Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XXXI. Solvent effect on the rate of 1-methyl-1-chlorocyclopentane heterolysis. Correlation analysis of solvation effects

Article abstract of DOI:10.1023/A:1023357716681

Heterolysis of 1-methyl-1-chlorocyclopentane in protic and aprotic solvents occurs by the E1 mechanism. The reaction rate in aprotic solvents or in a set of protic and aprotic solvents is satisfactorily described by the parameters of the polarity and electrophilicity or ionizing power of the solvents. In protic solvents, the reaction rate grows with increasing polarity or ionizing power of the solvent and decreases with increasing nucleophilicity.

Full text of DOI:10.1023/A:1023357716681

Russian Journal of General Chemistry, Vol. 72, No. 11, 2002, pp. 1797 1804. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 11, 2002,
pp. 1902 1909.
Original Russian Text Copyright
2002 by Dvorko, Koshchii, Prokopets, Ponomareva.
Kinetics and Mechanism of Monomolecular Heterolysis
of Commercial Organohalogen Compounds:
1
XXXI. Solvent Effect on the Rate
of 1-Methyl-1-chlorocyclopentane Heterolysis.
Correlation Analysis of Solvation Effects
G. F. Dvorko, I. V. Koshchii, A. M. Prokopets, and E. A. Ponomareva
Kiev Polytechnic Institute, National Technical University of Ukraine, Kiev, Ukraine
Received July 27, 2000
Abstract Heterolysis of 1-methyl-1-chlorocyclopentane in protic and aprotic solvents occurs by the E1
mechanism. The reaction rate in aprotic solvents or in a set of protic and aprotic solvents is satisfactorily
described by the parameters of the polarity and electrophilicity or ionizing power of the solvents. In protic
solvents, the reaction rate grows with increasing polarity or ionizing power of the solvent and decreases with
increasing nucleophilicity.
In one of the previous papers of this series [2], we
reported data on heterolysis kinetics of 1-bromo-1-
methylcyclopentane I and 10 alcohols and 31 aprotic
solvents. In aprotic solvents, this reaction occurs by
the E1 mechanism, and in protic solvents, by the E1 +
S 1 mechanism. The rate of E1 and S 1 reactions is
creasing polarizability. In a set consisting of equal
numbers of protic and aprotic solvents (10 + 10), the
reaction rate mainly depends on the solvent polarity
and electrophilicity and, to a lesser extent, on the
solvent nucleophilicity; in more nucleophilic solvents,
the reaction is slower.
N
N
described by a first-order kinetic equation (1) and is
determined by ionization of the covalent bond [1 3],
occurring via successive formation of contact, spacial-
ly separated, and solvation-separated ion pairs [4].
Correlations with the parameters of the ionizing
power of solvents, E and Z [5], were unsatisfactory;
T
however, good regression equations were obtained [2]
on adding the polarizability parameter (with a nega-
tive coefficient).
v = k[RX].
(1)
Proceeding with studies of solvation effects in het-
erolysis of 1-halo-1-methylcycloalkanes, we deter-
mined the rate of heterolysis of 1-methyl-1-chloro-
cyclopentane II in 10 alcohols and 26 aprotic solvents
Here, v is the rate; k, rate constant; and [RX], sub-
strate concentration.
RX
R+X
R+
Reaction products.
X
R+ Solv X
(
see table) and performed correlation analysis of the
solvation effects using the Koppel Palm equation (2)
8], with additionally introduced cohesion energy den-
[
2
The correlation analysis of solvation effects in
heterolysis of bromide I showed that, for the whole
set of solvents and separately for aprotic solvents, the
reaction rate is mainly described by the parameters of
the solvent polarity and electrophilicity, with an ap-
preciable influence of cohesion. In protic solvents, the
reaction rate is determined by the parameters of elec-
trophilicity and polarizability, growing with increas-
ing solvent electrophilicity and decreasing with in-
sity (parameter ) [9], and Eq. (3).
n2
1
1
log k = a + a1
+
0
2 + 1 n2 + 2
+
a E + a B + a 2,
(2)
(3)
3
4
5
log k = a0 + a E (Z) + a B + a 2.
1
T
2
3
Here, is the dielectric constant of the solvent; n,
refractive index; E and B are the empirical parameters
2
of electrophilicity and nucleophilicity;
= ( Hm
1
For communication XXX, see [1].
