Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XL. Nature of salt effects in dehydrobromination of 3-bromocyclohexene in γ-butyrolactone. Role of solvation effects of dipolar aprotic solvents

Article abstract of DOI:10.1007/s11176-005-0340-9

The influence of neutral salts on the rate of heterolysis of 3-bromocyclohexene at 31°C in γ-butyrolactone was studied by the verdazyl method; ν = k[C₆H₉Br], E1 mechanism. Additions of lithium picrate do not affect the reaction rate;

Full text of DOI:10.1007/s11176-005-0340-9

Russian Journal of General Chemistry, Vol. 75, No. 6, 2005, pp. 883 890. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 6, 2005,
pp. 937 944.
Original Russian Text Copyright
2005 by Ponomarev, Stambirskii, Dvorko.
Kinetics and Mechanism of Monomolecular Heterolysis
of Commercial Organohalogen Compounds:
XL.1 Nature of Salt Effects in Dehydrobromination
of 3-Bromocyclohexene in -Butyrolactone.
Role of Solvation Effects of Dipolar Aprotic Solvents
N. E. Ponomarev, M. V. Stambirskii, and G. F. Dvorko
Kiev Polytechnic Institute, National Technical University of Ukraine, Kiev, Ukraine
Received February 20, 2004
Abstract The influence of neutral salts on the rate of heterolysis of 3-bromocyclohexene at 31 C in
-butyrolactone was studied by the verdazyl method; v = k[C6H9Br], E1 mechanism. Additions of lithium
picrate do not affect the reaction rate; those of LiClO4 and Et4NClO4 increase it; and those of LiCl, Et4NCl,
and KNCS decelerate the reaction. The nature of salt and solvation effects in the heterolysis of 3-bromocyclo-
hexene in -butyrolactone, MeCN, and PhNO2 is discussed.
The rate of monomolecular heterolysis (SN1, E1,
solvolysis) is controlled by the ionization of a cova-
lent bond, occurring via successive formation of three
ion pairs: contact I, spacially separated II, and solva-
tion-separated III [2]:
ks = k(1 + b[salt]).
(2)
The linear character of the normal salt effect is due
to the low constant of association of a salt with a
covalent substrate; the parameter b reflects this asso-
ciation constant.
RX
R+X
I
R+
II
Reaction products.
X
R+|Solv|X
When a salt acts on pair I, the reaction rate first
sharply increases, but further additions of the salt do
not longer affect the reaction rate, and dk/d[salt] = 0
(special salt effect). The association constant of the
salt with pair I is high; therefore, even at low concen-
III
(1)
In the limiting step, pair I interacts with a solvent
cavity (cavities account for 10% of the volume of a
liquid [3]). Ion pair II is thus formed, which rapidly
transforms into ion pair III which, in turn, rapidly
yields the reaction products.
2
4
trations of the additive (10 10 M), the ion pair is
virtually fully bound by the salt to form a quadrupole
or a ternary ionic associate, and further additions of
the salt exert no effect on the reaction rate.
Neutral salts (ammonium or alkali metal salts)
affect the heterolysis rate strongly and specifically
[4 7]. The nature and intensity of the salt effect de-
pend on the substrate structure, salt, and solvent. The
observed diversity of the salt effects is due to the fact
that an ion pair of a salt or its ions can interact with
a covalent substrate or various ion-pair intermediates
[6, 7].
Usually a superposition of the normal and special
salt effects is observed: The reaction rate first sharply
increases at low salt concentrations, and then the in-
crease becomes more gradual. Extrapolation of the
linear portion of the curve to [salt] = 0 gives the max-
imal value of the special salt effect (kext). The param-
eters b1 and b2 characterizing the association of a salt
with pair I and covalent substrate are calculated from
the first and second portions of the curve by Eq. (2).
When a salt acts on a covalent substrate, the reac-
tion rate linearly grows with the salt concentration
(normal salt effect). The value of this effect (b,
Usually b1 : b2
10 102.
1
When a salt acts on intermediates formed after the
limiting step (pairs II , III), the reaction rate first dras-
tically decreases, but further addition of the salts do
not affect the rate, and dk/d[salt] = 0 (negative special
salt effect). Extrapolation of the second portion of the
curve to [salt] = 0 gives the maximal value of this salt
l mol ) is calculated by Winstein equation (2) [8],
where ks and k are the rate constants of the reaction in
the presence of a salt and without it.
1
For communication XXXIX, see [1].
1070-3632/05/7506-0883 2005 Pleiades Publishing, Inc.

