Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XLIII. Solvent effect on activation parameters of dehydrochlorination of 3-chloro-3-methylbut-1-ene. Correlation analysis of solvation effects

Article abstract of DOI:10.1134/S1070363207070110

The influence of temperature on the rate of dehydrochlorination of 3-chloro-3-methylbut-1-ene in 17 aprotic and 13 protic solvents, ν = k[C ₅H₉Cl], was studied by the verdazyl method. In aprotic solvents, the electrophilicity, ionizing power, and cohesion of solvents decrease ΔG ^≠ by increasing ΔS ^≠. The nucleophilicity and polarizability increase both ΔH ^≠ and ΔS ^≠ to equal extent and therefore do not affect ΔG ^≠. In protic solvents, the solvent nucleophilicity increases ΔH ^≠ to a greater extent than ΔS ^≠, and the overall effect of the nucleophilic solvation is small and negative.

Full text of DOI:10.1134/S1070363207070110

ISSN 1070-3632, Russian Journal of General Chemistry, 2007, Vol. 77, No. 7, pp. 1204 1214.
Pleiades Publishing, Ltd., 2007.
Original Russian Text N.E. Ponomarev, V.V. Zaliznyi, G.F. Dvorko, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 7, pp. 1120 1130.
Kinetics and Mechanism of Monomolecular Heterolysis
of Commercial Organohalogen Compounds:
XLIII.1 Solvent Effect on Activation Parameters
of Dehydrochlorination of 3-Chloro-3-methylbut-1-ene.
Correlation Analysis of Solvation Effects
N. E. Ponomarev, V. V. Zaliznyi, and G. F. Dvorko
Kiev Polytechnic Institute, National Technical University of Ukraine,
pr. Peremogi 37, Kiev, 03056 Ukraine
e-mail: m_ponomaryov@ukr.net
Received September 11, 2006
Abstract The influence of temperature on the rate of dehydrochlorination of 3-chloro-3-methylbut-1-ene in
17 aprotic and 13 protic solvents, v = k[C5H9Cl], was studied by the verdazyl method. In aprotic solvents,
the electrophilicity, ionizing power, and cohesion of solvents decrease
philicity and polarizability increase both and to equal extent and therefore do not affect G . In
protic solvents, the solvent nucleophilicity increases
G by increasing S . The nucleo-
H
S
H
to a greater extent than S , and the overall
effect of the nucleophilic solvation is small and negative.
DOI: 10.1134/S1070363207070110
In the previous papers of this series, we reported
The majority of relevant studies deal with the
data on the kinetics of dehydrochlorination of 3-meth-
yl-3-chloro-1-butene (I) at 25 C in 17 aprotic and 13
protic solvents [1, 2]. The reaction rate is satisfac-
torily described by a first-order rate equation (1), with
the E1 mechanism realized in aprotic solvents and the
E1 + SN1 mechanism, in protic solvents [1 3]:
solvent effect on the logarithm of the heterolysis rate
constant or, which is equivalent, on the activation free
energies [4, 5, 7 10]. In these studies, however, data
for a single temperature (usually 25 C) are considered,
because logk and
G
are strongly temperature-
dependent. The activation free energy is determined
by the activation enthalpy and entropy ( G = H
T S ), which are virtually temperature-independent
in the ranges usually examined. A study of the solvent
effect on the activation parameters of the heterolysis
would allow better understanding of the solvation
effects and reaction mechanism.
v = k[I].
(1)
The rate of monomolecular heterolysis reactions
(SN1, E1, solvolysis) is controlled by ionization of the
covalent bond, which occurs via successive formation
of three ion pairs: contact (A), spatially separated (B),
and solvation-separated (C) [4, 5]:
Protic and aprotic solvents affect the activation
parameters of the heterolysis differently. The forma-
tion of an ion pair is accompanied by solvent structur-
ing around this intermediate (electrostriction effect
[11]), which decreases S . In protic solvents, this
effect is superimposed by break-up of the solvent
structure ( S increases). In aprotic nonstructured
solvents, the nature of solvation of the transition state
can be judged from the sign and value of S , where-
as in protic solvents it is difficult to make such
conclusions.
k1
k
k2
k
R+| |X
R+X
R+|Solv|X
RX
1
2
A
B
C
Reaction products.
(2)
In the limiting step, ion pair A interacts with the
solvent cavity ( ); the cavities occupy about 10% of
the liquid volume [6]. In the process, ion pair B is
formed, which rapidly transforms into pair C. Pair C,
in turn, also rapidly transforms into reaction products.
The electrostriction effect is readily identified when
performing heterolysis in a binary mixture consisting
of a polar solvent and a low-polarity solvent, e.g., of
1
For communication XLII, see [1].
