Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XLII. Effect of aprotic solvent on the rate of 3-methyl-3-chloro-1-butene dehydrochlorination. Correlation analysis of solvation effects

Article abstract of DOI:10.1007/s11176-005-0461-1

The kinetics of 3-methyl-3-chloro-1-butene dehydrochlorination in propylene carbonate, γ-butyrolactone, sulfolane, acetone, MeCN, PhNO₂, PhCN, PhCOMe, MeCOEt, cyclohexanone, o-dichlorobenzene, PhCl, PhBr, 1,2-dichloroethane, dioxane, and AcOEt were studied; v = k[C₅H ₉Cl], E1 mechanism. The reaction rate is satisfactorily described by the parameters of the polarity, electrophilicity, and cohesion of the solvent; the solvent nucleophilicity and polarizability exert no effect on the reaction rate. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11176-005-0461-1

Russian Journal of General Chemistry, Vol. 75, No. 10, 2005, pp. 1517 1523. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 10, 2005,
pp. 1593 1599.
Original Russian Text Copyright
2005 by Ponomarev, Zaliznyi, Dvorko.
Kinetics and Mechanism of Monomolecular Heterolysis
of Commercial Organohalogen Compounds:
XLII.1 Effect of Aprotic Solvent on the Rate
of 3-Methyl-3-chloro-1-butene Dehydrochlorination.
Correlation Analysis of Solvation Effects
N. E. Ponomarev, V. V. Zaliznyi, and G. F. Dvorko
Kiev Polytechnic Institute, National Technical University of Ukraine, Kiev, Ukraine
Received July 13, 2004
Abstract The kinetics of 3-methyl-3-chloro-1-butene dehydrochlorination in propylene carbonate, -bu-
tyrolactone, sulfolane, acetone, MeCN, PhNO2, PhCN, PhCOMe, MeCOEt, cyclohexanone, o-dichlorobenzene,
PhCl, PhBr, 1,2-dichloroethane, dioxane, and AcOEt were studied; v = k[C5H9Cl], E1 mechanism. The reac-
tion rate is satisfactorily described by the parameters of the polarity, electrophilicity, and cohesion of the
solvent; the solvent nucleophilicity and polarizability exert no effect on the reaction rate.
The rate of monomolecular heterolysis (SN1, E1,
solvolysis) strongly depends on the solvent, which is
due to intense solvation of the polar transition state.
The nature of the solvation effects depends on the
substrate structure. As shown for 15 tertiary sub-
strates (t-BuX, 1-AdX, 1-halo-1-methylcycloalkanes,
ArCCl3, PhCMe2Cl, Ph2CCl2, 2-bromo-2-methylada-
mantane), the heterolysis rate increases with an in-
crease in the polarity and electrophilicity of the sol-
vent or in its ionizing power and decreases with an
increase in the solvent nucleophilicity and polarizabil-
ity [2 7]. The heterolysis rate of secondary substrates
is independent of the solvent nucleophilicity and is
satisfactorily described by the parameters of the sol-
vent polarity and electrophilicity, or of its ionizing
power [2, 3]. This was demonstrated by the examples
of Ph2CHBr [8], 1-phenyl-1-chloroethane [9], 7- -
bromocholesterol benzoate [10], and 3-bromocyclo-
hexeane [11]. The solvent cohesion usually increases
the rate of heterolysis of tertiary substrates and exerts
no effect on that of secondary substrates.
In the limiting step, ion pair A interacts with the
solvent cavity (cavities account for 10% of the liquid
volume [12]) to form ion pair B, which rapidly trans-
forms into ion pair C and then, also rapidly, into
reaction products.
The difference between secondary and tertiary sub-
strates is associated with the steric hindrance to nu-
cleophilic solvation of the substrate from the rear side.
In secondary substrates this solvation is sterically
possible, whereas in tertiary substrates it is hindered
or impossible. In heterolysis of tertiary substrates, it is
ion pair A forming before the limiting step that is sub-
ject to nucleophilic solvation. The solvation stabilizes
this intermediate and hinders formation of the transi-
tion state. In heterolysis of secondary substrates, equi-
librium solvation of the transition state is observed,
with the solvation shells of the initial and transition
states having similar structure (one of solvates of the
initial state reaches the transition state [13]). In the
case of tertiary substrates, we can speak of nonequi-
librium solvation, because the solvation shells of the
initial and transition states have different structures
[14].