884
PONOMAREV et al.
effect (kmin), and the ratio kmin : k indicates what part
of the associate of the salt with pair III gives the reac-
tion product. The parameter b reflecting the associa-
tion constant of the salt with pair III is calculated
from the first portion of the curve by Eq. (2). At a
normal salt effect, a salt catalyzes transformation of
a covalent substrate into pair I, and under the condi-
tions of a special salt effect a salt facilitates the ion
separation in pair I; the negative special salt effect is
associated with the catalysis of the return of the ex-
ternal pair (III II I RX) [6, 7].
The reaction rate was monitored spectrophotomet-
rically by the decrease in the ZVd. concentration; it is
satisfactorily described by first-order kinetic equation
(4) and is independent of the concentration and struc-
ture of the verdazyl indicator.
d[ZVd ]/2dt = k[(IV)].
(4)
We showed that additions of LiOPic do not affect
the reaction rate, perchlorates exert a normal salt ef-
fect, in the presence of bromides there is a superposi-
tion of the normal and special salt effects, and in the
presence of chlorides the k [salt] plot passes through
a maximum, suggesting a superposition of the positive
and negative salt effects. In all the cases, the acting
salt agent is the anion.
At a superposition of the positive and negative salt
effects, the k [salt] plot may pass through a maxi-
mum. This was the case in heterolysis of PhCHClMe
in -butyrolactone in the presence of Bu4NBr [9], of
Ph2CCl2 in MeCN in the presence of LiBr [10], and
of 7 -bromocholesterol benzoate in o-xylene in the
presence of N-cetylpyridinium bromide [11].
With perchlorates, the value of the normal salt
effect (b 16) is independent of particular verdazyl
indicator, and with bromides the values of the salt
effects (kext : k = 1.4, b1 400, b2 50) are independent
of the substituent in the C-phenyl ring of verdazyl.
With MeOVd., only the normal salt effect is observed
(b 60). In the presence of chlorides, the extent to
which the reaction rate increases in the first portion of
the k [salt] curve and the rate constant in the region
of the maximum grow with an increase in the elec-
tron-donor power of the verdazyl indicator in the seri-
es NO2Vd. ClVd. HVd., and the extent to which the
reaction rate decreases after passing through the max-
imum grows in the opposite order. Hence, the value of
the negative salt effect grows with an increase in the
electron-acceptor power of ZVd. With MeOVd. in the
presence of chlorides, only the normal salt effect is
observed (b 60).
The site of the effect of the salt anion on the heter-
olysis reaction coordinate is determined by the prin-
ciple of hard and soft acids and bases [12]. The harder
the anion, the more readily is interacts with a harder
electrophile in scheme (1). In particular, in heterolysis
of 1-AdOTs and 1-AdOPic (OPic is picrate) in aprotic
solvents in the presence of ammonium perchlorate,
bromide, and chloride, the ClO4 anion acts on the
covalent substrate (normal salt effect); Br , on pair I
(special salt effect); and Cl , on pair III (negative
special salt effect) [13].
In the previous studies of this series [14, 15], we
examined by the verdazyl method [16] the effect of
various salts on the rate of heterolysis of 3-bromo-
cyclohexene IV in PhNO2. As internal triphenylverda-
zyl indicators ZVd. we used 1,3,5-triphenylverdazyl
(HVd.), 1,5-diphenyl-3-(4-chlorophenyl)verdazyl
(ClVd.), 1,5-diphenyl-3-(4-nitrophenyl)verdazyl
(NO2Vd.), and 1-phenyl-3,5-di(4-methoxyphenyl)ver-
dazyl (MeOVd.), which rapidly and quantitatively
react with pair III of the substrate to form cyclohexa-
1,3-diene, verdazylium salt ZVd+Br , and leucoverda-
zyl ZVdH. The reaction follows Eq. (3).
The specific behavior of chlorides and bromides
with the MeOVd. indicator is accounted for by forma-
tion in heterolysis of bromide IV of two pairs I differ-
ing in the activity [16 18].