1204

KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XLIII.
1205
water and dioxane [12, 13]. The dependence of the
reaction rate on the mixture composition allows deter-
mination of the number of polar solvent molecules
participating in solvation of the transition state (so-
called solvate number). For SN1 (E1) reactions with
a polar transition state, the solvate number is usually
6 8, and for SN2 reactions occurring via low-polarity
transition state, it is 2 3.
reliable data were obtained in the correlation analysis
of the reactivity of tert-butyl halides separately in
protic and aprotic solvents [15, 16]. However, the
experimental data considered in these papers were ob-
tained by different authors using different procedures.
The goal of this study is correlation analysis of
the influence of solvent parameters on the activation
parameters of the heterolysis of 3-chloro-3-methylbut-
1-ene (I) whose carbocation has two electrophilic
centers (positions 1 and 3) for nucleophilic solvation.
We examined the influence of temperature on the rate
of heterolysis of chloride I in 13 protic and 17
aprotic solvents. Kinetic experiments were performed
by the verdazyl method [17]. As internal indicator we
used 1,3,5-triphenylverdazyl (Vd ), which rapidly and
quantitatively reacts with the solvation-separated ion
pair of the substrate after the limiting step to form in
an aprotic medium isoprene, verdazylium chloride
Vd+Cl , and leucoverdazyl VdH (E1 reaction). In a
protic medium (SOH), the reaction occurs in part
along the SN1 pathway with the formation of the
solvolysis product (ROS) instead of isoprene [3]. The
reaction follows the stoichiometric equation
Data on the solvent effect on H and S are very
important for understanding the nature of solvation
effects, but these data are few and insufficiently reli-
able. Abraham et al. [14], when performing correla-
tion analysis of solvation effects in the heterolysis of
t-BuCl and t-BuBr, used data for protic and aprotic
solvents in combination. The experimental data were
available for only 14 solvents. The other values (nine
solvents) were estimated; in most cases, not only H
and S , but also G were estimated. The data set
on the activation parameters of the heterolysis of
1-methyl-1-halocycloalkanes was insifficient for cor-
rect correlation analysis using multiparameter equa-
tions of the linear free energy relationship. The most
Ph
Ph
Me
Ph
NH
N
N
N
N
N
N
CH2=CH C=CH2
CH2=CHCMe2OS
N
N
N
N
+
+
N+
Cl
Ph
CH2=CHCMe2Cl + 2
Ph
Ph
Ph
Ph
Ph
I
Vd
ROS
VdCl
VdH
Irrespective of the reaction pathway (E1 and/or
SN1), 2 mol of Vd is always consumed per mole of
the substrate, and 1 mol of Vd+Cl is formed. The re-
action rate was monitored spectrophotometrically, by
The increase in the reaction rate in series (4) is
mainly associated with an increase in the stability of
the forming carbocationic intemediate. Indeed, the in-
crease in the rate is mainly due to a decrease in H .
1
For example,
H
(kJ mol ) for the heterolysis in
a decrease in the Vd concentration (
720 nm).
It is satisfactorily described by a first-omrdaxer rate equa-
tion (3):
-butyrolactone decreases in going from t-BuCl (103)
to 3-chloro-3-methylbut-1-ene (88) and then to cumyl
chloride (82), with
S
remaining essentially the
1
same ( 73 J mol 1 K ) [15, 19].
v = d[Vd ]/2dt = k[I].
(3)
The figure illustrates two separate compensation
The logk25 values, activation parameters of the
heterolysis of I in 30 solvents, and solvent parameters
are given in the table. Comparison of logk25 from this
table with the related data for t-BuCl [10, 15] and
cumyl chloride [18, 19] show that the reaction rate in
the series of substrates (4) increases by three orders of
magnitude.
effects
H
S (for protic and aprotic solvents) in
the heterolysis of I. For aprotic solvents, the correla-
tion is satisfactory (R 0.954), and for protic solv-
ents it is approximate (R 0.934).
The compensation effect is apparently caused by
the uniformity of solvation effects: dipolar solvation
in aprotic solvents and electrophilic solvation in pro-
tonic solvents. Analysis of the logk 1/T dependences
shows that the isokinetic relationship is not observed
in the heterolysis of I.
t-BuCl < CH2=CH CMe2Cl < PhCMe2Cl.
100 1000
(4)
1
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 7 2007

1206
PONOMAREV et al.