Monomolecular heterolysis usually occurs via suc-
cessive formation of three ion pairs: contact (A),
spacially separated (B), and solvation-separated (C)
[2, 3].
A kinetic study of the solvolysis of 3-methyl-3-
chloro-1-butene I showed that the solvation effects
observed with this compound in protic solvents differ
essentially from those observed with the previously
studied tertiary substrates [1]. The rate of heterolysis
of I in 13 alcohols (MeOH, EtOH, BuOH, CH2=CH
CH2OH, i-BuOH, i-PrOH, PentOH, HexOH, OctOH,
RX
R+X
R+
X
R+ Solv X
A
B
C
Reaction products.
For communication XLI, see [1].
1
1070-3632/05/7510-1517 2005 Pleiades Publishing, Inc.

1518
PONOMAREV et al.
cyclohexanol, s-BuOH, t-BuOH, t-PentOH) grows
with an increase in the solvent ionizing power ET and
is virtually independent of its nucleophilicity. Such
trends are typical of secondary substrates.
and aprotic solvents. This is due to the fact that the
negative effect of the nucleophilic solvation is mani-
fested in protic solvents considerably more strongly;
therefore, the values of logk in protic solvents are
lower than those estimated from the logk ET correla-
tions for aprotic solvents.
In this study we examined how the rate of heterol-
ysis of I depends on the nature of the aprotic solvent.
It is known [2, 3, 11] that, in cases when the solvent
nucleophilicity exerts no effect on the reaction rate
(secondary substrates), a satisfactory linear correlation
logk ET (k is the reaction rate constant), common for
protic and aprotic solvents, is observed. In cases when
the solvent nucleophilicity exerts a negative effect on
the reaction rate (tertiary substrates), two separate
linear correlations logk ET are observed for protic
We studied the kinetics of heterolysis of I in 17
aprotic solvents. The kinetic experiments were per-
formed by the verdazyl method [15]. As internal indi-
cator we used 1,3,5-triphenylverdazyl (Vd ), which
rapidly and quantitatively reacts with ion pair C of the
substrate to form isoprene, verdazylium salt (Vd+Cl ),
and leucoverdazyl (VdH). The reaction follows the
equation
Ph
Ph
Ph
Me
N
N
N
N
N
N
NH
N
N
N
N
N
+
CH2=CH C=CH2 +
CH2=CHCMe2Cl + 2
Ph
.
+ Cl
Ph
Ph
Ph
Ph
Ph
Vd
Vd+Cl
VdH
The reaction rate was monitored spectrophotometri-
where is the dielectric permittivity of the solvent;
n, refractive index; E and , empirical parameters of
the electrophilicity; B and , empirical parameters of
the nucleophilicity; *, dipolarity (polarity + polariz-
cally by a decrease in the Vd concentration (
max
720 nm). It is satisfactorily described by a first-order
kinetic equation, E1 mechanism.
ability) parameter; ET, solvatochromic parameter of
d[Vd ]
2d
2
the solvent ionizing power;
= ( HM
RT)/VM,
v =
= k[I].
parameter characterizing the energy of interaction of
solvent molecules with each other; HM, molar heat
of vaporization; VM, molar volume; and ai, coeffi-
cients characterizing the contributions of the corre-
sponding parameters. The solvent parameters were
taken from [16, 19, 20].
The substrate concentration in kinetic experiments
was 0.01 0.7 M; concentration of the verdazyl indi-
cator, (0.8 2.4) 10 M; substrate conversion, 0.1
4
0.001%; and verdazyl conversion, 10 30%.
The rate constants at 25 C and the solvent param-
eters required for the correlation analysis of the solva-
tion effects are given in the table. The correlation
analysis was performed using the Koppel Palm equa-
tions based on the linear free energy relationship [16]
with additional inclusion of the cohesion energy den-
The use of Eq. (1) leads to a satisfactory three-
parameter correlation (4):
logk = (12.3 0.7) + (2.83 0.98)f ( )
2
+ (0.0653 0.0064)E + (0.322 0.108)
,
(4)
R 0.960, S 0.52, F 101 (2.60), N 17.