RT)/V reflects the energy of the solvent self-associa-
m
1
070-3632/02/7211-1797$27.00 2002 MAIK Nauka/Interperiodica

1798
DVORKO et al.
Rate of heterolysis of 1-methyl-1-chlorocyclopentane at 25 C and solvent parameters
8
2,
kJ l
k 10 ,
s
Z,
E ,
E,
B,
T
20
No.
Solvent
log k
(20)
n
D
1
a
kJ mol 1 kJ mol
1
kJ mol 1 kJ mol
1
1
b
1
2
3
4
5
6
7
8
9
0
1
2
3
4
CF CH OH
MeOH
AcOH
EtOH
i-BuOH
HexOH
i-PrOH
BuOH
PrOH
Cyclohexanol
t-BuOH
t-AmOH
DMSO
Propylene
carbonate
-Butyro-
lactone
1.87
375
350
331
333
325
320
319
325
328
314
298
296
294
303
249
232
214
217
205
204
203
210
212
196
184
175
188
195
26.5
32.7
6.15 1.3716
1.2907
1.3286
81.2
62.3
61.1
48.5
37.7
39.7
33.6
43.1
44.4
28.9
21.8
22.6
13.4
20.5
1.3
644
941
427
703
531
485
565
552
607
515
460
460
636
736
3
2
c
1840 40
4.74
2.61
1.66
2.81
2.75
2.84
2.82
2.76
2.67
2.89
2.95
2.95
4.33
2.18
b
4.89
5.34
d
454 1
228 1
160 1
143 2
130 1
24.3
1.3614
1.3958
1.4182
1.3773
1.3992
1.3854
1.4674
1.3848
5.64
5.80
5.84
5.89
5.91
6.32
6.38
6.82
6.13
6.57
17.7
12.5
18.3
17.1
20.3
15.0
10.9
123 1
1
1
1
1
1
48.0 0.1
41.6 0.5
15.0 0.6
73.4 0.9
27.0 0.9
5.80 1.3859
48.9
1.4783
1.4189
70.0
39.0
37.5
1
5
24.7 0.1
6.61
290
185
1.4360
1.3416
12.1
2.48
695
1
1
6
7
MeCN
20.0 0.2
17.0 0.2
6.70
6.77
298
269
193
162
21.8
8.8
1.91
0.35
594
418
1,1,2,2-Tetra-
chloroethane
CHCl3
8.20 1.4944
1
1
2
2
2
8
9
0
1
2
4.24 0.03
3.20 0.06
1.86 0.04
1.81 0.07
1.06 0.02
7.37
7.49
7.73
7.74
7.97
264
269
272
278
265
164
172
176
176
175
4.81 1.4459
9.08 1.4246
25.2
34.8
10.4
13.8
11.3
3.3
0.80
12.6
0.17
0.28
1.85
0.80
0.48
341
408
515
477
411
CH Cl2
2
PhCN
PhNO2
1,2-Dichloro-
ethane
1.5282
1.5546
1.4451
23
24
25
26
MeCOEt
Acetone
PhCOMe
Cyclo-
0.852 0.003
0.625 0.003
0.453 0.001
0.442 0.002
8.07
8.20
8.34
8.35
268
275
274
271
173
177
173
171
18.5
20.7
17.4
18.3
1.3785
1.3588
1.5350
1.4510
5.4
8.8
2.9
2.1
2.50
2.68
2.42
2.89
362
393
464
431
hexanone
PhI
o-Dichloro-
benzene
PhBr
2
2
7
8
0.312 0.001
0.145 0.002
8.51
8.84
255
251
159
159
4.62 1.6212
9.80 1.5510
5.4
0.0
0.45
0.33
418
444
29
30
31
32
33
34
35
36
37
38
0.0822 0.0011
0.0759 0.0013
0.0568 0.0064
0.0553 0.0009
0.0551 0.0013
0.0263 0.0010
0.0254 0.0070
0.0175 0.0011
0.0163 0.0020
9.09
9.12
9.25
9.26
9.26
9.58
9.60
9.76
9.79
9.87
253
253
236
256
245
233
226
235
230
218
157
157
145
159
152
142
144
144
140
131
5.40 1.5560
5.62 1.5218
4.34 1.3527
6.02 1.5052
5.64 1.4379
2.34 1.4969
2.28 1.5011
2.57 1.5055
2.27 1.4958
2.02 1.4262
2.1
2.1
0.0
3.8
0.0
5.9
8.8
5.0
5.0
0.0
0.48
0.45
3.35
1.70
0.1
0.69
0.57
0.81
0.81
0.0
423
386
241
392
301
333
350
327
323
281
PhCl
Et O
2
PhCO Et
2
MeCCl3
PhMe
C6H6
o-Xylene
p-Xylene
Cyclohexane 0.0136 0.0008
a
b
c
d
Average of 2 3 determinations.