Solvent effect on the rate and activation parameters of heterolysis of chloride I
Solvent
E
ET
B
n
*
1 MeOH
3.83 94 2 80 4
49 3
75 4
69 4
81 4
62 232 2.61 8.58 32.7 1.329 0.6
0.98 0.66
2 CH2=CH CH2OH 4.19 97 1 75 4
3 EtOH
4 n-BuOH
5 i-BuOH
6 n-PentOH
7 n-PrOH
8 n-HexOH
9 Cyclohexanol
10 n-OctOH
11 2-BuOH
12 t-BuOH
13 t-PentOH
14 Sulfolane
15 Propylene carbonate 6.15 108 1 96 4
16 -Butyrolactone
17 MeCN
49 218 2.55 5.83 20.6 1.414 0.52 0.84 0.9
49 217 2.81 6.78 24.3 1.361 0.54 0.86 0.75
43 210 2.76 5.41 17.1 1.399 0.47 0.84 0.84
37 203 2.75 5.91 17.7 1.396 0.4
41 205 2.8 5.01 13.9 1.41 0.4
39 206 2.82 5.56 18.3 1.377 0.48 0.76 0.84
40 204 2.84 4.85 12.5 1.418 0.4 0.8 0.84
29 196 2.89 5.15 15 1.467 0.45 0.66 0.84
40 202 2.82 4.44 10.3 1.429 0.4
30 197 2.82 5.19 16.6 1.398 0.4
4.48 99 2 78 5
5.23 103 2 78 4
5.24 103 1 71 5 107 7
5.45 104 1 85 4 64 4
5.52 104 1 53 4 174 9
5.53 105 1 96 5
5.62 105 1 95 5
5.65 105 1 91 5
5.73 106 1 79 5
6.45 110 1 88 6
0.79 0.84
0.84 0.86
28 3
32 3
48 3
89 5
72 5
0.77 0.81
0.69 0.8
21 183 2.95 4.6
19 172 3.03 4.6
10 184 1.88 6.9
21 195 2.18 7.4
10.9 1.385 0.41 0.42 0.93
6.78 112 1 64 6 159 8
6.06 108 1 90 5
5.8 1.386 0.4
42.1 1.481 0.98
62.9 1.421 0.83
0.28 0.93
59 5
40 4
73 6
42 4
0
0
0
0.39
0.4
0.49
6.44 110 1 88 5
6.56 110 1 98 4
12 185 2.48 6.95 41
1.437 0.87
21 191 1.91 5.86 35.9 1.344 0.75 0.19 0.4
18 PhNO2
19 PhCN
20 PhCOMe
7.76 117 1 85 6 110 7
7.91 118 1 94 6 80 6
7.96 118 1 87 5 106 6
0
0
0
173
0.8 5.11 36.1 1.551 1.01
0
0
0.3
0.37
173 1.85 5.15 25.2 1.528 0.9
170 2.42 4.33 18.2 1.534 0.9
0.04 0.49
0.1
8.30 120 1 53 4 225 10 8.5 176 2.68 3.88 21.4 1.359 0.71 0.08 0.43
8.34 121 1 71 5 166 9 5.4 173 2.5 3.61 18.9 1.379 0.67 0.06 0.48
21 1,2-Dichloroethane 8.02 119 1 63 5 186 9 9.6 173 0.48 4.12 10.4 1.551 0.81
22 Acetone
23 MeCOEt
0
24 Cyclohexanone
25 o-Dichlorobenzene 8.79 123 1 52 7 237 10
26 PhBr
27 PhCl
28 Dioxane
29 THF
8.34 121 1 68 8 177 9
0
0
0
0
0
0
166 2.89 4.08 16
159 0.33 4.2 10.4 1.551 0.8
153 0.48 0.79
1.451 0.76
0
0
0
0
0
0
0
0.53
0.03
0.06
0.07
0.37
0.55
0.45
8.89 124 1 63 9 203 10
9.24 126 1 65 5 204 10
9.53 127 1 66 7 207 9
9.64 128 1 73 8 185 9
4
5.55 1.56
154 0.45 3.76
150 2.84 4.2
156 3.43 3.61
5.74 1.524 0.71
2.27 1.422 0.55
7.39 1.408 0.58
30 AcOEt
10.29 132 1 90 8 140 9 6.7 159 2.17 3.39
6
1.372 0.55
The correlation analysis of the effect of solvent
parameters on the activation parameters of the heterol-
ysis of I was performed using the Koppel Palm equa-
tion [20] with additionally included cohesion energy
Here
is the parameter being correlated ( G ,
H , S ); , dielectric constant of the solvent; n,
refractive index; E and , empirical parameters of
electrophilicity; B and , empirical parameters of
nucleophilicity; *, dipolarity parameter (polarity +
2
density parameter [21] [Eq. (5)], Eq. (6), and Kam-
let Taft equation (7) [22]:
polarizability); ET, solvatochromic parameter of the
2
solvent ionizing power;
= ( Hm RT)/Vm reflects
1
+ 1
n2
1
the energy of solvent self-association, Hm is the
molar heat of vaporization, and Vm, molar volume.