2
sity parameter [17] [Eq. (1)], Kamlet Taft equation
[Eq. (2)] [18], and Eq. (3):
Here f( ) = ( 1)/( + 1); F is the observed and
n2
1
critical (in parentheses) Fisher test [21]; the fact that
the observed Fisher test is higher than the critical
value indicates that the model is reliable.
1
log k = a0 + a1
+ a2
+ a3E
n2 + 1
+ 1
+ a4B + a5 2/100,
logk = a0 + a1 * + a2
+ a3 + a4 2/100, (2)
(1)
Exclusion of the most outlying point (ethyl acetate)
appreciably improves the correlation [Eq. (5)]:
2
n2
1
logk = (11.7 0.6) + (2.52 0.87)f ( )
log k = a0 + a1ET + a2
n2 + 1
++a3B + a4 2/100,
2
+ (0.0660 0.0051)E + (0.267 0.009)
,
(5)
(3)
R 0.972, S 0.41, F 143 (2.71), N 16.
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XLII.
1519
Solvent effect on the rate of heterolysis of 3-methyl-3-chloro-1-butene I and solvent parameters
Solvent
no.
k25 107,
ET,
kJ mol
E,
kJ mol
B,
kJ mol
Solvent
Sulfolane
Propylene carbonate
-Butyrolactone
Acetonitrile
logk25
n2D0
1
1
1
1
s
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
8.78
7.08
3.63
2.75
0.174
0.123
0.11
0.0955
0.0501
0.0457
0.0457
0.0162
0.0129
0.00575
0.00295
0.00229
6.06
6.15
6.44
6.56
7.76
7.91
7.96
8.02
8.30
8.34
8.34
8.79
8.89
9.24
9.53
9.64
184
195
185
191
173
173
170
173
176
173
166
159
153
154
150
156
159
42.1
62.9
41
1.481
1.421
1.437
1.344
1.551
1.528
1.534
1.551
1.359
1.379
1.451
1.551
1.56
10
21
12
21
0
0
0
9.6
8.5
5.4
0
0
0
0
0
0
6.7
1.88
2.18
2.48
1.91
0.8
1.85
2.42
0.48
2.68
2.5
2.89
0.33
0.48
0.45
2.84
3.43
2.17
35.9
36.1
25.2
18.2
10.4
21.4
18.9
16
10.4
5.55
5.74
2.27
7.39
6
Nitrobenzene
Benzonitrile
Acetophenone
1,2-Dichloroethane
Acetone
Methyl ethyl ketone
Cyclohexanone
o-Dichlorobenzene
Bromobenzene
Chlorobenzene
1,4-Dioxane
1.524
1.422
1.408
1.372
Tetrahydrofuran
Ethyl acetate
0.000513 10.29
1
Solvent no.
Solvent
2/100, kJ l
*
, D
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Sulfolane
6.9
7.4
0.98
0.83
0.87
0.75
1.01
0.9
0
0
0
0.19
0
0
0.04
0
0.08
0.06
0
0
0
0
0
0
0
0.39
0.4
0.49
0.4
4.81
4.38
4.12
3.44
4.03
4.05
2.96
1.75
2.64
2.76
3.01
2.77
1.55
1.54
0.45
1.75
1.88
Propylene carbonate
-Butyrolactone
Acetonitrile
Nitrobenzene
Benzonitrile
Acetophenone
1,2-Dichloroethane
Acetone
Methyl ethyl ketone
Cyclohexanone
o-Dichlorobenzene
Bromobenzene
Chlorobenzene
1,4-Dioxane
6.95
5.86
5.11
5.15
4.33
4.12
3.88
3.61
4.08
4.2
4
3.76
4.2
3.61
3.39
0.3
0.37
0.49
0.1
0.9
0.81
0.71
0.67
0.76
0.8
0.79
0.71
0.55
0.58
0.55
0.43
0.48
0.53
0.03
0.06
0.07
0.37
0.55
0.45
Tetrahydrofuran
Ethyl acetate
The effect of the polarizability and nucleophilicity
parameters on the correlation quality is insignificant;
with their inclusion, R 0.979.
