Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXIV. Solvent Effect on the Heterolysis Rate of 1-Chloro-1-methylcyclohexane. Correlation Analysis of Solvation Effects in Heterolysis of 1-Chloro-1-methylcyclohexane and 1-Chloro-1-methylcyclopentane

Article abstract of DOI:10.1023/A:1025112703599

The kinetics of heterolysis of 1-chloro-1-methylcyclohexane in 9 protic and 25 aprotic solvents at 25°C were studied by the verdazyl method. The kinetic equation is v = k[RCl] (El mechanism). The heterolysis rate of 1-chloro-1-methylcyclohexane in protic solvents is two orders of magnitude lower than that of 1-chloro-1-methylcyclopentane, whereas in low-polarity and nonpolar aprotic solvents the rates are close. A correlation analysis was made to reveal the solvation effects in heterolysis of both chlorides in a set of 9 protic and 25 aprotic solvents, and separately in protic and aprotic solvents.

Full text of DOI:10.1023/A:1025112703599

Russian Journal of General Chemistry, Vol. 73, No. 2, 2003, pp. 204 212. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 2, 2003,
pp. 224 232.
Original Russian Text Copyright
2003 by Dvorko, Koshchii, Ponomareva.
Kinetics and Mechanism of Monomolecular Heterolysis
of Commercial Organohalogen Compounds:
XXXIV.1 Solvent Effect on the Heterolysis Rate
of 1-Chloro-1-methylcyclohexane.
Correlation Analysis of Solvation Effects in Heterolysis
of 1-Chloro-1-methylcyclohexane
and 1-Chloro-1-methylcyclopentane
G. F. Dvorko, I. V. Koshchii, and E. A. Ponomareva
Kievskii Polytekhnicheskii Institut National Technical University of Ukraine, Kiev, Ukraine
Received December 29, 2000
Abstract The kinetics of heterolysis of 1-chloro-1-methylcyclohexane in 9 protic and 25 aprotic solvents
at 25 C were studied by the verdazyl method. The kinetic equation is v = k[RCl] (E1 mechanism). The hetero-
lysis rate of 1-chloro-1-methylcyclohexane in protic solvents is two orders of magnitude lower than that of
1-chloro-1-methylcyclopentane, whereas in low-polarity and nonpolar aprotic solvents the rates are close. A
correlation analysis was made to reveal the solvation effects in heterolysis of both chlorides in a set of 9 protic
and 25 aprotic solvents, and separately in protic and aprotic solvents.
The solvolysis kinetics of 1-chloro-1-methylcyclo-
NO2
hexane I in 80% aqueous ethanol was studied in [2,
6
1
3]; k25 = 2.94 10 s . This reaction is three times
slower than solvolysis of t-BuCl and 125 times slower
than solvolysis of 1-chloro-1-methylcyclopentane II.
Similar rate ratios were found for ethanolysis of these
compounds [4]. The rate of monomolecular heterol-
ysis of a six-membered substrate is always lower than
that of its five-membered analog. For example, in
solvolysis of 1-methylcycloalkyl p-nitrobenzoates in
80% aqueous acetone, the rate ratio is 39 [5], in sol-
volysis of 1-chloro-1-phenylcycloalkanes in the same
solvent it is 369 [6], and in ethanolysis of 1-bromo-
1-methylcycloalkanes it is 104 [1]. The difference in
the solvolysis rates of the cyclopentyl and cyclohexyl
derivatives is usually accounted for by variation of the
internal strain at the sp3 sp2 rehybridization of the
ring carbon atom [2, 4, 5]. The low solvolysis rate of
the six-membered compound is attributed to formation
of a carbocationic intermediate in the form of twist
conformer A [3].
+
(CH2)n
O C
Me
NO2
A
B
n = 0, 1.
CH2Cl2 [8, 9] (in these reactions, formation of the
carbocationic intermediate involves no rehybridiza-
tion) show that variation of the internal strain at rehy-
bridization is not a decisive factor. In the first case,
the six-membered substrate is less active than its five-
membered analog by a factor of 5; in the second case,
it is less active by a factor of 50 in addition of the
ditolylcarbenium ion [9] and by a factor of 37 [8] in
addition of the phenylanisylcarbenium ion. The high
activity of cyclopentyl substrates in these cases is
accounted for by a higher stability of the carbocati-
onic intermediate.