Data obtained in PhNO2 [14, 15] show that a salt
can interact simultaneously with both the covalent
substrate and various cationoid intermediates; the
intensity of this interaction is determined by the prin-
ciple of hard and soft acids and bases [12]. The poly-
functonal effect of a salt is revealed in some cases
when studying the normal salt effect. In particular, in
heterolysis of Ph2CHBr in acetone [19] and MeCN
[20] and also of Ph2CHCl in -butyrolactone [1] in the
presence of LiClO4, the normal salt effect grows with
an increase in the electron-acceptor power of the ver-
dazyl indicator in the series MeOVd. < HVd. <
NO2Vd.. This means that actually we deal with a
superposition of the positive and negative salt effects,
with the latter decreasing with an increase of the elec-
C6H4Z-p
Br
N
N
N
N
+ 2
Ph
C6H4Y-p
ZVd
C6H4Z-p
C6H4Z-p
NH
(3)
N
N
N
N
N
+
N+ Br
N
C6H4Y-p
C6H4Y-p
Ph
Ph
ZVd+X
ZVdH
ZVd , Z = Y = H (HVd ); Z = Cl, Y = H (ClVd ); Z =
NO2, Y = H (NO2Vd ); Z = Y = OMe (MeOVd ).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 6 2005

KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XL.
tronegativity of the verdazyl. In these cases, the acting
885
10
agent was the salt anion, as in the heterolysis of IV in
PhNO2. However, in the heterolysis of Ph2CHBr [21]
and 1-AdI [22] in -butyrolactone in the presence of
LiClO4, the acting agent of the salt is its ion pair or
the Li+ cation, and the negative salt effect is enhanced
with an increase in the electron-donor power of ZVd..
106,
1
I
II
k
1
9
s
III
IV
V
8
7
2
VI
VII
VIII
IX
X
XI
XII
The effect of salts on the rate of the heterolysis of
IV in MeCN [23] strongly differs from that observed
in PhNO2. Additions of LiClO4, LiBr, Et4NBr, and
Et4NCl do not affect the rate, and in the presence of
iodides (Et4NI, N-butylquinolinium iodide, KI) only
the normal salt effect is observed, which is indepen-
dent of the salt cation and substituent in the phenyl
ring of the verdazyl. Here the acting agent is iodide
ion, which nucleophilically assists the heterolysis.
6
3
5
4
3
2
4
5
1
0
Proceeding with studies of salt effects in the heter-
olysis of bromide IV, we examined by the verdazyl
method [16] the effect of LiOPic, LiClO4, Et4NClO4,
LiBr, Et4NBr, LiCl, Et4NCl, and KCNS additions on
the rate of the heterolysis of this substrate in -butyro-
lactone at 31 C. As internal indicators we used HVd.,
MeOVd., and NO2Vd.. The reaction follows stoichio-
metric equation (3). The reaction rate in the presence
of salts and without them is satisfactorily described by
kinetic equation (4).
6
1
2
3
4
5
[MY] 102, M
Fig. 1. Salt effect on the rate of heterolysis of 3-bromo-
cyclohexene in -butyrolactone, 31 C: (1) influence of
the concentration of (I) LiClO and (II) Et NClO , HVd
indicator; (2, III) influence of the LiOPic concentration,
HVd indicator; (3) influence of the concentration of
(IV) LiBr, HVd indicator, and of Et NBr, indicator
(V) HVd , (VI) MeOVd , and (VII) NO Vd ; (4) influence
of the concentrations of (VIII) LiCl and (IX) KCNS,
HVd indicator, and of (X) Et NCl, MeOVd indicator;
.
4
4
4
.
.
6
1
4
Without salt additions, k31 = (7.75 0.20) 10 s .
Additions of LiOPic exert no effect on the reaction
rate (Fig. 1, curve 2); at LiOPic concentrations of
.
.
.
2
.
.
3
6
1
4
(4 30) 10 M, k = (7.95 0.28) 10 s . In the
presence of perchlorates, a superposition of the normal
and special salt effects is observed (Fig. 1, curve 1),
.
(5, XI) influence of the Et NCl concentration, HVd
indicator; and (6, XII) influence of the Et NCl concentra-
tion, NO Vd indicator.
2
4
4
.
6
1
kext = 8.36 10 s , b1 150, b2 3.4. The maximal
value of the special salt effect exceeds the reaction
rate without salt additions by a factor of 1.1 (usually
this ratio is within 1.1 4 [6]). The reaction rate is
independent of the salt cation; the acting agent is the
ClO4 anion.
and at [HVd.] > 5 10 M is in-
6
1
5
5.2 10
s
dependent of the indicator concentration (Fig. 2,
curve 1). At lower HVd. concentrations, the reaction
rate depends on [HVd.], because such concentrations
are insufficient to adequately monitor the reaction rate
(at [HVd.] = 0, the measured reaction rate is zero).