Proceeding from Eq. (3), we obtain a satisfactory
two-parameter correlation (7):
logk = (18.7 1.6) + (0.0490 0.0117)ET
2
+ (0.465 0.123)
,
(7)
The use of Eq. (2) leads to a similar result [Eq. (6)]:
R 0.959, S 0.38, F 80 (2.52), N 17.
logk = (13.8 0.5) + (2.98 0.82) *
The correlation quality is considerably improved
after exclusion of AcOEt [Eq. (8)]:
2
+ (6.85 1.81) + (0.672 0.087)
,
(6)
R 0.967, S 0.35, F 62 (2.71), N 17.
logk = (18.4 1.1) + (0.0494 0.0080)ET
2
The nucleophilicity parameter exerts no effect on
+ (0.403 0.086)
,
(8)
the correlation quality: with its inclusion, R 0.968.
R 0.978, S 0.26, F 140 (2.60), N 16.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 10 2005

1520
PONOMAREV et al.
and hereinafter, the solvent numbering is the same as
in the table) are correlated [Eq. (10)]:
logk25
4
19
20
18
logk = (10.5 0.3) + (0.805 0.102) ,
R 0.916, S 0.49, F 63 (2.75), N 14.
(10)
5
22
21
23
The heterolysis rate in ethyl acetate is considerably
lower than that estimated from the logk ET correla-
tion for the other solvents (see figure); therefore,
exclusion of this point substantially improves the
quality of the correlations obtained using Eqs. (1) and
(3). The quantity logk25 for this solvent (see table)
was calculated by extrapolation of the dependence
log(k/T) 1/T (R 0.992) obtained for higher tempera-
tures and is beyond question. The observed deviation
is apparently caused by steric hindrance arising in
solvation of the substrate with solvents containing
acetyl group. Indeed, the points for acetone (no. 9)
and methyl ethyl ketone (no. 10) also lie below the
logk ET straight line (see figure).
26
28
24
25
1
27
6
7
29
2
30
3
4
a
5
6
8
7
8
9
11
12
9
10
13
14
18
16
17
10
c
b
In protic solvents, the contribution of the dipolar
solvation is apparently insignificant. Indeed, in 13
alcohols studied, the rate of solvolysis of chloride I
varies by three orders of magnitude at virtually con-
stant dipole moment of the solvent (1.7 1.8 D). The
major contribution in protic solvents is made by elec-
trophilic solvation via H-bonding of the nucleofuge.
The solvation effects in protic solvents are more uni-
form than in aprotic solvents. This is manifested in
the fact that the variance in the logk ET correlation
for protic solvents is considerably smaller than for
aprotic solvents (see figure).
160
180
200
220
ET, kJ mol
1
Effect of the solvent ionizing power E on the rate of
heterolysis of 3-methyl-3-chloro-1-butene
T
I (25 C).
(1 17) Figures are solvent numbers in the table;
(18) MeOH, (19) CH =CHCH OH, (20) EtOH,
(21) BuOH, (22) i-BuOH, (23) PentOH, (24) i-PrOH,
(25) HexOH, (26) cyclohexanol, (27) OctOH, (28) s-
BuOH, (29) t-BuOH, and (30) t-PentOH; (a c) for
comments, see text.
2
2
In aprotic solvents, it is difficult to reveal the effect
of the solvent nucleophilicity on the reaction rate [3].
Chloride I in aprotic solvents behaves in some cases
as a secondary substrate and in other cases, as a ter-
tiary substrate. Indeed, a satisfactory correlation of the
reaction rate with ET [Eq. (9)] in both protic and
aprotic solvents is characteristic of heterolysis of
secondary substrates [2, 3, 8 11], and the effect of the
cohesion on the reaction rate [Eqs. (5) (8)] is usually
observed in the heterolysis of tertiary substrates [2 7].
After exclusion of AcOEt and sulfolane, we obtain
a satisfactory one-parameter correlation (9):
logk = (20.7 1.1) + (0.0743 0.0066)ET,
R 0.953, S 0.34, F 12.7 (2.62), N 15.