The solvent parameters were taken from [23 25].
= a0 + a1
+ a2
n2 + 1
2
+ a3E + a4B + a5
,
(5)
(6)
n2
n2 + 1
1
2
= a0 + a1ET + a2
+ a3B + a4 ,
In aprotic solvents, application of Eq. (5) to the
activation free energy leads to a satisfactory five-
parameter correlation:
2
= a0 + a1 * + a2 + a3 + a4
.
(7)
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XLIII.
1207
2
G
= (163 12) (0.0306 0.0091)
(15.3 5.0)f ( )
(0.765 1.19)B;
(0.280 0.213)E (36.3 34.8)f(n)
R 0.959, S 2.43, F 25 (2.8), N 17.
17
15
6
Here and hereinafter, f( ) = (
1)/( + 1); f(n) =
100
80
8
2
(n2 1)/(n2 + 1); F is the calculated and critical (in
parentheses) Fisher test [26]. The model is reliable if
the calculated Fisher test is higher than the critical
value.
14 19
16
30
20
18
10
12
1
11
3
23
The polarizability, electrophilicity, and nucleo-
philicity parameters are insignificant, and on their
exclusion we obtain a satisfactory two-parameter
correlation:
4
29
27
2
5
24
13
28
60
21 26
22
2
G
= (153 3) (17.6 4.6)f ( ) (0.0399 0.0054)
R 0.952, S 2.3, F 68 (2.5), N 17.
;
1
7
25
40
The correlation analysis of the effect of solvent
paameters on and leads to correlations of
lower quality. Satisfactory correlation for was
0
100
200
S , J mol 1 K
1
H
S
S
obtained only after exclusion of three points (17, 25,
and 30; here and hereinafter, the solvent numbering is
the same as in the table):
H
S
plots for (1) protic and (2) aprotic solvents.
For solvent numbering, see table.
2
G
= (150 2) (13.9 3.3)f ( ) (0.0398 0.0091)
R 0.978, S 1.6, F 104 (3.1), N 13,
;
S
=
(853 100) + (1090 290)f(n) + (38.7 10.0)B
2
+ (0.466 0.052)
;
S = (610 113) + (136 33)f ( ) + (449 262)f(n)
2
R 0.950, S 2.4, F 31 (3.1), N 14.
+ (19.1 9.4)B + (0.319 0.051)
;
R 0.963, S 2.0, F 26 (3.6), N 13.
For the whole set of 17 solvents, R 0.858. The
electrophilicity and polarity parameters affect the cor-
relation quality insignificantly; their inclusion in-
creases R only to 0.961.
Thus, an increase in the heterolysis rate with an
increase in the solvent polarity and cohesion is due to
the effect of these paameters on S , and the indepen-
dence of the rate from the solvent nucleophilicity and
polarizability means that these parameters increase
H and S to the same extent (full compensation
effect). Similarly, after exclusion of four points (17,
20, 23, 30), application of Eq. (6) leads to the follow-
ing correlations:
Even after exclusion of four points (17, 23, 25, 30),
we obtained for H only an approximate correlation:
H
= (97.7 31.0) + (302 72)f(n) + (9.82 2.67)B
2
+ (0.0879 0.0127)
;
R 0.934, S 5.9, F 21 (3.3), N 13.
2
G
= (176 9) (0.261 0.067)ET (0.0253 0.0071)
R 0.976, S 1.7, F 99 (3.1), N 13,
;
Thus, in aprotic solvents, the polarizability, nucleo-
philicity, and cohesion increase both and S ,
H
S
= (1010 136) + (1.29 0.65)ET + (33.7 7.8)B
and only the cohesion affects the reaction rate, in-
creasing it. This fact indicates that the nucleophilicity
and polarizability affect both activation parameters to
similar extent, which leads to the compensation effect,
whereas the cohesion affects S more strongly than
H . It is unclear why the reaction rate depends on
the solvent polarity if this parameter affects neither
H nor S . The cause may be the fact that, when
constructing correlations for G , H , and S , we
used different sets of solvents. To eliminate this in-
consistency, we performed a correlation analysis for
G and S in the same set of solvents as for H :
2
+ (1130 210)f(n) + (0.346 0.066)
;
R 0.978, S 17, F 43 (3.6), N 13,
H
= (98.0 37.0) + (302 17)f(n) + (9.82 1.67)B
2
+ (0.0879 0.0127)
;
R 0.934, S 5.9, F 21 (3.3), N 13.