In the presence of Et4NCl, the reaction rate depends
on both the concentration and structure of the verdazyl
indicator; in the presence of NO2Vd. (Fig. 2, curve 3),
the rate is lower than in the presence of MeOVd.
(Fig. 2, curve 2).
Chlorides, bromides, and KCNS decelerate the
reaction (Fig. 1, curves 2 5). The values of these salt
effects are independent of the cation; hence, the acting
agent of these salts is the anion.
The negative salt effect is accounted for by the
action of a salt on pair III or II [6, 7]. In the first case,
the reaction rate should depend on the concentration
and structure of the verdazyl indicator, since ZVd. and
the salt act on the same intermediate; in the second
case, such dependences should be lacking [24].
These data show that a decrease in the reaction
rate in the presence of chlorides and bromides is due
to the action of chlorides on pair III and of bromides
on pair II or another intermediate preceding formation
of pair II. These conclusions are consistent with data
in Fig. 1 showing how the rate of heterolysis of IV
depends on the salt concentration with various verda-
zyl indicators.
Figure 2 shows how the concentration and structure
of the indicator affect the reaction rate in the presence
of 0.025 M Et4NBr or 0.020 M Et4NCl. In the pres-
ence of the bromide, the reaction rate decreases to
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 6 2005

886
PONOMAREV et al.
formed from pair II and subsequently transforms into
1
k
106, s
6
ion pair III separated by two solvent molecules [5].
A quantum-chemical analysis of the separation of ions
in a liquid registers the formation of both these ion
pairs [28].
1
5
4
3
2
Chlorides decrease the reaction rate more strongly
than do bromides (Fig. 1, curves 3 5); Cl acts on a
later intermediate, pair III. This is consistent with the
principle of hard and soft acids and bases [13 15]. In
this case, the reaction rate depends on the concentra-
tion and structure of ZVd. and at [Et4NCl] 0.015 M
is independent of the salt concentration. At these con-
centrations, Cl completely binds ion pair III to form
a ternary anionic associate Cl R+ Solv Br . There-
fore, the rate constants plotted in Fig. 1, curves 3, 5 at
2
3
1
0
dk/d[salt] = 0 and [ZVd.] 1.5 10 M and the rate
4
1
2
3
4
5
constants plotted in Fig. 2, curves 2, 3 at the same
concentrations of verdazyl indicators essentially coin-
cide, which indicates that, at [Et4NCl] 0.015 M, the
verdazyl indicator is completely bound by the ternary
associate Cl R+ Solv Br to form charge-transfer
complex V:
[ZVd ] 104, M
.
Fig. 2. Influence of the ZVd concentration on the rate
of heterolysis of 3-bromocyclohexene in -butyrolactone,
.
.
31 C: (1) HVd , 0.025 M Et NBr; (2) MeOVd , 0.020 M
4
.
Et NCl; and (3) NO Vd concentration, 0.020 M Et NCl.
4
2
4
ZVd
The rate of heterolysis of bromide IV at [Et4NBr]
0.003 M (Fig. 1, curve 2) is independent on the salt
concentration and structure of the verdazyl indicator
Cl R+|Solv|Br
V
1
6
1
(b 25.0 l mol , kmin 5.2 10 s , kmin : k = 0.67).
Hence, the bromide anion acts on an intermediate pre-
ceding the formation of pair III and at [Et4NBr]
0.003 M completely binds this intermediate. There-
fore, the minimal rate constant in Fig. 1, curve 2 and
the maximal rate constant in Fig. 2, curve 1 are equal:
The parameters of the negative special salt effect in
the presence of bromides are as follows: with MeOVd.,
1
6
1
b
190 l mol , kmin 3.2 10 s , kmin : k = 0.41;
with HVd., b 120 l mol , kmin 2.0 10 s ,
1
6
1
kmin : k = 0.26; with NO2Vd., b 80 l mol , kmin
1
6
1
6
1
5.2 10 s . Under these conditions, 33% of the
salt intermediate associate transforms back into the
covalent compound. Direct action of Br on pair II
is improbable, since the lifetime of this intermediate is
restricted by the time of translation rotation relaxation
0.6 10 s , kmin : k = 0.08. The above values of b
show that the constants of formation of complexes V
increase with in an increase in the electron-donor
power of ZVd., and the degree of transformation of
these complexes into the covalent substrate increases
in the opposite direction: 59% for MeOVd., 74% for
HVd., and 92% for NO2Vd..