In [1, 10, 11], we studied in detail the solvent
effect on the heterolysis rates of 1-bromo-1-methyl-
cyclopentane and 1-bromo-1-methylcyclohexane. We
found that, in protic solvents, the ratio of the rate
constants of solvolysis of the five- and six-membered
substrates only weakly depends on the ionizing power
However, data on solvolysis of allyl dinitrobenzo-
ates of structure B in 80% aqueous acetone [7] and
on electrophilic addition to methylenecycloalkanes in
1
For communication XXXIII, see [1].
1070-3632/03/7302-0204$25.00 2002 MAIK Nauka/Interperiodica

KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XXXIV.
205
of the solvent, being about 102, whereas in aprotic
solvents this ratio decreases from 102 to about 101 in
going to -butyrolactone (a solvent with a high ioniz-
ing power) to chlorobenzene [1]. Correlation analysis
of the solvation effects showed that this is due to a
higher sensitivity of the five-membered compound to
electrostatic and electrophilic solvation.
reacts with the solvent-separated ion pair of the
substrate, which allows spectrophotometric monitor-
ing of the reaction rate by a decrease in the verdazyl
concentration (
720 nm). The reaction yields
verdazylium salt mIVax and the verdazyl alkylation prod-
uct, which rapidly decomposes into 1-methylcyclo-
hexene and leucoverdazyl V. Equation (1) satisfactori-
ly describes the reaction rate in all the solvents.
The solvent effect on the heterolysis rate of chlo-
ride II is studied more extensively as compared to I
[12, 13]; the rate constants at 25 C have been deter-
mined in 12 protic and 26 aprotic solvents.
The rate constants of heterolysis of chlorides I and
II (for II, data are taken from [13]) in 34 solvents and
of t-BuCl in 22 solvents [17] are listed in the table
together with solvent parameters [15, 18, 19].
Monomolecular heterolysis of tertiary derivatives
of cycloalkanes occurs as a E1 reaction in aprotic sol-
vents and as E1 + SN1 reaction in protic solvents
[1 6, 10 13]. The rate of this reaction is controlled
by ionization of the covalent bond, which occus via
successive formation of contact, loose, and solvent-
separated ion pairs [14].
The correlation analysis of solvation effects was
performed with the Koppel Palm equation [18] (with
2
the additionally included cohesion energy density
[19]) [Eq. (2)] and Eq. (3).
n2
n2 + 2
1
1
log k = a0 + a1
+ a2
2 + 1
R+X
R+
Reaction products.
X
R+ Solv X
2
(2)
(3)
RX
+ a3E + a4B + a5
,
2
log k = a0 + a1ET (Z) + a2B + a3
.
Here, is the dielectric permittivity of the solvent;
n, refractive index; E and B, empirical electrophilicity
and nucleophilicity parameters; ET and Z, solvato-
chromic parameters of the ionizing power of a solvent
In the limiting stage, the contact ion pair interacts
with the solvent cavity [14, 15]. A loose ion pair is
formed, which rapidly transforms into a solvent-
separated ion pair, which, in turn, rapidly transforms
into the reaction products.
[15], reflecting its polarity, polarizability, and electro-
2
philicity [20];
= ( Hm RT)/Vm, parameter charac-
The rate of SN1 and E1 reactions is described by
a first-order kinetic equation
terizing the energy of the solvent self-association;
Hm, molar heat of vaporization; Vm, molar volume;
and a0 a5, empirical coefficients.
v = k[RX].
(1)
The figure illustrating the dependences of logkI,
logkII, and logkt-BuCl on Z and the table show that the
heterolysis rates of II and t-BuCl similarly vary de-
pending on the ionizing power of the solvent; in all
the solvents, the heterolysis rate of II is approximately
1.5 orders of magnitude higher than that of t-BuCl.
The heterolysis rate of I depends on Z considerably
less significantly. The heterolysis of t-BuCl is faster
than that of I only in the most polar solvent, MeOH;
in the other alcohols, the rates are similar, and in di-
polar aprotic solvents, the heterolysis of t-BuCl is
slower than that of I; the difference in the heterolysis
rates grows with decreasing Z. In polar solvents, het-
erolysis of II is faster than that of I by approximately
two orders of magnitude, in dipolar aprotic solvents,
by one order of magnitude, and in aprotic low-polarity
(halobenzenes) and nonpolar (alkylbenzenes, cyclo-
hexane) solvents the rates are similar.
In this work, we studied the solvent effect on the
heterolysis rate of chloride I and performed a correla-
tion analysis of solvation effects for chlorides I and
II in the same set of solvents.