Thus, the heterolysis rate increases with an increase
in the ionizing power and cohesion of the solvent. The
effect of the solvent ionizing power is due to an in-
crease in S , and the effect of cohesion, due to a
more pronounced increase in S than in H . The
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 7 2007

1208
PONOMAREV et al.
solvent nucleophilicity and polarizability do not affect
the reaction rate, because they increase both H and
effect of the nucleophilic solvation on the activation
entropy and enthalpy is apparently due to the occur-
S
to an equal extent.
rence of the compensation effect
ure). Indeed, the quality of correlations obtained with
Eqs. (5) (7) increases upon exclusion of solvents that
H
S (see fig-
After exclusion of three points (19, 22, 30), Eq. (7)
takes the following forms:
deviate from the
H
S correlation to the greatest
extent (AcOEt, MeCOEt, MeCN).
H
= (17.9 10.9) + (24.9 15.3) * + (90.3 32.0)
2
+ (29.5 9.3) + (0.0551 0.0156)
;
Application of Eq. (5) to protic solvents, after
exclusion of three points (7, 11, 12), leads to the fol-
lowing relationships:
R 0.944, S 5.8, F 17.3 (3.3), N 14,
(8)
(9)
G
S
= (150 3)
(15.3 5.2) *
(0.0387 0.0052)
(34.1 11.1)
2
;
2
G
S
= (50.4 14.4) + (22.3 4.7)B (0.0172 0.0051)
R 0.963, S 1.5, F 44 (4.1), N 10,
;
R 0.968, S 2.0, F 49 (3.1), N 14,
(12)
=
(447 33) + (139 49) * + (413 102)
=
(1900 270) + (2100 320)f(n) + (5.85 0.91)E
2
+ (108 30) + (0.308 0.048)
;
2
+ (296 67)B + (0.140 0.075)
R 0.969, S 19, F 72 (6.0), N 10,
;
R 0.974, S 19, F 42 (3.3), N 14.
(10)
(13)
As the parameters of Eq. (7) are normalized, from
correlations (8) and (10) we can calculate the relation-
ship describing the effect of solvent parameters on
G [Eq. (11)]; this equation reasonably agrees with
correlation (9) obtained using Eq. (7):
H
=
(41.8 3.9) + (515 81)f(n) + (1.64 0.30)ET
+ (95.1 22.7)B;
R 0.938, S 4.5, F 15 (4.8), N 10.
(14)
In all the three correlations, the nucleophilic effect
of the solvent is significant: Upon exclusion of B,
R decreases to 0.902, 0.728, and 0.830 in the first,
second, and third cases, respectively.
2
G
= 151
16.5 *
32.7
0.0360
.
(11)
The adequacy of relationships (9) and (11) con-
firms the correctness of our conclusions that the lack
of the nucleophilic effect of a solvent in the heterol-
ysis of chloride I is due to the compensation of the
effects of nucleophilic solvation on H and S .
Thus, the electrophilicity and polarizability of a
protic solvent increase both
equal extent and therefore do not affect the solvolysis
rate; the solvent nucleophilicity increases to a
H
and
S
to an
H
greater extent than S and thus decreases the reac-
tion rate; an increase in the reaction rate with increas-
ing cohesion is due to the effect on S .
Thus, analysis of the solvation effects using three
multiparameter equations shows that in an aprotic
solvents the ionizing power and also the dipolarity
and polarity of the solvent decrease H and thus in-
crease the heterolysis rate. The solvent cohesion also
After exclusion of three solvents (3, 7, 12), Eq. (6)
takes the following forms:
increases the reaction rate, as it affects
strongly than H .
S
more
G
= (82 20)
(0.107 0.043)ET2 + (17.1 4.4)B
(0.00818 0.00308)
;
The most important conclusion following from our
study for aprotic solvents is as follows: In the heterol-
ysis of chloride I, which, like secondary substrates,
experiences no steric hindrance in nucleophilic solva-
tion of the covalent substrate, the nucleophilicity and
R 0.992, S 0.71, F 128 (4.8), N 10,
H
=
(636 121) + (1.32 0.23)ET + (122 26)B
(354 68)f(n);
R 0.945, S 4.2, F 17 (4.8), N 10,
polarizability of the solvent increase both
H
and
S
=
(2260 430) + (4.28 1.11)ET + (1430 330)f(n)
+ (301 109)B;
S to an equal extent and, therefore, do not affect the
reaction rate. Abraham et al. [14] attributed this com-
pensation effect to break-up of the solvent structure,
accompanying formation of an ion pair, which leads
to an increase in both S and H . This explanation
contradicts the modern views on the heterolysis mech-
anism, according to which the break of the solvent
structure facilitates formation of cavities in liquids
and thus increases the reaction rate [4, 5]. Further-
more, only in structured protic solvents the effect
of structure break-up can be significant. The equal
R 0.959, S 15, F 19 (4.8), N 10.