11
of the solvent dipole ( 10 s [25, 26]). This means
that pair II transforms into pair III at a rate of
1
1011 s , whereas the rate of diffusion contact of the
A decrease in the reaction rate in the presence of
chlorides is due to the fact that complex V formed
when Cl and ZVd. act on pair III can transform along
two pathways: into the reaction products and back
into the covalent substrate [6, 7].
salt anion with pair II is considerably lower {5 109
1
l mol 1 s ) [27]}. The negative salt effect in this case
can be accounted for by the action of salt M+Y on
pair I with the formation of a ternary anionic complex
Y R+X , which, upon interaction with a solvent cavity,
transforms into a spacially separated ternary associate
Y R+ X , which, in turn, can transform back into the
covalent substrate with the formation of the nucleo-
fuge exchange product RY (initial substrate in our
case).
Formation of the reaction products is a multistep
process in which the limiting step is either formation
of complex V or its decomposition into ZVd+Br and
alkyl radical which rapidly reacts with a second verda-
zyl molecule to form the alkylation product Vd R,
decomposing by an E1 reaction into the the olefin and
leucoverdazyl. If formation of V is the limiting step,
the reaction rate should grow with an increase in the
electron-donor power of ZVd., and if the decomposi-
A more probable explanation of the negative salt
effect of the bromide is the action of the salt on an ion
pair separated by one solvent molecule, which is
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 6 2005

KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XL.
887
tion of V is the limiting step, the trend will be oppo-
site.
Effect of aprotic solvent on the kinetic parameters of the
heterolysis of 3-bromocyclohexene and Ph2CHBr
The second reaction pathway (return) is due to
strong delocalization of the carbocation charge in V
over three ligands: Br , Cl , and ZVd. As a result, the
bond with the fourth ligand, a solvent molecule occur-
ring in the interionic space, becomes weaker, and this
molecule readily passes into the cavity arising in the
vicinity of the complex. Pair II thus formed trans-
forms into pair I, which then rapidly collapses to
form the covalent substrate [6, 7]. The rate of the
reaction along this pathway should grow with an in-
crease in the electron-donor power of ZVd.
logk251,
k in s
H ,
kJmol
S ,
G ,
kJmol
Solvent
1
1
1
Jmol 1 K
-Butyro-
lactone
MeCN
5.38
(5.63)a
5.50
(5.60)
7.00
92 (87)
72 (69) 110 (122) 104 (105)
86 (68) 90 (157) 113 (115)
40 (64) 104 (105)
PhNO2
(7.39)
a
In parentheses are data for Ph CHBr.
2
Thus, the rate of formation of reaction products
under the conditions of the negative special salt effect
can grow with an increase in both electron-donor and
electron-acceptor power of ZVd., and the return rate,
only with an increase in the electron-donor power of
the indicator. As a result of a competition between
these two pathways, the dependences of the negative
salt effect on the substituent in the verdazyl indicator
may be diverse, which is actually the case [6, 7, 22].
parameters [30]: MeCN, 37.5, E 22, B 1.9; -butyro-
lactone, 39.0, E 12, B 2.5; PhNO2, 34.5, E 1.0,
B 0.8.
In the heterolysis of IV in MeCN, which is a strong
(compared to nitrobenzene and -butyrolactone) CH
acid (pKa 24 [31]), bromides and chlorides do not
affect the reaction rate, and iodides exert a normal salt
effect due to the nucleophilic assistance to the heterol-
ysis, rendered by I [23]. Presumably, the bromide
and chloride anions are deactivated owing to forma-
In our case, the negative special salt effect becomes
stronger with an increase in the electron-acceptor
power of ZVd. This fact indicates that both reaction
pathways involve formation of complex V as the lim-
iting step and that an increase in the electron-donor
power of verdazyl exerts a stronger effect on the prod-
uct formation than on the return.
tion of an H complex Y
+H CH2CN , and only
the iodide ion, thanks to its large size and high polar-
izability, preserves its activity. However, in the het-
erolysis in MeCN of such secondary substrates as
Ph2CHBr [32] and Ph2CHCl [33], when the nucleo-
philic attack from the rear side is possible, additions
of chlorides, bromides, iodides, and nitrates strongly
decelerate the reaction, with the salt anion being the
acting agent.