The reaction kinetics was monitored by the verda-
zyl method [16], with 1,3,5-triphenylverdazyl III as
indicator. The reaction occurs as follows:
Ph
Me
Me
N
N
N
N
Cl
+ 2
Ph
Ph
I
III
Ph
Ph
NH
N
N+
N
N
N
+
Cl
+
N
N
Comparison of the solvent effects on the heterol-
ysis of I, II, and t-BuCl shows that the effect of sol-
vation on the heterolysis rate of I is less pronounced
as compared to II and t-BuCl.
Ph
Ph
Ph
Ph
IV
V
The verdazyl indicator rapidly and quantitatively
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 2 2003

206
DVORKO et al.
Heterolysis rates of 1-chloro-1-methylcyclohexane I, 1-chloro-1-methylcyclopentane II, and t-BuCl at 25 C in various
solvents, in relation to solvent parameters
2
ET,
E,
B,
,
Z,
kI 1010,a
No.
Solvent
logkI logkII logkt-BuCl
(20)
n2D0
1
s
1
2
3
4
5
6
MeOH
EtOH
BuOH
HexOH
PrOH
Cyclo-
hexanol
i-PrOH
t-BuOH
t-AmOH
2110 10
533 8
261 4
253 2
223 10
181 1
6.68
7.27
7.58
7.60
7.65
7.74
4.74
5.34
5.89
5.80
5.91
6.32
6.07
7.06
7.28
7.45
7.33
8.07´
32.7
1.3286 350 232 62.3
1.3614 333 217 48.5
1.3992 325 210 43.1
1.4182 320 204 39.7
1.3854 328 212 44.4
1.4674 314 196 28.9
2.61
2.81
2.76
2.84
2.67
2.89
941
703
552
485
607
515
24.3
17.1
12.5
20.3
15.0
7
8
9
161 1
97.8 0.2
28.8 0.2
7.79
8.01
8.54
7.57
5.84
6.38
6.82
6.77
7.83
8.39
8.77
18.3
10.9
1.3773 319 203 33.6
1.3848 298 184 21.8
2.82
2.95
2.95
0.35
565
460
460
418
5.80 1.3859 296 175 22.6
8.20 1.4944 269 162 8.8
10 1,1,2,2-Tetra- 296 1
chloroethane
11 DMSO
12 Propylene
carbonate
244 1
202 2
7.61
7.69
6.13
6.57
7.88
8.31
48.9
70.0
1.4783 294 188 13.4
1.4189 303 195 20.5
4.33
2.18
636
736
13 CHCl3
14 CH2Cl2
15 MeCN
16 -Butyro-
lactone
150 1
117 6
105 1
7.82
7.93
7.98
8.22
7.37
7.49
6.70
6.61
9.72
9.54
8.64
8.58
4.81 1.4459 264 164 13.8
9.08 1.4246 269 172 11.3
0.17
0.28
1.91
2.48
341
408
594
695
37.5
1.3416 298 193 21.8
1.4360 290 185 12.1
59.7 0.3
39.0
17 PhNO2
18 MeCOEt
19 1,2-Dichloro- 18.0 0.1
ethane
20 Cyclo-
hexanone
25.0 1
23.6 0.4
8.60
8.63
8.74
7.74
8.07
7.97
9.86
10.64
9.61
34.8
18.5
10.4
1.5546 278 176
1.3785 268 173
1.4451 265 175 12.6
0.80
5.4
0.80
2.50
0.48
477
362
411
10.0 0.2
9.00
8.35
18.3
1.4510 271 171
2.1
2.89
431
21 Acetone
22 PhCN
23 PhCl
7.82 0.01 9.11
7.26 0.16 9.14
7.30 0.03 9.14
6.52 0.27 9.19
5.95 0.02 9.23
8.20
7.73
9.12
8.51
8.34
8.84
9.61
10.00
20.7
25.2
1.3588 275 177
1.5282 272 176
8.8
3.3
2.1
5.4
2.9
0.0
2.68
1.85
0.45
0.45
2.42
0.33
393
515
386
418
464
444
5.62 1.5218 253 157
4.62 1.6212 255 159
17.4
24 PhI
25 PhCOMe
26 o-Dichloro-
benzene
10.13
1.5350 274 173
5.36 0.2
9.27
9.80 1.5510 251 159
27 PhBr
28 C6H6
29 PhMe
30 PhCO2Et
31 p-Xylene
32 Et2O
4.51 0.07 9.35
2.66 0.07 9.58
2.50 0.30 9.60
2.00 0.07 9.70
1.92 0.24 9.72
1.87 0.02 9.73
1.53 0.07 9.82
9.09
9.60
9.58
9.26
9.79
9.25
9.76
9.87
5.40 1.5560 253 157
2.28 1.5011 226 144
2.34 1.4969 233 142
6.02 1.5052 256 159
2.27 1.4958 230 140
4.34 1.3527 236 145
2.57 1.5055 235 144
2.02 1.4262 218 131
2.1
8.8
5.9
3.8
5.0
0.0
5.0
0.0
0.48
0.57
0.69
1.70
0.81
3.35
0.81
0.0
423
350
333
392
323
241
327
281
12.70
33 o-Xylene
34 Cyclohexane 1.34 0.02 9.88
a
b
Average of 2 3 determinations.