(15)
In this case, the polarizability increases both
H
and
S
to an equal extent and therefore does not
affect G ; the solvent ionizing power increases S
to a greater extent than and thus increases the
reaction rate; and the nucleophilicity increases
to a greater extent, so that the reaction rate decreases.
The effect of the nucleophilicity of protic solvents
on the reaction rate is significant: Exclusion of this
H
H
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XLIII.
1209
factor decreases R to 0.703 for H and to 0.885 for
S . However, the compensation effect reduces the
influence of the nucleophilicity on G .
ic solvents, a weak negative effect of nucleophilic
solvation is observed, i.e., the solvent nucleophilicity
affects
H
more strongly than S . The negative
effect of nucleophilic solvation is observed only in
heterolysis of tertiary substrates [4, 5, 9, 10, 27, 28].
This effect increases with an increase in the steric
hindrance to nucleophilic solvation from the rear side
and reaches a maximum in heterolysis of adamantyl
substrates in which the nucleophilic solvation from
the rear side is impossible. In heterolysis of I, the
decrease in the reaction rate with increasing solvent
nucleophilicity is relatively weak.
With Eq. (7), no satisfactory correlations for
and were obtained.
H
S
Thus, in a protic medium the nucleophilic solva-
tion exerts a weak negative effect, as H increases to
a greater extent than S .
Application of Eqs. (5) (7) to the whole set of
protic and aprotic solvents gave satisfactory correla-
tions only for G .
The negative effect of nucleophilic solvation is
better manifested in protic than in aprotic solvents.
This fact can be accounted for as follows. Apparently,
in protic solvents the nucleophilic solvation of pair A
yields cyclic solvate II in which hydrogen bonds are
formed between the molecules that nucleophilically
solvate the carbocation and electrophilically solvate
the anion, which hinders ion separation in the transi-
tion state [4, 5].
The heterolysis of a covalent bond involves forma-
tion of a polar transition state; therefore, it is com-
monly believed that all types of solvation should favor
this process. The recently identified negative effect of
nucleophilic solvation in heterolysis of tertiary sub-
strates [4, 5] contradicts this rule and requires detailed
consideration. Analysis of solvent effect on activation
parameters offers such an opportunity.
The solvent nucleophilicity increases both H and
S of heterolysis of I. As the first factor decreases
the reaction rate and the second factor increases it,
a compensation effect is observed. In aprotic solvents,
there is full compensation, and the solvent nucleo-
philicity does not affect the heterolysis rate. In proton-
McLennan and Martin [29] analyzed the solvent
effect on the rate of solvolysis of benzhydryl deriva-
tives. They concluded that transformation of pair A
into pair B is accompanied by cleavage of hydrogen
bonds between the solvent molecules solvating pair A.
solv
solv
t-Bu+Cl
Me
Cl
S
R+X
H
+
Me
Cl
O
H
+
H
O
O
H
H
H
O
CH2
+C
CH
CH
CH2
C
1
3
1
3
solv
H
S
solv
Me
Me
O
H
O
H
S
n
2
II
III
IV
V
An ab initio analysis of the mechanism of t-BuCl
hydrolysis made McLennan and Martin [29] to sug-
gest intermediate formation of a ten-membered cyclic
solvate of contact ion pair III with four water mole-
cules one of which nucleophilically solvates the car-
bocation from the rear side and another, the chloride
ion [30]. Similar conclusions were made by Yamabe
and Tsuchida [31] who studied the reaction of t-BuCl
with a water cluster. It follows from [32] that solvate
III is formed by reorganization of the solvation shell
of the covalent substrate, suggesting nonequilibrium
solvation.
of covalent chloride I occurs at both sites, with the
formation of solvate IV. In position 1, there is no
steric hindrance to nucleophilic solvation, and the
solvent molecule is in the direct contact with the elec-
trophilic center, whereas in position 3, in which there
is strong steric hindrance, the nucleophilically solvat-
ing solvent molecule is remote from the electrophilic
center. When the covalent substrate transforms into
pair A, the arrangement of the nucleophilically solvat-
ing molecule at position 1 does not change, whereas
the nucleophilically solvating solvent molecule at
position 3 comes in direct contact with the electro-
philic center, possible because of the planar structure
of the carbocation [33]; in the process, solvate V is
formed.
Owing to delocalization of the partial positive
charge in positions 1 and 3, the nucleophilic solvation
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 7 2007

1210
PONOMAREV et al.
