Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XLI. Solvent effect on the rate of 3-methyl-3-chloro-1- butene solvolysis. Correlation analysis of solvation effects and role of solvent nucleophilicity

Article abstract of DOI:10.1007/s11176-005-0441-5

The kinetics of 3-methyl-3-chloro-1-butene solvolysis at 25°C in MeOH, EtOH, BuOH, i-BuOH, PentOH, 2-PrOH, 2-BuOH, HexOH, OctOH, t-BuOH, t-PentOH, cyclohexanol, and allyl alcohol was studied by the verdazyl method; v = k[C ₅H₉Cl], SN1 + E1 mechanism. The reaction rate shows a satisfactory correlation with the parameter of the solvent ionizing power E _T and is independent of the solvent nucleophilicity. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11176-005-0441-5

Russian Journal of General Chemistry, Vol. 75, No. 9, 2005, pp. 1430 1436. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 9, 2005,
pp. 1503 1509.
Original Russian Text Copyright
2005 by Ponomarev, Zaliznyi, Dvorko.
Kinetics and Mechanism of Monomolecular Heterolysis
of Commercial Organohalogen Compounds:
XLI.1 Solvent Effect on the Rate
of 3-Methyl-3-chloro-1-butene Solvolysis. Correlation Analysis
of Solvation Effects and Role of Solvent Nucleophilicity
N. E. Ponomarev, V. V. Zaliznyi, and G. F. Dvorko
Kiev Polytechnic Institute, National Technical University of Ukraine, Kiev, Ukraine
Received April 27, 2004
Abstract The kinetics of 3-methyl-3-chloro-1-butene solvolysis at 25 C in MeOH, EtOH, BuOH, i-BuOH,
PentOH, 2-PrOH, 2-BuOH, HexOH, OctOH, t-BuOH, t-PentOH, cyclohexanol, and allyl alcohol was studied
by the verdazyl method; v = k[C5H9Cl], SN1 + E1 mechanism. The reaction rate shows a satisfactory correla-
tion with the parameter of the solvent ionizing power ET and is independent of the solvent nucleophilicity.
Studies of the solvent effect on the rate of mono-
molecular heterolysis (SN1, E1, solvolysis) are usually
performed with such weakly active reference sub-
strates as t-BuCl and 1-AdCl [2, 3]. In such a polar
because of steric factors; in heterolysis of tert-butyl
halides, weak nucleophilic assistance is assumed,
although the nucleophilic solvation from the rear side
is sterically hindered [3, 7, 8]. A study of the solva-
tion effects in heterolysis of adamantyl and tert-butyl
substrates by the verdazyl method [9] revealed no
nucleophilic assistance of the solvent in both cases
[5]. The nucleophilic solvation of tertiary substrates
occurs in the step of formation of pair A formed be-
fore the limiting step. It stabilizes the intermediate
and complicates removal of the nucleofuge by the SN1
mechanism. The reaction rate decreases, and with
adamantyl substrates this effect is stronger than with
tert-butyl substrates. Since only the relative effect of
the solvent nucleophilicity was determined, it was
erroneously concluded that a solvent renders nucleo-
philic assistance in the heterolysis of tert-butyl sub-
strates [4, 7, 8].
solvent as MeOH, the conversion half-time
25 C is 21 h for t-BuCl and 150 years for 1-AdCl;
in dipolar aprotic MeCN, of t-BuCl is about
at
1/2
1/2
5 years [3, 4]. Therefore, the kinetics of the heterolys-
is of t-BuCl in aprotic solvents is studied insufficient-
ly, and for 1-AdCl data are few for protic solvents and
lacking at all for aprotic solvents.
Monomolecular heterolysis involves successive
formation of three ion pairs: contact (A), spacially
separated (B), and solvation-separated (C) [5]:
RX
R+
AB
X
R+ Solv X
Reaction products.
C
In the limiting step, pair A interacts with a solvent
cavity (cavities occupy about 10% of the volume of
a liquid [6]) to form pair B, which rapidly transforms
into pair C and then, also rapidly, into the reaction
products.
The negative effect of the nucleophilic solvation is
observed only in the heterolysis of tertiary substrates;
it is indicative of the nonequilibrium solvation of the
transition state when the solvation shells of the ground
and excited states have different structures. This effect
is usually pronounced in protic solvents, which is
apparently associated with hydrogen bonding between
molecules solvating nucleophilically and electrophili-
cally pair A. In heterolysis of secondary substrates,
e.g., Ph2CHBr [10], 1-phenyl-1-chloroethane [11],
and 3-bromocyclohexane [12], when the nucleophilic
solvation from the rear side is possible, the reaction
rate is independent of the solvent nucleophilicity.