Data of [17].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 2 2003

KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XXXIV.
207
13
32
12
(a)
11
(b)
(c)
22 25
31
33
17
21
32
20
30
22
10
9
25
34
21
28
19 26
2017
15
32
27
24
9
12
4
26
6
16
21 8
7
28
8
25
3
2
(a)
11
15
22 17
9
7
6
3
1
3
16
5
2
12
8
6
(c)
11
7
4
1
5
4
(b)
200
225
250
275
300
325
350
Z, kJ/mol
Effect of the solvent ionizing power Z on (a) logk , (b) logk , and (c) logk
. The solvent numbering is the same as
I
II
t-BuCl
in the table.
The correlation analysis of solvation effects in het-
erolysis of II using Eqs. (2) and (3) showed [13] that,
in protic solvents (n = 12), the effects of the solvent
polarity and ionizing power on the reaction rate are
positive, and the effect of the nucleophilicity is nega-
tive. In aprotic solvents (n = 26), the effects of the
solvent polarity, electrophilicity, and ionizing power
are positive, and the effect of nucleophilicity is insig-
nificant. However, within a narrower range of fairly
nucleophilic aprotic solvents (ketones, nitriles, etc.),
the negative effect of nucleophilic solvation can be
revealed [21]. A similar pattern of solvation effects is
observed in heterolysis of t-BuCl [17].
log kI = (4.13 0.23) + (0.580 0.030)log kII; R 0.960,
S 0.253, F 378 (250), n 34.
Here, F is the actual and critical (in parentheses)
Fisher test for the confidence level of 95%; since the
actual Fisher test exceeds the critical value, the corre-
lation is reliable.
After excluding four polychlorinated organic sol-
vents (nos. 10, 13, 14, 19; here and hereinafter, the
solvent numbering is the same as in the table), which
are not commonly used in correlation analysis of sol-
vation effects [22], the correlation becomes noticeably
better:
The goal of this study was to elucidate the differ-
ences between the solvation effects in heterolysis of I
and II; to this end, we performed a correlation anal-
ysis for both chlorides in the same set of solvents.
log kI = (4.26 0.18) + (0.570 0.023)log kII; R 0.978,
S 0.195, F 602 (249), n 30.
The values of logkI and logkII in 34 solvents show
a satisfactory mutual correlation:
It is seen that, in this set of solvents, the logarithm
of the heterolysis rate constant of the five-membered
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 2 2003

208
DVORKO et al.
substrate is by a factor of 1.8 more sensitive to the
solvent effect as compared to the six-membered sub-
strate.
Application of Eq. (2) to 30 solvents gives satisfac-
tory five-parameter correlations:
log kI = (11.2 0.6) + (2.71 0.98)f( ) + (0.468 1.88)f(n)
+ (0.0285 0.0050)E + (0.0772 0.0730)B
+ (0.00149 0.00100) 2; R 0.963, S 0.269,
F 61.8 (4.53), n 30.
The correlation between log kII and log kt-BuCl in 22
solvents (see table) is also satisfactory, but the Fisher
test shows that the dependence is not sufficiently
reliable.
log kII
=
(0.590 0.461) + (0.720 0.052)log kt-BuCl
;
log kII = (12.5 0.9) + (5.25 1.43)f( ) + (1.98 2.76)f(n)
+ (0.0491 0.0080)E + (0.262 0.107)B
+ (0.00177 0.00100) 2; R 0.973, S 0.394,
F 86.2 (4.53), n 30.
R 0.952, S 0.359, F 192 (240), n 22.