Data of [6].
In [6], logk
4.33.
In [6, 7], logk
25
5.25.
2
5
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 11 2002

KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XXXI.
1799
(5)
tion; Hm is the molar heat of vaporization; Vm,
log k/k0 = mYCl + lNT + C.
molar volume; and E and Z, solvatochromic param-
T
eters of the ionizing power of solvents, satisfactorily
described by their polarity, polarizability, and electro-
philicity parameters [10].
Here, k and k are the rate constants of the reaction
in the given solvent and in 80% aqueous ethanol, re-
0
spectively; N , solvent nucleophilicity [16]; Y ,
T
Cl
ionizing power of the solvent calculated from data on
solvolysis of 1-AdCl [17] for which m = 1 and l = 0
The reaction progress was monitored using 1,3,5-
triphenylverdazyl III [11] as internal indicator. The
reaction follows stoichiometric equation (4).
(for MeOTs, l = 1); and C, free term.
Takeuchi et al. [6] concluded that solvolysis of II
Ph
occurs with nucleophilic assistance of the solvent,
l 0.16. Similar analysis of the solvation effects by
Eq. (5) with the same set of solvents showed that
t-BuCl is more sensitive to the nucleophilic assistance
of the solvent, l 0.32. The nucleophilic assistance of
the solvent in heterolysis of t-BuCl was shown previ-
ously with Eq. (5) [18 20]. These data are in contra-
Me
N
N
N
N
Me
+
2
Ph
Cl
Ph
II
III
(
4)
Ph
Ph
NH
N
N
N
N
N
N
+
Cl
+
+
diction with the nature of S 1 and E1 reactions which,
N
N
Ph
Ph
Ph
Ph
according to [3], should occur without nucleophilic
assistance of the solvent [4].
IV
V
The verdazyl indicator rapidly and quantitatively
reacts with the solvation-separated ion pair of the
substrate, which allows the reaction to be monitored
spectrophotometrically, by the consumption of III
( max 720 nm). Initially, verdazylium chloride IV
and, probably, the verdazyl alkylation product are
formed; in aprotic solvents, the latter rapidly and
quantitatively decomposes into 1-methyl-1-cyclopen-
tene and leucoverdazyl V. In protic solvents, mono-
molecular solvolysis of the substrate occurs in parallel
with the E1 reaction [12], but this does not affect the
stoichiometric ratios in reaction (4).
Application of Eq. (5) to monomolecular solvolysis
often leads to contradictory conclusions [4]. For ex-
ample, correlations of the solvolysis rates of neophyl
bromide and 1-phenylneopentyl methanesulfonate, for
which the nucleophilic attack from the rear side is
sterically impossible, gave absurd values of l: 1.15
and 1.55, respectively [21, 22]. In some studies,
Eq. (5) led to negative values of l [17, 19, 22, 23],
i.e., the negative effect of nucleophilic solvation was
observed [4, 24]. This effect is due to nucleophilic
solvation of the contact ion pair, an intermediate
formed before the rate-determining stage. In this case,
nonequilibrium solvation of the transition state takes
place, in which the solvation shells of the initial and
transition state have different structures.
In all the solvents, the reaction rate is satisfactorily
described by Eq. (1). The concentration of II in the
kinetic experiment was 0.01 0.5 M, and that of the
4
verdazyl indicator, (1 3) 10 M. The degree of
Equation (5) is of little use for analysis of solvation
substrate conversion in the kinetic experiments was
effects, since the parameters N and Y do not reflect
T
Cl
1
0.001%.
any specific single property of a solvent [25]. The
parameter N depends on both the nucleophilicity and
T
Kinetic data are available for solvolysis of II in
dipolarity (polarity + polarizability) of a solvent, and
YCl, on its electrophilicity, polarity, and polarizability
8
0% aqueous ethanol [13, 14], mixtures EtOH H O
2
[
15], EtOH [6, 7], MeOH, AcOH, and CF CH OH
3 2
[10]. These parameters are strongly correlated.