In this case, the covalent substrate (initial state) is
The effect of the solvent nucleophilicity on the
rates of these reactions was discussed in numerous
papers [3 5, 7, 8]. It is usually believed that, in heter-
olysis of adamantyl substrate, the solvent renders no
nucleophilic assistance, because the nucleophilic
solvation from the rear side in this case is impossible
1
For communication XL, see [1].
1070-3632/05/7509-1430 2005 Pleiades Publishing, Inc.

KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XLI.
1431
subject to nucleophilic solvation, and its solvation
vinyl group should appreciably reduce the steric hin-
drance to the nucleophilic solvation from the rear side
[17] and thus should alter the nucleophilic effect of
the solvent.
shell has no time to rearrange within the time of for-
13
mation of the transition state ( 10
rangement time 10
s; typical rear-
10
11
10 s [13]). Thus, heterolysis
of secondary substrates is characterized by equilibri-
um solvation.
A kinetic study of the solvolysis of I at 32 C in
80% aqueous acetone showed that HCl is formed in
the solution by two pathways: direct solvolysis of
The rate of heterolysis of secondary substrates is
satisfactorily described by the polarity and electro-
philicity parameters, or by the parameter of the sol-
vent ionizing power ET and Z. In heterolysis of ter-
tiary substrates, the reaction rate depends, along with
the polarity and electrophilicity, also on the param-
eters of the solvent nucleophilicity, polarizability, and
cohesion, or on some of them. The nucleophilicity and
polarizability exert a negative effect on the reaction
rate, and the cohesion, a positive effect. The negative
effect of the polarizability is due to the fact that, the
higher the solvent polarizability, the easier the nucleo-
philic solvation of pair A; the positive effect of the
cohesion is due to the fact that, the higher the cohe-
sion, the more difficult it is to single out the solvent
molecule for nucleophilic solvation of pair A [5].
4
1
chloride I (kI 4.81 10 s ) and rearrangement of
chloride I into 2-methyl-4-chloro-2-butene II (krear
4
1
1.75 10 s ) followed by its solvolysis (kII 1.33
4
1
10 s ) [15].
krear
CH2=CHCMe2Cl
CMe2=CH CH2Cl
I
II
H2O
H2O
kI
kII
|
|
C5H9OH + HCl
The first-order rate constant of HCl formation
(v = kHCl[C5H9Cl]) decreases in the course of a kinet-
ic experiment; kI was found by extrapolation of kHCl
to zero time.
For a comparative analysis of the solvation effects
in heterolysis of organochlorine compounds, it is nec-
essary to choose a more active reference substrate than
t-BuCl and 1-AdCl. One of such substrates is 3-meth-
yl-3-chloro-1-butene I, which is more active than
t-BuCl by approximately three orders of magnitude
(80% aqueous acetone, 0 C) [14, 15]. The solvent
effect on the rate of heterolysis of chloride I has not
been examined; along with the data for aqueous ace-
tone, there are only data for 50% aqueous alcohol at
0 C [16].
We studied the solvolysis kinetics of I by the ver-
dazyl method in 13 alcohols. As an internal indicator
we used 1,3,5-triphenylverdazyl (Vd ), which rapidly
and quantitatively reacts after the limiting step with
pair B of the substrate to form the verdazylium salt
(Vd+Cl ) and the verdazyl alkylation product (Vd R),
which, in turn, rapidly decomposes into leucoverdazyl
(VdH) and diene (E1 reaction) or reacts with the sol-
vent to give the solvolysis product C5H9OR and VdH
(SN1 reaction). The reaction follows the stoichiometric
equation given below.
Replacement of the methyl group in t-BuCl by the
Ph
Ph
Ph
N CMe2 CH=CH2
N
N
N
N
N
N
N
N
N
+
CH2=CHCMe2Cl + 2
Ph
Cl
Ph
N+
N
Ph
Ph
Ph
Ph
Vd
Vd+Cl
Vd R
SN1
E1
ROH
CH3
CH2=CH C OR
CH2=CH C=CH2
CH3
CH3
|
|
+
Ph
NH
N
N
N
Ph
Ph
VdH
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 9 2005

1432
PONOMAREV et al.