(9)
Thus, the rate of heterolysis of chloride I in aprotic
solvents, as in protic solvents, depends on the polarity
(dipolarity) and electrophilicity of the solvent, or on
its ionizing power. In aprotic solvents, the solvent
cohesion makes a significant contribution. When this
parameter is excluded, the correlation becomes con-
siderably worse or is broken at all.
In some cases, the effect of the solvent nucleo-
philicity on the heterolysis rate can be revealed when
only dipolar aprotic solvents are considered in the
correlation analysis of the solvation effects. For ex-
ample, the activation free energy of heterolysis of
1-methyl-1-chlorocyclopentane in eight solvents
(nos. 3 9, 11) grows with an increase in the solvent
nucleophilicity and polarizability [22]:
In an aprotic medium, the major factor is electro-
static (dipolar) solvation occurring via formation of
linear or cyclic quadrupoles from the substrate and
solvent dipoles. In this case, there should be a strong
correlation between the heterolysis rate and dipole
moment of the solvent ( ). Indeed, we found that
these quantities in 14 solvents (1 3, 5 7, 9 16; here
2
G
= 122000 + 35700f (n) + 1.72B 0.0357
R 0.993, S 0.64, F 94 (6.1), N 8;
, (11)
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XLII.
1521
G
= 176000
0.288ET + 1.03B
0.0216 2, (12)
for the heterolysis of I in protic and aprotic solvents
is shown in the figure. The numbering of the protic
solvents is given in the figure caption. We can dis-
tinguish two separate linear correlations: one for
protic solvents (straight line a) and another one for
aprotic solvents (straight line b), as in the case of ter-
tiary substrates. Straight line c corresponding to the
whole set of data virtually coincides with the straight
line for aprotic solvents, as in the case of secondary
substrates.
R 0.989, S 0.18, F 61 (6.1), N 8.
A decrease in the reaction rate with an increase in
the solvent polarizability is associated with the nega-
tive effect of the nucleophilic solvation, since an in-
crease in the solvent polarizability favors the nucleo-
philic solvation of the contact ion pair.
With logk used instead of G , we obtain for the
same substrate in the same solvents similar correla-
tions (13) and (14) based on Eqs. (1) and (3):
Application of Eqs. (1) (3) to the whole set of the
solvents shows that the nucleophilicity and polariz-
ability exert no effect on the reaction rate [Eqs. (17)
(19)]:
logkC H Cl
=
(8.06 0.50)
+ (0.243 0.055)B (0.564 0.039)
R 0.993, S 0.11, F 97 (8.89), N 8;
(5.37 0.97)f (n)
5
9
2
,
(13)
(14)
logkI
+ (2.83 0.98)f ( ) + (0.322 0.108)
R 0.960, S 0.52, F 101 (1.95), N 30;
=
(12.3 0.7) + (0.0653 0.0064)E
2
logk
=
(7.63 0.67) (6.07 1.42)f (n)
(0.253 0.083)B + (0.535 0.046)
R 0.987, S 0.14, F 52 (8.89), N 8,
where f(n) = (n2 1)/(n2 + 1).
C5H9Cl
,
(17)
(18)
2
,
logkI (11.8 0.6) + (1.12 0.99) *
+ (3.52 0.56) + (0.600 0.114)
R 0.958, S 0.53, F 97 (1.95), N 30;
logkI (2.11 0.80) + (0.0770 0.0041)ET, (19)
=
2
,
A similar pattern is observed in the heterolysis of
cumyl chloride. In a wide range of aprotic solvents
(N 20), the reaction rate is independent of the solvent
nucleophilicity, and in dipolar aprotic solvents (nos.
2 4, 7 9, MeNO2, DMF) the solvent nucleophilicity
decreases the reaction rate. Using Eqs. (1) and (3), we
obtain relationships (15) and (16):
=
R 0.962, S 0.49, F 344 (1.93), N 30.
Thus, in contrast to the previously studied tertiary
substrates, the solvent nucleophilicity exerts no effect
on the rate of the heterolysis of I. This does not mean
that the nucleophilic solvation does not take place in
the heterolysis of this substrate: it occurs always.