[6]. The rate constant in EtOH determined by us was
in reasonable agreement with the value reported in
It should also be noted that Eq. (5) is used, as a
[
[
6, 7]. In MeOH, our value is slightly lower than in
6] (see table). In [6], methanol might contain traces
rule, for mixed solvents, which strongly complicates
interpretation of solvation effects, since the structure
of the solvation shells of the initial and transition
states can depend differently on the composition of
the mixed solvent [4].
of water. We purified methanol by refluxing for 12 h
over CaO, followed by addition of magnesium and a
crystal of I , refluxing for 8 h, and fractional distilla-
2
tion. The rate constants obtained in two series of
independent experiments were in good agreement.
The use of multiparameter equations (2) and (3)
leads to more reliable conclusions on the nature of
solvation effects. In particular, it was shown that the
rate of t-BuCl heterolysis in protic solvents decreases
with increasing solvent nucleophilicity, whereas in
aprotic solvents it is independent of this parameter
Takeuchi et al. [6], when analyzing the solvation
effects in solvolysis of II in the above solvents, used
the extended two-parameter Grunwald Winstein equa-
tion (5) [15].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 11 2002

1
800
DVORKO et al.
In protic solvents, the rate of heterolysis of I is
log kII
only 1.7 orders of magnitude higher compared to II.
In this case, the driving force of ionization of the
carbon halogen covalent bond is the electrophilic
solvation of the nucleofuge with the formation of
H complex B. With chloride I, this solvation is more
efficient; therefore, in alcohols the difference between
the chloride and bromide is an order of magnitude
weaker than in dipolar aprotic solvents in which for-
mation of a type A solvate should not be appreciably
dependent on the electron-withdrawing properties of
the halogen.
(
a)
3
(b)
36
37
1
1
0
36
3
8
3
5
3
34
33
1
38
3
3
32
2
3
4
35
2
9
3
0
3
1
31
9
27
25
2
7 26
2
9
3
0
24
2
5
2
8
28
2
6
22
20 23
22
24
21
2
3
8
7
2
1
19
2
9
1
8
20
12
1
6
16
14
18
17
1
7
14
1
0
1
2
1
0
8
15
8
1
5
9
6
9
11
In organochlorine solvents that are CH acids, the
rate of heterolysis of II is 2.3 orders of magnitude
lower than that of bromide I. Apparently, in these
solvents both dipolar solvation and electrophilic sol-
vation of the nucleofuge are manifested.
1
1
1
7
6
6
13
5
3
4
2
4
7
5
4
3
5
3
2
3
In nonpolar and weakly polar aromatic solvents
nos. 27 30 and 34 37), the rate of heterolysis of II
(
1
exceeds that of I by only 1.8 orders of magnitude.
Here, apparently, the type of solvation of the transi-
tion state is quite different: Short-lived complexes of
type C are formed, in which the aromatic molecule
approaches the substrate from the side of the positive
end of the C Hlg dipole, with the benzene ring plane
oriented perpendicular to this dipole [31]. In dipolar
aromatic solvents (nos. 20 and 21), ionization of the
covalent bond is accompanied by formation of a quad-
rupole of type A.
1
2
1
1
00
150 200
250
300
350
Z(E ), kJ mol
400
1
T
Fig. 1. Plots of (a) logk vs. E and (b) log k vs. Z
for solvent numbering, see table).
II
T
II
(
[
1, 10, 26, 27]. A similar pattern is observed in het-
erolysis of 1-bromo-1-methylcyclohexane [28]. Abra-
ham et al. [29, 30] analyzed the solvation effects in
heterolysis of tert-butyl halides using a four-parameter
Kamlet Taft equation similar to Eq. (2). It was shown
that the rate of heterolysis of tert-butyl halides is
independent of the solvent nucleophilicity.
In the first case, we should expect an upfield shift
of the H NMR signals of the substrate -protons, and
in the second case, a downfield shift as compared to
1
CCl in which no complexation occurs. Indeed, it was
4
shown that, in the spectra of t-BuI in benzene, tolu-
ene, and halobenzenes, the proton signals are shifted
upfield, and in PhNO and PhCN, downfield relative
2
Comparison of the rate constants of the heterolysis
of bromide I and chloride II allows some conclusions
on the nature of solvation effects in various groups of
solvents. In dipolar aprotic solvents (nos. 13 16, 20,
to CCl [32].