Table 1. Solvent effect on the rate of solvolysis of CH2=CH CMe2Cl, t-BuCl, and PhCMe2Cl and solvent parameters
CH2=CH CMe2Cl
Solvent
no.
t-BuCl,
logk25
PhCMe2Cl,
ET,
kJ mol
Solvent
b
c
1
logk25
a
k25 105
logk25
1
2
3
4
5
6
7
8
9
10
11
12
13
MeOH
CH2=CH CH2OH
EtOH
BuOH
i-BuOH
PentOH
HexOH
2-PrOH
Cyclohexanol
OctOH
14.7 0.5
6.43 0.05
3.28 0.01
3.83
4.19
4.48
5.23
5.24
5.45
5.53
5.52
5.62
5.65
5.73
6.45
6.78
6.06
2.33
232
218
217
210
203
205
204
206
196
202
197
183
172
7.07
7.28
3.42
4.20
3.99
0.585 0.006
0.574 0.007
0.351 0.003
0.297 0.002
0.305 0.003
0.242 0.001
0.224 0.004
0.188 0.002
0.0357 0.001
0.0168 0.0002
7.45
7.83
8.07
7.52
4.87
4.35
4.93
4.83
4.87
5.16
2-BuOH
t-BuOH
t-PentOH
8.39
8.77
Table 1. (Contd.)
2
Solvent
no.
R,
kJmol
B,
kJmol
,
d
Solvent
n2D0
*
Es
1
1
1
kJl
1
2
3
4
5
6
7
8
9
10
11
12
13
MeOH
CH2=CH CH2OH
EtOH
BuOH
i-BuOH
PentOH
HexOH
2-PrOH
Cyclohexanol
OctOH
32.7
20.6
24.3
17.1
17.7
14.7
12.5
18.3
15.0
10.3
16.6
10.9
5.8
1.329
1.414
1.361
1.399
1.396
1.408
1.418
1.377
1.467
1.429
1.398
1.385
1.386
62
49
49
43
37
41
40
39
29
40
30
21
19
2.61
2.55e
2.81
2.76
2.75
2.75
2.84
2.82
2.89
2.82
2.82
2.95
3.03
358
583f
678
541
591
510
485
556
515
444
519
460
460
0.60
0.52
0.54
0.47
0.40
0.40
0.40
0.48
0.45
0.40
0.40
0.41
0.40
0.98 0.66 0.00
0.84 0.90 0.31g
0.86 0.75 0.08
0.84 0.84 0.31
0.79 0.84 0.93
0.84 0.86 0.31
0.80 0.84
0.76 0.84 0.48
0.66 0.84 0.69
0.77 0.81
2-BuOH
t-BuOH
t-PentOH
0.69 0.80
0.42 0.93 1.13
0.28 0.93 2.28
a
b
c
d
e
Mean result of 2 3 determinations. Taken from [18]. Taken from [19]. Taken from [20]. Estimated from data for other
f
2
g
alcohols. Calculated by the equation
= ( H
RT)/V .
Taken by analogy with BuOH and PentOH.
M
M
The kinetic experiments were performed at 25 C;
v = (d[Vd ]/2dt) = kI[I].
the effect of temperature on the reaction rate was
studied in MeOH, EtOH, and n-BuOH. Irrespective of
the reaction mechanism (SN1 or E1), 1 mol of the
substrate always reacts with 2 mol of Vd .
Since the substrate conversion in our kinetic exper-
iments is very low, chloride II is not formed in notice-
able amounts, and the rate constant does not change
in the course of the experiment.
The reaction rate was monitored spectrophotomet-
rically by the decrease in the Vd concentration (
The rate constants of solvolysis of I in 13 solvents
are given in Table 1 together with published data for
t-BuCl and PhCMe2Cl and with the solvent param-
eters required for the correlation analysis of the solva-
tion effects. The correlation analysis was performed
with Eqs. (1) (Koppel Palm equation [21] with addi-
max
720 nm). The substrate concentration in kinetic ex-
periments was 0.01 0.5 M, and that of the verdazyl
4
indicator, (1 3) 10 M; the conversion of I was
0.01 1%. The reaction rate is satisfactorily described
by a first-order kinetic equation
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XLI.
1433
Table 2. Activation parameters of solvolysis of I, t-BuCl, and PhCMe2Cl
MeOH
t-BuClb
EtOH
BuOH
t-BuClb
Parameter
Ia
Ia
t-BuClb
PhCMe2Cld
Ia
1
G , kJ mol
H , kJ mol
94 2
80 4
49 3
108
102
19
99 2
78 5
69 4
113
108
18
103 2
79 4
81 4
115
99
54
1
83
33
S , J mol 1 K
1
a
5
5
4
4
4
1 b
Calculated from k
2.83 10 , k
6.05 10 , k
1.05 10 , k
1.47 10 , and k
2.56 10
s
.