In the heterolysis of Ph2CHBr [22] and 1-AdI [21]
in -butyrolactone in the presence of LiClO4, the
negative special salt effect becomes stronger with an
increase in the electron-donor power of ZVd. In these
cases, the acting agent of the salt is the ion pair which,
apparently, forms a cyclic quadrupole with pair III:
To account for the differences observed in the salt
effects in the heterolysis of bromide IV and benz-
hydryl halides, it is necessary to consider the solvation
effects for these substrates. The rate of the heterolysis
of both 3-bromocyclohexene and diphenylbromometh-
ane depends on the polarity and electrophilicity of the
solvent, or on its ionizing power [29, 34]; the solvent
effect is considerably stronger with Ph2CHBr. For 9
protic and 14 aprotic solvents, the following correla-
tion was obtained:
R+|Solv|Br
ClO4|Solv|Li+
Complex V formed when ZVd. acts on this quadru-
pole has also two reaction pathways; the product for-
mation depends on the electron-donor power of ZVd.
to a lesser extent than does the return.
As expected, the salt effects in the heterolysis of IV
strongly depend on the solvent. In going from MeOH
to toluene, the rate of the heterolysis of IV decreases
by 5 orders of magnitude [29]. In dipolar aprotic sol-
vents, various solvates can form, which react at differ-
ent rates and differently interact with salts. We studied
the salt effects in three dipolar aprotic solvents having
similar dielectric permittivity ( ) but strongly differing
in the electrophilicity (E) and nucleophilicity (B)
logkIV = 1.71 + 0.719logkPh CHBr
R 0.989, S 0.24, n 23.2
,
The kinetic parameters of the heterolysis of bro-
mide IV and Ph2CHBr in MeCN, PhNO2, and -buty-
rolactone [14, 15, 22, 35, 36] are listed in the table. In
all the solvents, the rate of the heterolysis of Ph2CHBr
is slightly lower than that of IV, although the activa-
tion enthalpy in heterolysis of Ph2CHBr is always
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 6 2005

888
PONOMAREV et al.
lower than that of IV. The decrease in the heterolysis
rate in going from IV to Ph2CHBr is due to a decrease
in the activation entropy.
In nitrobenzene, which is a weak nucleophile and a
weak electrophile simultaneously, the k [salt] depen-
dence in the heterolysis of IV in the presence of chlo-
rides passes through a maximum [14, 15] reflecting
a superposition of positive and negative salt effects.
However, in the heterolysis of 1-AdOPic [42] and
7 -bromocholesterol [11], when solvation is
weaker than in the heterolysis of IV (as judged from
Low values of S in the heterolysis of Ph2CHBr
are indicative of strong steric hindrance arising in the
solvation of the transition state.
A specific feature of bromide IV is that it is more
accessible than Ph2CHBr to a nucleophilic attack from
the rear side. The most stable conformer of cyclohex-
ene is half-chair VI in which the allyl bonds (pseudo-
axial a and pseudo-equatorial e ) form a torsion angle
of 59 , which makes the carbon atom in these posi-
tions readily accessible to a nucleophilic attack from
the rear side [37]. As a result, bromide IV is readily
solvated from the rear side with an MeCN molecule,
which prevents the nucleophilic assistance of the het-
erolysis by such ions as Br and Cl , and only the
large and readily polarizable iodide anion can over-
come the solvation effect of MeCN. It is known that,
in solvolysis of optically active PhCHClMe, additions
of alkyl cyanides favor the configuration retention,
which is attributed to the blocking effect of RCN
molecules on the nucleophilic attack from the rear
side [38].
1
S , about 50 J mol 1 K ), only a negative special
salt effect is observed in the presence of chlorides.
Thus, a combined consideration of the salt and
solvation effects allows more comprehensive under-
standing of the mechanisms of both effects.
EXPERIMENTAL
3-Bromocyclohexene was prepared and purified as
in [29], and triphenylverdazyls, as in [45]. -Butyro-
lactone was kept for 40 h over anhydrous CaCl2,
filtered, kept for 1 h over P2O5, decanted, distilled in
a vacuum, kept over K2CO3 for 30 h, filtered, and
double-distilled in a vacuum; bp 70 C (8 mm Hg).
Salts were recrystallized from ethanol or acetone and
dried in a vacuum (2 3 mm Hg) at 110 C.
a
The kinetic experiments were performed in a tem-
perature-controlled cell of an SF-26 spectrophotom-
eter. The reaction rate was monitored by a decrease
a
e
e
e
e
a
in the ZVd. concentration (for MeOVd.,
750 nm,
720 nm,
a
max
.
1
4470 l mol 1 cm ; for HVd ,
VI
max
.
1
Among the solvents under consideration, -butyro-
lactone is the most basic. In this solvent, salts exist
in form III, whereas in MeCN they form pair I [39].