The correlation between logkI and logkt-BuCl is bad
(R 0.889). A satisfactory correlation is obtained only
after exclusion of the five most outlying points (nos.
13, 14, 20, 21, 25).
Here, f( ) = (
1)/(2 + 1) and f(n) = (n2 1)
1
(n2 + 2) .
log kI
=
(4.36 0.307)
+
(0.435 0.036)log kt-BuCl;
The effects of the polarizability, nucleophilicity,
and cohesion parameters are insignificant; their exclu-
sion only slightly affects the regression coefficients.
R 0.953, S 0.230, F 148 (246), n 17.
Thus, the logarithm of the heterolysis rate constant
of t-BuCl is more sensitive to the solvent effects by a
factor of 1.4 as compared to II and by a factor of 2.3
as compared to I.
log kI
+ (0.0364 0.0030)E; R 0.954, S 0.282, F 138 (19.5), n 30.
log kII (12.0 0.4) + (8.30 1.00)f( )
=
(11.0 0.2) + (4.36 0.64)f( )
Application of Eqs. (2) and (3) to 34 solvents gives
similar dependences. After exclusion of four polychlo-
rinated organic solvents, Eq. (3) gives the following
correlations:
=
+ (0.0597 0.0050)E; R 0.964, S 0.429, F 179 (19.5), n 30.
Thus, the heterolysis rate of I and II in a set of 9
protic and 21 aprotic solvents depends on the solvent
ionizing power or (which actually is the same [20]) on
the solvent polarity and electrophilicity; the logarithm
of the heterolysis rate constant of II, as compared to I,
is more sensitive to the polarity parameter by a factor
of 1.9 and to the electrophilicity parameter by a factor
of 1.6.
log kI
=
(14.6 0.6) + (0.0191 0.0030)Z
2
+ (0.0242 0.0580)B + (0.00134 0.00100)
R 0.966, S 0.250, F 120 (8.64), n 30.
;
log kII
+ (0.148 0.072)B + (0.00143 0.00100)
R 0.982, S 0.312, F 234 (8.64), n 30.
=
(18.2 0.7) + (0.0343 0.0040)Z
2
;
In 25 aprotic solvents, the correlation between
logkI and logkII is approximate. After exclusion of
polychlorinated organic solvents, the following corre-
lation is obtained:
The errors for the coefficients at the solvent nu-
cleophilicity and cohesion parameters show that the
effect of these parameters is insignificant. Indeed,
exclusion of these parameters makes the correlations
only slightly worse:
log kI = (4.250 0.327) + (0.570 0.038)log kII; R 0.959,
S 0.200, F 220 (249), n 21.
log kI = (15.4 0.4) + (0.0243 0.0010)Z; R 0.958,
S 0.265, F 315 (249), n 30.
Aprotic solvents affect the logarithm of the het-
erolysis rate constant of II by a factor of 1.75 more
strongly as compared to I.
log kII
=
(19.5 0.5) + (0.0426 0.0020)Z;
R 0.978, S 0.335, F 602 (249), n 30.
Application of Eqs. (2) and (3) to correlation anal-
ysis of solvation effects in 25 aprotic solvents leads to
unsatisfactory results. After exclusion of polychlori-
nated organic solvents, Eq. (3) gives the following
correlations:
The logarithm of the heterolysis rate constant of II
in a set of 9 protic and 21 aprotic solvents is by a
factor of 1.8 more sensitive to the solvent ionizing
power as compared to heterolysis of I.
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XXXIV.
209
log kI
=
(13.5 1.2) + (0.0128 0.0070)Z
log kI
R 0.907, S 0.298, F 88.3 (246), n 21.
log kII (20.0 0.9) + (0.0444 0.0030)Z;
R 0.950, S 0.374, F 175 (246), n 21.
log kI (14.6 0.6) + (0.0335 0.0040)ET;
R 0.905, S 0.301, F 86.2 (246), n 21.
log kII (18.1 0.7) + (0.0592 0.0040)ET;
R 0.950, S 0.372, F 177 (246), n 21.
=
(15.6 0.7) + (0.0252 0.0030)Z;
2
+ (0.0475 0.0710)B + (0.00235 0.00100)
;
R 0.928, S 0.279, F 35.4 (8.64), n 21.
=
log kII
=
(16.0 1.4) + (0.0215 0.0070)Z
2
+ (0.173 0.077)B + (0.00384 0.00100)
R 0.971, S 0.302, F 93.1 (8.64), n 21.