Taken from
11.6
16.7
21.2
25.0
30.2
c
5
5
4
5
5
[26]. Calculated from k
1.46 10 , k
2.28 10 , k
3.28 10 , k
4.64 10 , k
7.79 10 , and k
18.5
22.0
25.0
26.7
32.6
39.8
5
4
1
d
e
6
6
5
1.38 10
s
.
Taken from [27]. Calculated from k
4.02 10 , k
5.87 10 , k
1.23 10 , k
1.92 10
,
22.0
25.0
30.6
36.3
5
1
k
3.12 10
s
.
41.5
2
tionally included cohesive energy density
(2) (Kamlet Taft equation [23]), and (3).
[22]),
S , more than compensating for an increase in H .
This trend may be due to more difficult steric accessi-
bility of the reaction center in PhCMe2Cl, compared
to I, to nucleophilic solvation from the rear side.
1
+ 1
n2
1
2
log k = a0 + a1
+ a2
n2 + 1
+ a3E + a4B + a5
,
(1)
(2)
The use of Eq. (3) for the correlation analysis of
the solvation effects in the solvolysis of chloride I
leads to a satisfactory three-parameter regression:
2
logk = a0 + a1 * + a2 + a3 + a4
n2
,
1
2
log k = a0 + a1ET + a2
+ a3E + a4 .
(3)
n2 + 1
logkI = (0.0187 0.0076)ET
(3.42 0.19)B
2
+ (0.00157 0.000041) ,
Here is the solvent dielectric permittivity; n, re-
fractive index; E and , empirical parameters of the
electrophilicity; B and , those of the nucleophilicity;
*, dipolarity (polarity + polarizability) parameter;
R 0.977, S 0.19, F 69 (2.91), N 13.
Here F is the Fisher test, observed and critical for
the significance level of 0.05 (in parentheses). As the
ET, solvatochromic parameter of the solvent ionizing
2
power;
= ( HM RT)/VM, parameter characteriz-
log k25
ing the energy of interaction of solvent molecules
with each other; HM, molar heat of vaporization; and
VM, molar volume [24, 25].
1
2
3
4
5
6
7
8
3
1
5
The figure shows the logkI ET correlation for sol-
volysis of chloride I and the related data for t-BuCl
and PhCMe2Cl; the activation parameters of the sol-
volysis of I and t-BuCl in MeOH, EtOH, and BuOH
and of PhCMe2Cl in EtOH are given in Table 2. As
seen from the figure and Table 1, the rate of solvol-
ysis of I is two orders of magnitude higher than that
of t-BuCl and an order of magnitude lower than that
of PhCMe2Cl. The increase in the solvolysis rate in
going from t-BuCl to I or PhCMe2Cl is due to a de-
crease in the activation enthalpy, which more than
compensates for a decrease in the activation entropy.
A decrease in H is indicative of an increase in the
stability of the forming cationoid intermediates, and
2
3
4
3
4
8
10
7
7
6
11
11
12
5
8
1
(b)
13
10
12
7
8
(a)
4
6
12
(c)
13
9
240
220
200
180
160
ET, kJ mol
1
a decrease in
S is apparently caused by the more
intense solvation of the cationoid intermediates in the
heterolysis of I and PhCMe2Cl, due to weaker steric
hindrance to the nucleophilic solvation from the rear
side. In turn, an increase in the solvolysis rate in
going from I to PhCMe2Cl is due to an increase in
Correlation of the heterolysis rates of (a) I,
(b) PhCMe Cl, and (c) t-BuCl with the solvent
2
ionizing power ET. The solvent numbering is
the same as in Table 1.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 9 2005

1434
PONOMAREV et al.
observed Fisher test exceeds the critical value, the
model is reliable.
the following correlations based on Eq. (1) were ob-
tained for solvolysis of t-BuCl in 20 solvents and of
cumyl chloride in 10 solvents [23, 24]:
The nucleophilicity and cohesion parameters exert
a relatively weak effect on the correlation quality.
1
log kt-BuCl = 13.6 + 16.6
+ 0.0370E 1.24B
2 + 1
Exclusion of the cohesion parameter results in R
2
2
0.972, and exclusion of
and B leads to a satisfac-
+ 0.00140
,
tory one-parameter correlation:
R 0.964, S 0.63, N 20,
logkPhCMe Cl = 4.49 3.30B + 0.0319
R2 0.988, S 0.165, N 10.
2
,
logkI = (16.0 0.9) + (0.0522 0.0042)ET,
R 0.966, S 0.22, F 155 (2.69), N 13.