4
Application of Eq. (2) to heterolysis of chloride II
in the whole set of solvents leads to a satisfactory
two-parameter correlation.
2
1, 23 26, 32; here and hereinafter, the numbering is
the same as in table) and Et O, the rate of heterolysis
2
of bromide I is 2.7 orders of magnitude higher com-
pared to chloride II. In these solvents, formation of a
dipolar complex of type A should be expected in the
transition state.
log k = 11.9 + 7.98f ( ) + 0.0660E; R 0.962, S 0.497.
The polarizability, nucleophilicity, and cohesion
parameters in this case are insignificant: for the five-
parameter correlation, R 0.961.
Cl
+
CH2
+
CH2 C +
CH2
Equation (3) for the whole set of solvents gives
two one-parameter correlations.
R Cl
R Cl HOR
Me
CH2
N C R
+
log k = 20.2 + 0.0454Z; R 0.965, S 0.475, (6)
A
B
X
C
log k = 18.5 + 0.0621E ; R 0.954, S 0.541. (7)
T
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 11 2002

KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XXXI.
1801
The nucleophilicity and cohesion parameters are
insignificant. The dependences logk Z and logk ET
are shown in Fig. 1.
Exclusion of the electrophilicity or cohesion pa-
rameter appreciably deteriorates the correlation; how-
ever, it remains satisfactory.
Exclusion of the three most outlying points (nos. 2,
3, 17) slightly improves all the three correlations:
2
log k = 12.5 + 4.29f ( ) + 0.00486 ; R 0.950, S 0.338,
1
R becomes 0.969, 0.975, and 0.964, respectively.
Exclusion of organochlorine solvents that are CH
acids (nos. 17 19, 22), which considerably improved
the correlation for I [2], had only a slight effect in the
case of II.
log k = 11.9 + 2.54f ( ) + 0.0757E; R 0.951, S 0.334.
Application of Eq. (3) to aprotic solvents leads to
unsatisfactory correlations with Z and E (R 0.916
T
and 0.888). After exclusion of four solvents (nos 17,
1
8, 25, 32 in the first case and 13, 15, 17, 18 in the
log k = 12.0 + 8.26f ( ) + 0.0671E; R 0.973, S 0.449,
log k = 20.5 + 0.0462Z; R 0.976, S 0.414, (8)
second), satisfactory one-parameter correlations were
obtained.
log k = 18.8 + 0.0637E ; R 0.971, S 0.452. (9)
T
log k = 20.2 + 0.0454Z; R 0.959, S 0.335,
The solvent parameters in Eqs. (2) and (3) are
independent variables. For the whole set of solvents,
the correlation between the parameters of Eq. (2) is
low. The highest correlation coefficients were ob-
log k = 17.4 + 0.0540E ; R 0.960, S 0.276.
T
For aprotic solvents, the most correlated parameters
2
of Eq. (2) are f( ) and
(R 0.673). In Eq. (3), the
2
2
tained for the following pairs:
E (R 0.632), f( )
mutual correlation of the parameters is considerably
(R 0.664), and f(n) E (R 0.694). For the other pairs
2
2
stronger:
ET, R 0.851;
Z, R 0.853.
of parameters, this coefficient is within 0.34 0.52.
After exclusion of three or four solvents, the maximal
correlation coefficient between the parameters remains
below 0.7. The parameters of Eq. (3) are more corre-
For 12 protic solvents, Eq. (2) gives a satisfactory
three-parameter correlation
2
2
lated:
ET, R 0.781;
Z, R 0.768. For the other
log k = 4.20 + 13.9f ( )
12.4f (n)
1.78B;
pairs of parameters, R 0.34 0.54.
R 0.978, S 0.314.
Thus, in the whole set of protic and aprotic sol-
vents, the rate of heterolysis of chloride II is satisfac-
torily described by the polarity and electrophilicity
parameters, or by the parameter of the ionizing power
of the solvents.
Exclusion of the nucleophilicity parameter breaks
the correlation (R 0.81), and exclusion of the polariza-
bility parameter noticeably deteriorates it; however,
the correlation remains satisfactory:
Application of Eq. (2) to 26 aprotic solvents results
in unsatisfactory correlation (R 0.924). After exclu-
sion of four solvents (nos. 15, 17, 22, 32), a satisfac-
tory three-parameter correlation was obtained.
log k = 7.25 + 16.4f ( ) 2.14B; R 0.964, S 0.375.