;
=
=
Similar correlations were obtained with another
parameter of the ionizing power:
log kI
=
(13.0 0.9) + (0.0169 0.0080)ET
In these cases, the ionizing power of aprotic sol-
vents affects the logarithm of the heterolysis rate
constant of II by a factor of 1.8 more strongly as
compared to I.
2
+ (0.0479 0.0690)B + (0.00241 0.00100)
R 0.930, S 0.276, F 36.0 (8.64), n 21.
;
log kII
=
(15.2 1.0) + (0.0293 0.0090)ET
Application of Eq. (2) to 21 aprotic solvents gives
five-parameter correlations:
2
+ (0.169 0.073)B + (0.00382 0.00100)
R 0.973, S 0.290, F 101 (8.64), n 21.
;
log kI
=
(10.9 0.7) + (2.16 1.25)f( )
(0.644 2.60)f(n) + (0.0321 0.0220)E
+ (0.0304 0.080)B + (0.00226 0.00100)
R 0.936, S 0.280, F 21.4 (4.60), n 21.
In these cases, the effect of the solvent nucleophi-
licity is insignificant; exclusion of this parameter only
slightly affects the correlation quality.
2
;
log kI
=
(14.0 1.0) + (0.0154 0.0050)Z
log kII
=
(11.9 0.8) + (3.66 1.43)f( )
+ (0.00209 0.00100) 2; R 0.926, S 0.274,
(1.20 2.99)f(n) + (0.0246 0.0250)E
2
F 54.5 (19.4), n 21.
+ (0.155 0.092)B + (0.00461 0.00100)
;
log kII
=
(17.8 1.2) + (0.0309 0.0060)Z
R 0.971, S 0.322, F 49.1 (4.60), n 21.
+ (0.00289 0.00100) 2; R 0.962, S 0.335,
F 112 (19.4), n 21.
The effects of the nucleophilicity and polarizability
parameters are insignificant; exclusion of these para-
meters only slightly affects the correlation quality.
log kI
=
(13.3 0.8) + (0.0201 0.0070)ET
log kI
=
(11.1 0.3) + (2.64 0.92)f( )
+ (0.00217 0.00100) 2; R 0.927, S 0.272,
2
+ (0.0391 0.0150)E + (0.00188 0.00100)
R 0.935, S 0.266, F 39.3 (8.69), n 21.
;
F 55.4 (19.4), n 21.
log kII
=
(16.4 0.9) + (0.0408 0.0080)ET
log kII
=
(12.4 0.3) + (5.50 1.18)f( )
+ (0.00299 0.00100) 2; R 0.965, S 0.324,
2
+ (0.0457 0.019)E + (0.00345 0.00100)
R 0.962, S 0.344, F 70.4 (8.69), n 21.
;
F 120 (19.4), n 21.
In aprotic solvents, polarity, electrophilicity,
and cohesion affect the logarithm of the heterolysis
rate constant of II more strongly as compared to I by
factors of 2.1, 1.2, and 1.8, respectively.
The effect of the ionizing power and cohesion of
aprotic solvents on the logarithm of the heterolysis
rate constant of II, compared to I, is stronger by fac-
tors of 2 and 1.4, respectively.
In nine protic solvents, the correlation of logkI
with logkII is satisfactory.
The effect of the cohesion of aprotic solvents is
relatively weak; exclusion of this parameter only
slightly affects the correlation quality, but the result-
ing one-parameter correlations are insufficiently reli-
able according to the Fisher test.
log kI
=
(2.89 0.47) + (0.808 0.080)log kII;
R 0.967, S 0.137, F 102 (237), n 9.
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210
DVORKO et al.
The effect of protic solvents on the heterolysis rate
constant of II as compared to I is stronger by a factor
of 1.25.
of protic solvents and polarity of aprotic solvents may
be due to weaker conformational effects in forma-
tion of the transition state from II. Indeed, the initial
envelope conformer of II only slightly differs in the
geometry from the nearly planar carbocationic transi-
tion state, whereas the initial chair conformer of I
strongly differs from the carbocationic twist conform-
er [23]. As for the stronger effect of the polarity of pro-
tic solvents on the heterolysis rate of I compared to II,
this is difficult to explain. Presumably, the electrostatic
solvation in these solvents strongly shifts the con-
formational equilibrium toward the twist conformer.
