator XI eliminates the special salt effect, while
(k + k8 + k
+ k13)(k + k2).
7
12
1
Then we can write Eq. (12):
kmin = k1k8/(k + k8 + k13
k7k ).
(12)
(13)
7
7
On condition that k13 >> k , we get Eq. (13).
7
kmin = k1k8/(k8 + k13).
In this case, the reason for the special salt effect chlorides eliminate both the special and negative
may be suppression of the internal ion pair return
stage (k ) at practically complete binding
of the contact ion pair by the salt, yielding interme-
diate V. Actually, at k8 >> k13 we have kS = k1 [7]. If
k13 >> k8, complex VIII can completely decompose
to the starting reagents, and the reaction by pathway
(b) will not occur.
special salt effects. These results cannot be in terms
of pathways a and b of scheme (4). One of the
reasons may be that the reaction of three-ion inter-
mediate VII with indicator XI involves complete
conversion of the resulting charge-transfer complex
to the starting reagents, k11 << k13 >> k8. This
conclusion is not unexpected in view of the known
examples of a 90% return by pathway k13 [8, 10].
a
1
Thus, the salt effects observed in dehydrobromina-
tion of compound I in the presence of perchlorates,
bromides, and chlorides and indicators II, IX, and X
are fairly described by scheme (4), pathways a
and b.
If our suggestion is valid, then the elimination of
special and negative special salt effects in the presence
of inidicator XI may imply that heterolysis of organic
substrates involves formation of a variety of contact
ion pairs, followed by ionization of one of them under
the action of salt to give an intermediate capable of
quantitatively reacting with indicator XI only.
In the presence of the most electron-donor indicator
XI with all the salts we observe a normal salt effect
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 1 2002

84
PONOMAREV et al.
The intermediate formation of contact ion pairs
that the indicator reacts with a space-separated ion
with different degrees of charge separation has been pair. One of the reasons for this situation may lie a
11
repeatedly mentioned in the literature [11 16].
Winstein et al. reasoned that the ion pair is formed as
a result of electrophilic solvation of the nucleofuge,
while nucleophilic solvation of such ion pair does not
occur [17]. Raber et al. [18] in their review on ions and
ion pairs in solvolysis reactions proposed formation
of electrophilically and nucleophilically solvated ion
pair XII which can mediate contact and solvent-
separated ion pairs.
fairly short lifetime of this intermediate (>10 s).
In the presence of perchlorates, pathway c is
insignificant.
EXPERIMENTAL
Methods of synthesis and purification of reagents
are given in [2]. Kinetic measurements were per-
formed on an SF-26 spectrophotometer in a tempera-
ture-controlled cell. The reaction rate was controlled
by the decrease in the concentration of the verdazyl
indicator. The errors in k were 2 4%.
Y
Solv
R+X
Solv
Y R+X
Solv.
XII
XIII
1
Dannenberg suggested [19] that under the action
of salt on such ion pair the solvent molecule that
effects nucleophilic solvation is replaced by the anion
to form intermediate XIII, and this favors ion separa-
tion in the ion pair. Okamoto [20] and Kinoshita et
al. [21], when interpreting the salt effects in SN1(E1)
phenolysis reactions, too, proposed formation of an
ion pair like XII, but they held that it undergoes
frontal nucleophilic attack of the salt.
Given are [MY] 102 (M) and k 107 (s ). Expe-
riments with indicator X (
730 nm, 3320) in the
presence of Et4NClO4: 0.6m2a,x1.08; 0.93, 1.12; 1.39,
1.16; and 2.08, 1.25; experiments with indicator XI
(
750 nm, 4120) in the presence of Et4NClO4:
0.29, 1.07; 0.58, 1.12; 0.87, 1.20; 1.3, 1.22; and 1.95,
1.28.
max
Experiments with indicator IX (
730 nm,
max
4570) in the presence of Bu4NBr: 0.12, 1.2; 0.31,
1.37; 0.469, 1.51; 0.938, 1.81; 1.407, 2.06; 2.11, 2.44;
and 3.02, 2.75; experiments with indicator X in the
presence of Bu4NBr: 0.305, 1.51; 0.611, 1.75; 0.685,
1.70; 1.03, 1.82; 1.54, 2.14; 1.64, 2.16; 2.31, 2.54;
and 3.62, 3.2; experiments with indicator XI in the
presence of Bu4NBr: 0.66, 1.32; 0.725, 1.35; 0.99,
1.50; 1.485, 1.74; 2.227, 2.19; 3.02, 2.53; and 3.02,
2.53.
The quantum-chemical analysis of ion separation
in liquids led the authors of [22, 23] to conclude that
contact and solvent-separated ion pairs are mediated
by one more ion pair and they called it a contact ion
pair which has began to separate. In [24, 25], this ion
pair was called a space-separated ion pair.
Thus, heterolysis of organic compounds may give
rise to different contact pairs with different reactivity.
Organic compounds capable of giving complex car-
bocationic intermediates can form several ion pairs.
Apparently, one of such compounds is 3-bromocyclo-
hexane whose heterolysis gives rise to an allylic
carbocation. It thus can be concluded than bromide I
under the action of a salt anion forms at least two
different ion pairs incorporated in three-ion species
VI. One of these ion pairs, more active, gives solvent-
separated three-ion species VII which partially (under
the action of indicators II, IX, and X) or completely
(under the action of inidicator XI) converts to the
covalent substrate. Concurrently, by pathway c, a
more stable three-ion species XIV is formed, which
gives rise to an inactive space-separated three-ion
species XV, and the latter can react with such an
active indicator as XI. As a result, with indicator XI
in the presence of bromides and chlorides, only normal
salt effect is observed. The absence of special salt
effect in this case suggests that three-ion species XIV
is formed only under the action of the salt on the
covalent substrate, while the absence of negative
special salt effect is consistent with the conclusion
Experiments with indicator X in the presence of
Et4NCl: 0.06, 1.41; 0.11, 1.54; 0.23, 1.90; 0.46, 2.03;
0.68, 1.67; 1.03, 1.42; 1.54, 1.32; 2.04,1.13; and 2.31,
0.96; experiments with indicator XI in the presence
of Et4NCl: 0.74, 1.29; 1.11, 1.79; 1.66, 1.93; and
2.49, 2.52.
Experiments with indicator IX in the presence of
Et3PhCH2NCl: 0.15, 1.69; 0.3, 1.91; 0.44, 2.07; 0.59,
2.19; 0.66, 2.27; 0.88, 2.25; 1.33, 1.93; 1.48, 1.84;
2.22, 1.42; 2.99, 1.05; 3.33, 0.99; and 3.52, 0.90;
experiments with indicator X in the presence of
Et3PhCH2NCl: 0.49, 1.74; 0.73, 1.61; 1.63, 1.34; and
2.49, 0.99; experiments with indicator XI in the
presence of Et3PhCH2NCl: 0.22, 1.07; 0.45, 1.26;
0.64, 1.37; 0.67, 1.34; 0.67, 1.42; 0.96, 1.56; 1.10,
1.77; 1.44, 1.94; 1.56, 1.92; 3.24, 3.10; and 4.25, 3.70.
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