Therefore, in -butyrolactone salts readily interact
with pair III of the substrate to form cyclic quadru-
poles. Also, in heterolysis of various substrates in
4360 l mol 1 cm ; and for NO2Vd ,
740 nm,
max
1
4200 l mol 1 cm ). The conversion of IV in the
kinetic runs did not exceed 0.6%; the determination
errors were (3 4)%.
1
Below are the temperature ( C) and k 106 (s ):
26.5, 4.94; 31.0, 7.75; 34.6, 14.1; 40.0, 24.9; 44.0,
40.5.
1
-butyrolactone, S (about 50 J mol 1 K ) is usu-
ally appreciably higher than in other dipolar aprotic
solvents (about 120 J mol 1 K ) [10, 11, 21, 22,
Below are the salt, ZVd., salt concentration (M),
1
.
[ZVd.] 104 (M), and k 106 (s ): LiClO4, HVd ,
1
40 44]. This fact suggests relatively weak solvation
of the transition state, which makes the cationoid
intermediate readily accessible for association with
an ion pair or anion of a salt. Therefore, in -butyro-
lactone, the majority of salts (even perchlorates [20,
21], which, as a rule, accelerate the reaction [6, 22])
exert a negative special salt effect. This was observed
in the heterolysis of Ph2CHBr [19, 40], t-BuCl and
2-chloro-2-methyladamantane [41], 7 -bromochole-
sterol benzoate [11], Ph2CCl2 [10], 1-AdOTs and
1-AdOPic [42], cumyl chloride [43], and 2-bromo-2-
methyladamantane [44]. As in the heterolysis of bro-
mide IV, the negative special salt effect usually be-
comes stronger with an increase in the electron-accep-
tor power of ZVd.; only in the presence of LiClO4 the
trend is opposite.
1
3
1.07 10 , 1.50, 8.22; 7.75 10 , 2.00, 8.56;
1.55 10 , 2.50, 8.75; 2.55 10 , 1.60, 9.21;
2
2
2
2
3.11 10 , 2.5, 9.40; 3.50 10 , 1.50, 9.53.
Et4NClO4, HVd., 1.88 10 , 1.82, 8.43; 3.75 10 ,
3
3
3
2
1.82, 8.51; 7.50 10 , 1.82, 8.63; 1.50 10 , 1.82,
.
2
8.72; 3.00 10 , 1.82, 9.20. LiOPic, HVd , 3.95
3
3
2
.
10 , 1.71, 8.13; 7.90 10 , 1.97, 8.34; 1.58 10 ,
2
1.50, 7.63; 3.16 10 , 1.50, 7.72. LiBr, HVd ,
3
3
1.43 10 , 1.34, 5.65; 2.87 10 , 1.63, 5.44;
3
2
5.75 10 , 1.63, 5.49; 1.15 10 , 1.63, 5.06;
.
2
4
2.30 10 , 1.63, 4.53. Et4NBr, HVd , 9.10 10 ,
3
3
1.31, 6.14; 1.82 10 , 1.43, 5.79; 3.65 10 , 1.55,
5.69; 7.30 10 , 1.85, 5.41; 1.46 10 , 1.35, 5.05;
3
2
.
2
2.92 10 , 1.35, 4.89. Et4NBr, MeOVd , 4.80
4
4
3
10 , 1.47, 6.38; 9.60 10 , 1.47, 6.38; 1.92 10 ,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 6 2005

KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XL.
889
3
3
1.47, 6.26; 3.85 10 , 1.47, 5.46; 7.70 10 , 1.37,
9. Dvorko, G.F. and Cherevach, T.V., Zh. Obshch. Khim.,
2
2
5.33; 1.54 10 , 1.37, 4.91; 1.73 10 , 1.50, 4.98;
1988, vol. 58, no. 6, p. 1371.
2
2
2.01 10 , 3.10, 4.79; 3.08 10 , . 1.37, 5.04;
10. Dvorko, G.F. and Cherevach, T.V., Zh. Obshch. Khim.,
2
3
3.47 10 , 1.60, 4.97. Et4NBr, NO2Vd , 1.92 10 ,
1991, vol. 61, no. 2, p. 415.
3
3
1.89, 5.69; 3.85 10 , 2.10, 5.56; 7.70 10 , 1.70,
11. Ponomarev, N.E. and Dvorko, G.F., Zh. Obshch. Khim.,
2
2
5.33; 1.54 10 , 1.70, 5.45; 1.80 10 , 2.20, 5.06;
1991, vol. 61, no. 2, p. 428.