Application of Eq. (3) to nine protic solvents gives
satisfactory three-parameter correlations:
log kI = (23.3 6.5) + (0.0340 0.0110)Z + (1.60 1.28)B
+ (0.000578 0.00100) 2; R 0.964,
S 0.171, F 21.7 (9.01), n 9.
log kII
=
(23.3 7.4) + (0.0368 0.0130)Z
2
+ (1.78 1.43)B + (0.00107 0.00100)
;
In protic solvents, the heterolysis rate of both sub-
strates is independent of the solvent nucleophilicity.
This is due to the fact that as protic solvents we used
aliphatic alcohols only; their nucleophilicity (param-
R 0.968, S 0.192, F 24.8 (9.01), n 9.
In protic solvents, the effects of cohesion and nu-
cleophilicity are insignificant. Their exclusion only
slightly affects the correlation coefficients, but the
correlations become insufficiently reliable.
1
eter B) varies in a narrow range, 2.61 2.95 kJ mol .
In [13], along with aliphatic alcohols, we also consid-
1
ered AcOH (B 1.66 kJ mol ) and CF3CH2OH (B
1
1.3 kJ mol ) and revealed a decrease in the reaction
rate with increasing nucleophilicity of the solvent:
log kI
R 0.949, S 0.171, F 62.8 (237), n 9.
log kII (16.9 1.4) + (0.0342 0.0040)Z;
R 0.950, S 0.202, F 64.5 (237), n 9.
log kI (13.4 0.7) (0.0282 0.0030)ET;
R 0.957, S 0.157, F 75.6 (237), n 9.
log kII (12.7 0.8) + (0.0337 0.0040)ET;
=
(16.8 1.2) + (0.0286 0.0040)Z;
log kII
(2.14 0.21)B; R 0.964, S 0.375, F 58.9 (19.3), n 12.
log kII (15.4 3.1) + (0.0381 0.0080)Z
=
(7.25 1.68) + (16.4 3.6)f( )
=
=
=
+
(0.948 0.300)B; R 0.969, S 0.347, F 69.4 (19.3), n 12.
In the set of protic solvents considered here, B
approximately correlates with both the electrophilicity
parameter and the parameter of the solvent ionizing
power:
=
R 0.955, S 0.191, F 72.9 (237), n 9.
The ionizing power of protic solvents affects the
heterolysis rate constant of II by a factor of 1.2 more
strongly compared to I.
B = (3.12 0.055)
R 0.914, S 0.0506, F 35.5 (237), n 9.
B = (4.84 0.342) (0.00635 0.001)Z;
(0.00814 0.001)E;
Application of Eq. (2) to nine protic solvents gives
satisfactory two-parameter correlations:
R 0.914, S 0.0506, F 35.4 (237), n 9.
log kI
+ (0.0246 0.0070)E; R 0.960, S 0.164, F 35.0 (19.3), n 9.
log kII (9.60 1.48) + (5.29 3.79)f( )
=
(11.6 1.2) + (6.78 3.07)f( )
This correlation explains why the heterolysis rate
of both substrates in our set of protic solvents depends
only on Z (or ET) or on f( ) and E.
=
The negative effect of nucleophilic solvation is
more clearly revealed in a set consisting of equal
numbers of protic and aprotic solvents [1, 13, 17].
In a set of seven protic (nos. 1 3, 5 7, 9) and seven
aprotic solvents (nos. 12, 15 17, 20 22), we obtain
the following correlation:
+ (0.0349 0.0080)E; R 0.957, S 0.203, F 32.6 (19.3), n 9.
The polarity of protic solvents affects the heterol-
ysis rate constant of I by a factor of 1.3 more strongly
compared to II, whereas the effect of electrophilicity
is stronger (by a factor of 1.4) in the case of II. Hence,
the stronger effect of the ionizing power of a protic
solvent on the heterolysis rate of II is due to the elec-
trophilic assistance effect exerted by the solvent.
log kI
=
(3.75 0.38) + (0.653 0.057)log kII;
R 0.957, S 0.220, F 132 (244), n 14.
The higher sensitivity of II to the electrophilicity
Application of Eq. (3) to these 14 solvents gives
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XXXIV.
211
good three-parameter correlations [in this set of sol-
vents, there is no correlation between B and the other
solvent parameters appearing in Eqs. (2) and (3)].
log kII
=
(8.31 2.73)
(3.34 4.92)f( )
+ (3.45 3.98)f(n) + (0.0458 0.0090)E
(0.0470 0.231)B + (0.00228 0.00100)
R 0.968, S 0.342, F 23.9 (4.82), n 14.