Exclusion of solvent nos. 6 and 9 from the set of
protic solvents considerably improves the correlations:
log k = 12.6 + 7.58f ( ) + 2.56f (n) + 0.0953E;
R 0.949, S 0.377.
log k = 4.35 + 14.7f ( )
13.4f (n)
1.77B;
R 0.990, S 0.237,
The major contribution is made by the polarity and
electrophilicity parameters. Exclusion of the polariza-
bility parameter relatively weakly affects the correla-
tion quality.
log k = 7.68 + 17.5f ( ) 2.15B; R 0.975, S 0.350.
The dependence of the reaction rate in a protic sol-
vent on the solvent polarity and nucleophilicity only
and its independence from the electrophilicity param-
eter are apparently due to the following correlation
between the parameters of these solvents:
log k = 12.1 + 8.05f ( ) + 0.0916E; R 0.947, S 0.368.
After exclusion of four other solvents (nos. 13, 17,
8, 19), another three-parameter correlation was ob-
1
tained.
E = 25.6 + 208f ( )
29.1B; R 0.933, S 7.88.
log k = 12.2 + 5.61f ( ) + 0.0461E + 0.00288 2;
R 0.970, S 0.272.
Application of Eq. (3) to protic solvents leads to
satisfactory two-parameter correlations:
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 11 2002

1
802
DVORKO et al.
protic solvents, the polarity has a positive effect, and
log k = 15.8 + 0.0390Z 0.915B; R 0.975, S 0.351,
the nucleophilicity, a weak negative effect on the reac-
tion rate. The solvent polarizability has a positive
effect in aprotic solvents and a negative effect in
protic solvents.
log k = 10.4 + 0.0376ET 1.09B; R 0.964, S 0.417.
Exclusion of the nucleophilicity parameter de-
creases the correlation coefficient to 0.943 in the first
case and to 0.911 in the second case.
A decrease in the rate of the E1 (or S 1) reaction
N
with increasing solvent nucleophilicity in protic sol-
vents is due to the fact that contact ion pair D formed
in the course of electrophilic solvation of the nucleo-
fuge can transform into cyclic solvate E by nucleo-
philic solvation of the carbocation with the second
solvent molecule [4, 33], which stabilizes the interme-
diate and prevents removal of the nucleofuge by the
SN1 mechanism [24, 34].
The following parameters of Eqs. (2) and (3) for
1
2 protic solvents show the strongest mutual correla-
2
2
2
tion:
ET, R 0.649;
Z, R 0.620; f( ) , R 0.685;
f(n) B, R 0.650; B E , R 0.715; B E, R 0.854; f(n)
T
E, R 0.758; f(n) Z, R 0.735.
Thus, in aprotic solvents, the reaction rate depends
on the polarity, polarizability, and electrophilicity, or
on the ionizing power of a solvent; it decreases as the
nucleophilicity and polarizability increase.
R
O
R+
H
R OH
R
Application of Eq. (2) to a set of equal numbers
8 + 8) of protic (nos. 2, 4, 5 10) and dipolar aprotic
nos. 14, 16, 20, 21, 23 26) solvents shows that the
solvent electrophilicity increases the reaction rate, the
solvent nucleophilicity has an opposite effect, and the
other parameters are insignificant.
R+X HOR
O
(
(
X
H
D
E
Probably, solvate E is a product of blind equilib-
rium, and the reaction occurs via solvate D. Such an
effect of blind equilibrium is observed in nucleo-
philic solvation of the contact ion pair formed in het-
erolysis of 2-bromo-2-methylpentane in DMF [35].
log k = 7.49 + 0.0629E 0.323B; R 0.975, S 0.288.
The major contribution is made by the electrophi-
licity parameter.