2
2
3.07 10 , 1.70, 5.26; 3.60 10 , 1.90, 5.10.
Et4NBr, HVd., 2.54 10 , 0.52, 4.21; 2.54 10 ,
2
2
12. Pearson, R., Coord. Chem. Rev., 1990, vol. 100,
2
2
no. 2, p. 403.
0.86, 5.16; 2.54 10 , 1.26, 5.38; 2.54 10 , 1.88,
.
2
13. Dvorko, G.F. and Ponomareva, E.A., Dokl. Akad.
Nauk SSSR, 1990, vol. 315, no. 4, p. 877.
5.50; 2.54 10 , 2.75, 5.52. KCNS, HVd , 2.00
3
3
3
10 , 1.40, 4.95; 3.97 10 , 1.88, 4.28; 7.94 10 ,
2
2
1.88, 3.79; 1.59 10 , 1.88, 3.41; 3.18 10 , 1.88,
14. Ponomarev, N.E., Stambirskii, M.V., and Dvorko, G.F.,
3.19. LiCl, HVd., 2.66 10 , 1.50, 4.88; 5.32 10 ,
Zh. Obshch. Khim., 1998, vol. 68, no. 1, p. 106.
3
3
2
2
1.50, 4.44; 1.06 10 , 1.50, 3.35; 2.13 10 , 1.50,
15. Ponomarev, N.E., Stambirskii, M.V., and Dvorko, G.F.,
.
2
2.80; 4.26 10 , 1.46, 2.84. Et4NCl, HVd , 4.60
Zh. Obshch. Khim., 2002, vol. 72, no. 1, p. 86.
3
3
2
10 , 1.60, 3.85; 9.20 10 , 1.60, 2.93; 1.50 10 ,
16. Dvorko, G.F. and Ponomareva, E.A., Usp. Khim.,
2
2
1.60, 2.22; 1.84 10 , 1.60, 2.29; 2.77 10 , 2.15,
1991, vol. 60, no. 10, p. 2089.
.
2
2.10; 4.55 10 , 2.15, 1.83. Et4NCl, MeOVd ,
1.83 10 , 1.68, 4.99; 3.66 10 , 1.47, 4.74;
7.35 10 , 1.47, 3.86; 1.47 10 , 1.47, 3.32;
3
3
17. Cramer, J.A. and Iwett, J.G., J. Am. Chem. Soc., 1972,
3
2
vol. 94, no. 6, p. 1377.
2
2
18. Panchenko, B.V. and Gershikov, A.G., Teor. Eksp.
Khim., 1987, vol. 23, no. 1, p. 91.
2.30 10 , 1.90, 2.89; 2.95 10 , . 1.47, 2.99;
2
3
4.60 10 , 1.40, 2.64. Et4NCl, NO2Vd , 1.97 10 ,
3
3
2.10, 3.16; 3.94 10 , 1.90, 2.10; 7.87 10 , 1.36,
19. Dvorko, G.F., Golovko, N.N., and Ponomareva, E.A.,
2
2
1.19; 1.57 10 , 1.36, 1.38; 2.28 10 , 1.64, 0.713;
Zh. Obshch. Khim., 1996, vol. 66, no. 9, p. 1506.
2
2
3.15 10 , 1.36, 0.448; 4.57 10 , 1.64, 0.422.
20. Pervishko, T.L., Koshchii, I.V., Leunov, D.I., and
Dvorko, G.F., Dopov. Nats. Akad. Nauk Ukr., 1995,
no. 3, p. 86.
Et4NCl, MeOVd., 2.05 10 , 0.22, 0.540; 2.05
2
2
2
2
10 , 0.47, 1.43; 2.05 10 , 0.96, 2.93; 2.05 10 ,
.
2
2.05, 3.35; 2.05 10 , 4.02, 3.99. Et4NCl, NO2Vd ,
21. Dvorko, G.F., Pervishko, T.L., Leunov, D.I., and Po-
nomareva, E.A., Zh. Org. Khim., 1997, vol. 33, no. 4,
p. 565.
2
2
2.02 10 , 0.31, 0.123; 2.02 10 , 0.60, 0.375;
2.02 10 , 1.24, 0.442; 2.02 10 , 2.50, 0.964;
2.02 10 , 3.10, 1.23.
2
2
2
22. Dvorko, G.F., Golovko, N.N., Pervishko, T.L., and
Ponomareva, E.A., Zh. Obshch. Khim., 1994, vol. 64,
no. 8, p. 1281.
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