Without steric hindrance, the relative contributions
In heterolysis of I in any specially selected set of
dipolar aprotic solvents, the reaction rate is independ-
ent of the solvent nucleophilicity. Presumably, in an
aprotic medium only the nucleophilic solvation in
position 1 is significant. In protic solvents, where
the solvent nucleophilicity decreases the heterolysis
rate, the nucleophilic solvation in position 3 is also
significant. Hence, heterolysis of I in aprotic and pro-
tonic solvents models the effect of solvent nucleo-
philicity in heterolysis of secondary and tertiary sub-
strates, respectively.
of the enthalpy and entropy terms of the energy of
nucleophilic solvation change only slightly in going
from the covalent substrate to pair A, full compensa-
tion is observed, and the reaction rate is independent
of the solvent nucleophilicity. At a strong steric hin-
drance, the relative contributions of the enthalpy and
entropy terms of the energy of nucleophilic solvation
change appreciably in going from the covalent sub-
strate to pair A, because of additional decrease in the
entropy upon reorientation of the solvent molecule
toward the electrophilic center in position 3. Since the
nucleophilic solvation of a carbocation is entropy-
controlled [34], this reorientation will result in a
weaker increase in S , compared to H , and hence
in a decrease in the reaction rate with increasing nu-
cleophilicity of the solvent.
Four types of solvolysis reactions occurring with
the nucleophilic assistance of the solvent are known:
SN2-classical (one-step reaction of a nucleophile with
a covalent substrate), SN2-intermediate (reaction of a
nucleophile with a covalent substrate with intermedi-
ate formation of a contact ion pair), SN2-ion pair (the
limiting step is the reaction of a nucleophile with a
contact ion pair), and SN2(C+) (the limiting step is the
reaction of a nucleophile with a solvation-separated
ion pair or, according to Ingold, with a free carbo-
cation) [4, 5, 38, 39]. In these reactions, the solvent as
a nucleophile participates in the formation of the tran-
sition state.
In aprotic solvents, the negative effect of nucleo-
philic solvation is considerably more difficult to reveal
than in protic solvents. In a broad set of aprotic
solvents (N 26), it was found only for 2-bromo-2-
methyladamantane [5] in which the nucleophilic sol-
vation from the rear side is impossible, and also for
p-methoxyneophyl tosylate in ten solvents [35]. How-
ever, this effect can be relatively readily revealed
when only dipolar aprotic solvents are considered.
This was demonstrated for heterolysis of cumyl chlo-
ride [5], Ph2CCl2 [36], and 1-chloro-1-methylcyclo-
pentane [37].
The heterolysis of secondary substrates, apparently,
is also accompanied by nucleophilic assistance by the
solvent through nucleophilic solvation of the covalent
substrate from the rear side [40, 41], but it is compen-
sated by solvation effects hindering the ion separation
(solvates II and VI). In the heterolysis of tertiary
substrates in which the nucleophilic solvation from
the rear side is impossible or strongly hindered, a neg-
ative effect of the nucleophilic solvation is observed,
caused by formation of solvates II and VI.
This trend can be illustrated by the heterolysis of
1-AdOTs. As shown in [7], in a set of 22 aprotic solv-
ents the reaction rate is independent of the solvent nu-
cleophilicity. We found that, in a set of eight aprotic
solvents (propylene carbonate, MeCN, PhNO2, PhCN,
PhCOMe, acetone, cyclohexanone, 1,2-dichloroeth-
ane), the rate of this reaction decreases with an in-
crease in the solvent polarizability and nucleophilicity.
Entelis and Tiger [42] conclude that in nonpolar
solvents the solvation of the transition state in heterol-
ysis is equilibrium, whereas in dipolar aprotic and
especially in protic solvents nonequilibrium solva-
tion of the transition state prevails. Our results render
this assumption more concrete: Only nucleophilic sol-
vation is nonequilibrium in protic and dipolar aprot-
ic solvents, because just this kind of solvation in-
volves break-up of the solvent structure. However,
electrophilic and dipolar solvation is equilibrium.
Gorodynskii and Morachevskii [43], and Kim and
Hynes [44] believe that the solvation of the transition
state is nonequilibrium. This is caused by orientation
polarization, which changes sufficiently rapidly (in
logk = 0.0289ET
0.55f(n)
0.399B;
R 0.987, N 8.
Exclusion of B decreases R to 0.928, and exclusion
of f(n), to 0.697.
The negative effect of nucleophilic solvation in
dipolar aprotic solvents can be accounted for by elec-
trostatic solvation of pair A, leading to the formation
of cyclic quadrupole VI by interaction of the dipole of
pair A with that of the solvent molecule. Such interac-
tion stabilizes the intermediate and hinders the ion
separation in the transition state.