(4)
In our case, the negative effect of the nucleophilic
solvation [Eqs. (5) and (6)] may be an artefact, because
in the alcohols used the parameter ET relatively
strongly correlates with the nucleophilicity and elec-
trophilicity parameters:
Such a correlation is usually observed in the sol-
volysis of secondary substrates, when the reaction rate
is independent of the solvent nucleophilicity and is
satisfactorily described by the parameter of the solvent
ionizing power or by the polarity and electrophilicity
parameters [5, 10, 11] when the reaction occurs under
the conditions of the equilibrium solvation of the
transition state. The dependence of the reaction rate
on the solvent polarity and electrophilicity is due to
the fact that the parameter ET itself is satisfactorily
described by these parameters [18, 21]. In our case,
the correlation between ET and the polarity and elec-
trophilicity parameters is poor (R 0.9); therefore,
these parameters only approximately describe the rate
of solvolysis of I:
ET = (411 65) (28.7 21.5)B + (0.358 0.190)E, (7)
R 0.920, S 6.68, F 26 (2.80), N 13.
However, Eq. (7) gives also grounds to an opposite
conclusion that the lack of the nucleophilic effect of
the solvent [Eq. (4)] is an artifact.
Equations (1) and (3) are interrelated, because E
was calculated from ET [21]. Therefore, the dilemma
that arose can be solved using Eq. (2); it shows that
the rate of solvolysis of I is independent of the solvent
nucleophilicity:
1
log kI = (12.8 1.9) + (7.51 2.40)
+ 1
+ (0.0267 0.0099)E,
logkI = (10.0 0.5) + (6.20 1.02) * + (2.56 0.43)
R 0.965, S 0.24, F 67.2 (2.80), N 13.
R 0.905, S 0.38, F 31 (2.80), N 13.
For solvolysis of t-BuCl, Eq. (2) reveals a negative
effect of nucleophilic solvation:
Using Eq. (1), we obtained a satisfactory three-
parameter correlation:
2
logkt-BuCl
=
4.44
4.47 + 0.0017
,
logkI = (0.0291 0.0075)E
(2.65 0.09)B
,
2
R 0.961, S 0.57, N 21.
+ (0.00174 0.00078)
(5)
R 0.977, S 0.19, F 69 (2.91), N 13.
In cases when the solvolysis rate decreases with an
increase in the nucleophilicity of a protic solvent, it
depends on the volume of the alkyl group in the alco-
hol molecule [11]; as its volume grows, the reaction
rate increases because of growing steric hindrance to
nucleophilic solvation of pair A [5]. Therefore, we
performed the correlation analysis, based on Eq. (8),
of solvation effects in solvolysis of I, t-BuCl, and
PhCMe2Cl:
Exclusion of the nucleophilicity parameter makes
the correlation appreciably worse (R 0.953), and ex-
clusion of the cohesion parameter affects the correla-
tion quality less significantly:
logkI = (0.0431 0.0019)E
(2.50 0.070)B, (6)
R 0.965, S 0.22, F 75.0 (2.80), N 13.
Exclusion of the nucleophilicity parameter in this
case substantially decreases the correlation quality:
R 0.945.
logk = a0 + a1ET + a2B + a3ES.
(8)
where ES is the steric constant of the alkyl group in
the alcohol molecule (Table 1).
Equation (1) reveals the negative effect of the
nucleophilic solvation in solvolysis of I, as in solvol-
ysis of tertiary substrates [5, 10, 28]. In particular,
In solvents for which ES values are available
(Table 1), the rate of solvolysis of all the three chlo-
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XLI.
1435
rides is satisfactorily described by a one-parameter
equation:
refluxing over calcined CaO, distilled off, and frac-
tionated from sodium metal. Allyl alcohol was dried
over calcined K2CO3. Viscous hexanol and octanol
were heated at 100 C with granules of sodium metal
and fractionated in a vacuum.
logkI
=
(15.9 0.8) + (0.0521 0.0042)ET,
R 0.972, S 0.22, F 181 (2.97), N 11,
logkt-BuCl (16.4 0.9) + (0.0431 0.0013)ET
=
The kinetic experiments were performed in the
temperature-controlled cell of an SF-26 spectropho-
tometer. The mathematical treatment of the experi-
mental data was performed with EXCEL-97 program
package at 95% confidence level.
R 0.972, S 0.22, F 101 (3.44), N 9,
logkPhCMe l
=
(16.5 1.5) + (0.0602 0.0074)ET,
R 0C.957, S 0.29, F 65 (3.18), N 10.
Application of Eq. (8) to t-BuCl and PhCMe2Cl
shows that inclusion of ES substantially improves the
correlation quality:
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=
(22.0 3.6) + (0.0846 0.0463)ET
+ (0.774 0.474)ES,
2
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+ (0.630 0.226)ES,
R 0.989, S 0.15, F 122 (3.73), N 9.
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