There are stoichiometric evidences of the nucleophilic
solvation of the initial state of substrates reacting by
the SN1 or E1 mechanism [25]. In particular, such
solvation causes partial or complete retention of the
configuration in the phenolysis of optically active
substrates [26]. Experiments on the effect of neutral
salts on the rate of heterolysis of 3-bromocyclohexene
in MeCN revealed a strong influence of the nucleo-
philic solvation of the covalent substrate on the mani-
festation of the salt effects [27].
logkPhCMe Cl
=
(15.8 0.4) + (0.077 0.0249)E
2
2
(0.186 0.113)B + (0.285 0.175)
R 0.969, S 0.29, F 20 (8.89), N 8;
,
(15)
logkPhCMe Cl (15.7 3.8) + (0.0476 0.0249)ET
=
2
2
+ (0.282 0.175)
R 0.969, S 0.29, F 20 (8.89), N 8.
(0.186 0.113)B,
(16)
Application of Eqs. (1) and (3) to the correlation
analysis of the heterolysis of chloride I in nine di-
polar aprotic solvents (nos. 1 6, 8, 9, 11) shows that
the solvent nucleophilicity exerts no effect on the re-
action rate.
In the heterolysis of I, as in the case of secondary
substrates, specifically the covalent substrate (initial
state) is subject to the nucleophilic solvation, and the
solvation does not change on reaching the transition
state. Such a pattern is possible only when the solva-
tion is not hindered sterically. Since our substrate is
an allyl derivative, the nucleophilic solvation of the
hydrocarbon radical can occur at positions 1 and 3,
with no steric hindrance to the solvation at position 1
(Solv CH2=CHCMe2Cl); therefore, this tertiary sub-
strate behaves as a secondary substrate.
Thus, the negative effect of the nucleophilic solva-
tion in the heterolysis of chloride I in aprotic solvents
is not revealed. Manifestation of this effect strongly
depends on the solvent set chosen, substrate structure
(including the nature of the nucleofuge), and choice
of the correlation equation [3]. This effect is more
pronounced in protic solvents than in aprotic solvents.
It is frequently revealed by combined consideration of
data for protic and aprotic solvents [3, 8, 23, 24].
We performed a correlation analysis of the solva-
tion effects in 17 aprotic (nos. 1 17) and 13 protic
(nos. 18 30) solvents. The correlation logk = f(ET)
Comparative analysis of solvation effects is usually
performed with tert-butyl halides as reference sub-
strates. However, the rate of their heterolysis depends
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 10 2005

1522
PONOMAREV et al.
on the solvent nucleophilicity, which complicates the
analysis of the solvation effects. With adamantyl sub-
strates [28, 29], the pattern is still more complex,
since these substrates are still less active, and the rate
of their heterolysis depends on the solvent nucleophi-
licity still more strongly [2, 3]. Chloride I is more
suitable as reference substrate. It is by two orders of
magnitude more active than t-BuCl, and the rate of its
heterolysis in protic and aprotic solvents is virtually
independent of the solvent nucleophilicity and is satis-
factorily described by the solvatochromic parameter of
the solvent ionizing power.
8. Dvorko, G.F., Pervishko, T.L., Golovko, N.I., Va-
sil’kevich, A.I., and Ponomareva, E.A., Zh. Org.
Khim., 1993, vol. 29, no. 9, p. 1805.
9. Dvorko, G.F. and Cherevach, T.V., Zh. Obshch. Khim.,
1988, vol. 58, no. 6, p. 1371.
10. Ponomarev, N.E., Yakhimovich, R.I., and Kulik, N.I.,
Zh. Obshch. Khim., 1988, vol. 58, no. 5, p. 1106.
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EXPERIMENTAL
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ment with dry HCl at 20 C; the product was purified
by double distillation under reduced pressure (bp 43
44 C/230 mm Hg); n2D0 1.4192 (published data [30]:
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according to [31].
The aprotic solvents were purified according to
[32], and the alcohols, according to [1]. The kinetic
experiments were performed in a temperature-con-
trolled cell of an SF-26 spectrophotometer. The math-
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