2
;
log kI
=
(15.8 0.6) + (0.0240 0.0030)Z
2
(0.108 0.090)B + (0.00120 0.00010)
R 0.981, S 0.163, F 84.1 (8.81), n 14.
;
The effects of polarity, polarizability, and nu-
cleophilicity are insignificant; exclusion of these pa-
rameters only slightly affects the correlation quality:
log kII
=
(18.3 1.0) + (0.0360 0.0050)Z
2
(0.0135 0.137)B + (0.00133 0.00100)
R 0.979, S 0.248, F 77.6 (8.81) n 14.
;
log kI
=
(9.79 0.30) + (0.00254 0.0050)E
+ (0.00185 0.00100) 2; R 0.954, S 0.238,
F 56.0 (19.4), n 14.
The negative sign of the coefficient at the nucleo-
philicity parameter in these equation and the positive
sign of the coefficient at the cohesion parameter are
indicative of the negative effect of nucleophilic solva-
tion [14]. However, the nucleophilicity parameter is
of low significance (see above); its exclusion has no
noticeable effect on the correlation quality.
log kII
=
(8.83 0.40) + (0.0428 0.0060)E
+ (0.00193 0.00100) 2; R 0.962, S 0.317,
F 69.1 (19.4), n 14.
Here, cohesion affects the logarithms of the
heterolysis rate constants of both substrates to approx-
imately equal extent, whereas the effect of electrophi-
licity is by a factor of 1.7 stronger in the case of II.
log kI
=
(15.6 0.6) + (0.0221 0.0030)Z
+ (0.00141 0.00010) 2; R 0.978, S 0.166,
F 120 (19.4), n 14.
log kII
=
(18.3 0.9) + (0.0358 0.0040)Z
+ (0.00136 0.00100) 2; R 0.979, S 0.237,
Thus, variation of the ratio of the heterolysis rates
of II and I in the range from 100 in polar solvents to
1 in nonpolar solvents is due to specific features of
solvation effects in heterolysis of I, namely, to lower
sensitivity of the heterolysis rate of I to electrostatic
and especially electrophilic solvation, which, in turn,
is associated with conformational features. For exam-
ple, the ionizing power of aprotic and protic solvents
affects the logarithm of the heterolysis rate constant
of II, compared to I, more strongly by factors of 1.8
and 1.2, respectively; both the polarity and electro-
philicity of aprotic solvents exert a stronger effect on
II also. At the same time, in protic solvents, polar-
ity affects more strongly the heterolysis of I, and
electrophilicity, the heterolysis of II.
F 128 (19.4), n 14.
Cohesion affects the logarithms of the heterol-
ysis rate constants of both substrates to approximately
equal extent, whereas the effect of the solvent ionizing
power is stronger by a factor of 1.6 in the case of II.
Exclusion of the cohesion parameter makes the cor-
relations appreciably worse; they remain satisfactory
but become insufficiently reliable.
log kI
R 0.957, S 0.220, F 132 (244), n 14.
log kII (19.2 0.9) + (0.0414 0.0030)Z;
=
(16.5 0.7) + (0.0278 0.0020)Z;
=
R 0.970, S 0.269, F 193 (244), n 14.
1-Chloro-1-methylcyclohexane was prepared and
purified according to [2]; bp 42 C (15 mm), nD20
1.4578; published data [2]: bp 83 84 C (100 mm).
The other chemicals and solvents were prepared and
purified as described in [1, 13]. The kinetic experi-
ments were performed in a temperature-controlled cell
of an SF-26 spectrophotometer. The concentration of I
in kinetic runs was 0.01 0.8 M, and that of the verda-
In this case, the solvent ionizing power affects the
heterolysis rate constant of II by a factor of 1.5 more
strongly compared to I.
Application of Eq. (2) to 14 solvents gives satisfac-
tory five-parameter correlations:
4
zyl indicator, (1 2) 10 M. The degree of substrate
log kI = (10.9 2.0) + (2.05 3.69)f( ) + (1.75 2.99)f(n)
conversion in kinetic runs was 0.01 0.0005%. Calcu-
lation with Eqs. (2) and (3) were performed by the
least-squares method using Spss program package;
confidence range 95%.
+ (0.0309 0.0070)E
+ (0.00140 0.00100) 2; R 0.961, S 0.257,
(0.0644 0.173)B
F 19.5 (4.82), n 14.
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212
DVORKO et al.
13. Dvorko, G.F., Koshchii, I.V., Prokopets, A.M., and
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