A decrease in the reaction rate with increasing
polarizability of a protic solvent is apparently due to
higher stability or easier formation of complex E [4].
log k = 8.16 + 0.0585E; R 0.966, S 0.321. (10)
Application of Eq. (3) to this set of solvents gives
good one-parameter correlations:
Solvation effects in heterolysis of bromide I [2]
and chloride II are essentially different. Equation (10)
shows that the rate of heterolysis of chloride II is
mainly determined by the solvent electrophilicity,
which is due to higher sensitivity of chlorine to elec-
trophilic solvation. This effect compensates a decrease
in the reaction rate in protic solvents due to nucleo-
philic solvation; therefore, for a set consisting of both
protic and aprotic solvents, the correlation with Z and
ET is satisfactory or good [Eqs. (6) (9), (11), (12)].
log k = 20.0 + 0.0438Z; R 0.984, S 0.219, (11)
log k = 18.9 + 0.0627E ; R 0.975, S 0.274. (12)
T
In this set of solvents, the strongest mutual corre-
lation is observed in the parameter pairs f( ) B (R
2
2
0
0
.614), f(n) E (R 0.632),
.737), and
E (R 0.716),
Z (R
2
ET (R 0.793), and the weakest correla-
tion, in the pairs f(n) Z (R 0.061) and f(n) f( ) (R
.086).
It is interesting that the correlations logk Z(E ) in
T
0
aprotic solvents are unsatisfactory for both cyclopen-
tenyl substrates. This indicates that the solvation ef-
fects in these solvents are nonuniform. Indeed, along
with solvates A and C, dipole dipole solvation can
yield linear quadrupoles of structure B, hexupoles,
and various cyclic complexes [26].
Thus, in a set consisting of equal numbers of protic
and aprotic solvents, the reaction rate mainly depends
on the electrophilicity or ionizing power of the sol-
vent, which, in turn, depends mainly on its electrophi-
licity. The nucleophilicity has a weak negative effect
on the reaction rate.
Figure 2 shows the correlations logkII logk and
I
Thus, in the whole set of solvents, separately in
aprotic solvents, and in a set consisting of equal num-
bers of protic and aprotic solvents, the rate of heterol-
ysis of II grows with increasing polarity and electro-
philicity of the solvent (or its ionizing power). In
logkII logkt-BuCl. There are 36 common solvents in
the first case (nos. 2, 4 38) [2] and 25, in the second
case (nos. 1 4, 6 16, 18 22, 24 26, 31) [1]. It is
seen that, in the first case, the points are strongly
scattered owing to different sensitivity of bromide and
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 11 2002

KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XXXI.
1803
nature of the nucleofuge, requires the use in every
case of a reference substrate with the same leaving
group when performing correlation analysis of solva-
tion effects using kinetic parameters of the ionizing
power of solvents (Y) [17, 20].
log kII
1
1
(
a)
3
7
(
b)
3
3
5
1
3
8
1
0
3
2
25 28
0
36
3
34
Another difference in the solvation effects in het-
erolysis of I and II is that, for the bromide, the het-
erolysis rate depends on the solvent polarizability
more strongly; in protic solvents, the polarizability
has a negative effect on the reaction rate, and in some
cases in correlation equations the polarizability param-
eter with a negative coefficient appears instead of
parameter B with a negative coefficient. This may be
due to the fact that, in more polarizable solvents, the
readily polarizable contact ion pair of the bromide
forms solvate E more readily. In heterolysis of chlo-
ride II, the negative effect of the solvent nucleophilic-
ity is more pronounced, since formation of solvate E
in this case depends more strongly on the solvent
nucleophilicity than on its polarizability.
3
3
2
5
33
9
26
2
9
2
4
2
2
7
24
2
2
6
2
2
8
7
21
19
2
1
2
0
2
3
1
9
1
8
2
8
2
0
1
5 16 17
16
4
12
1
1
4
1
1
5
1
1
1
1
3
9
10
9
8
6
1
0
6
5
8
11
1
3
7
6
5
4
4
2
3
2
4
3
EXPERIMENTAL
2
1
1
1
-Methyl-1-chlorocyclopentane was prepared and
2
0
0
2
4
6
9
10
12
14
purified as described in [11] (bp 21 C/14 mm Hg, nD
1.4467). 1,3,5-Triphenylverdazyl was prepared and
purified as described in [35]. Solvents were purified
according to [2]. Kinetic experiments were performed
in a temperature-controlled cell of an SF-26 spectro-
photometer. Calculations by Eqs. (2) and (3) were
performed by the least-squares method using the Spss
program package; confidence level 95%.
log k ( log kt-BuCl)
I
Fig. 2. Correlations (a) logk logk and (b) logk
II
II
I
logk
(for solvent numbering, see table).
t-BuCl
chloride to electrophilic solvation, whereas in the sec-
ond case a satisfactory linear correlation is observed.
The correlation logkII logk for 36 solvents is un-
satisfactory (R 0.944). After exclusion of three points
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