16
10 s) to affect the formation of the transition state
13
R+X
( 10
s) [45]. This idea is convenient for quantum-
chemical analysis of heterolysis products, but it is
difficult to prove. We believe that the nonequilibrium
solvation is caused by the multistep course of heterol-
solv
+
VI
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS... : XLIII.
1211
ysis of a covalent bond. The nonequilibrium solvation
of the transition state is suggested by low values of
S . Coordination of one monodentate ligand de-
ance by a solvent originates from the use of relative
quantities. The observed differences between frame-
work and tert-alkyl substrates show that the influence
of the solvent nucleophilicity on the rate of solvolysis
of these compounds is different, but give no informa-
tion about the causes of their difference. Direct evalu-
ation of the influence of the solvent nucleophilicity on
the heterolysis rate confirmed that the nucleophilic
effect of the solvent, indeed, depends on the steric
factor, but an increase in the steric hindrance leads to
an increase in the negative effect of nucleophilic sol-
vation, rather than to a decrease in the nucleophilic
assistance by the solvent [4, 5, 7, 9, 10, 27, 35 37,
49, 50, 62 64]. In other words, the solvent nucleo-
philicity decreases, rather than increases, the heterol-
ysis rate and this effect becomes stronger with an in-
crease in the steric hindrance to nucleophilic solva-
tion.
1
creases the entropy by 45 J mol 1 K [46]. As seen
from the table, in heterolysis of I from one to five
solvent molecules additionally participate in formation
of the transition state. Similar pattern is observed in
heterolysis of secondary substrates [47].
Conclusions about the occurrence of the nucleo-
philic assistance by the solvent are based either on
application of the Grunwald Winstein dependences
unsuitable for this purpose or on incorrect interpreta-
tion of experimental data on relative rate constants
[4, 5, 17]. The nucleophilicity parameter in the Grun-
wald Winstein equation is a complex quantity deter-
mined by the polarity, electrophilicity, and nucleo-
philicity parameters [27, 48]. Its application leads to
contradictory, and in some cases to absurd results
[11]. Correct interpretation of the experimental data
shows that the nucleophilic assistance by the solvent
is lacking [4, 5, 27, 48 50].
Abboud et al. [65, 66] showed that the rate of sol-
volysis of various framework substrates well corre-
lates with the stability of their carbocations in the gas
phase. They believed that the solvent nucleophilicity
did not affect the solvolysis rate of these compounds
and therefore suggested to use the framework line
for revealing the occurrence or lack of nucleophilic
assistance by a solvent with other substrates. The
deviation from the framework line for tert-alkyl
substrates was attributed to the nucleophilic assistance
by the solvent in solvolysis of these compounds [61,
67]. This conclusion was confirmed by the fact that
deviations from the framework line decreased with
an increase in the steric hindrance to nucleophilic
solvation from the rear side [61].
The idea of nucleophilic assistance by the solvent
is based in incorrect conclusions of Ingold and Win-
stein that the return from the product-forming inter-
mediate exerts a significant effect on the heterolysis
rate [4, 5, 39, 51]. It was assumed that the nucleo-
philic solvation of a carbocation (free or incorporated
in an ion pair) should increase the reaction rate by
shifting the equilibrium toward reaction products.
These conclusions were based on erroneous interpre-
tation of the so-called salt effect of the law of mass
action [52] and special salt effect [53]. At that time,
another interpretation of these effects could not be
offered, but now there is a good reason to offer a
correct interpretation [4, 5, 52, 53]. Nevertheless, even
relatively recent papers [54, 55] refer only to clas-
sical studies.
In one of their later studies [68], Abboud et al.
showed that using the framework line for revealing
the nucleophilic effect of a solvent leads to absurd
conclusions. They showed that data for PhCH2Cl are
well fitted by the framework line , whereas data for
Ph2CHCl strongly deviate from it. However, an oppo-
site effect should be expected, because the solvolysis
of PhCH2Cl occurs with a strong nucleophilic assist-
ance by a solvent, as the rate of this reaction is limited
by interaction of the solvent with the contact ion pair
(SN2-ion pair mechanism) [69, 70], whereas the rate
of heterolysis of benzhydryl halides is independent of
the solvent nucleophilicity [4, 70, 71]. Thus, applica-
tion of the framework line to revealing the nucleo-
philic assistance by a solvent leads to results opposite
to those expected.
Search for nucleophilic assistance by a solvent was
started more than 50 years ago and still continues with
increasing intensity [56 58]. Following a Russian
proverb, this search resembles catching a black cat in
a dark room in which this cat is absent. And since it
is believed that this cat (nucleophilic assistance)
does exist, any deviations from common trends are at-
tributed to the nucleophilic assistance by the solvent.
For example, comparison of the solvolysis rates of
adamantyl and tert-butyl substrates in various solvents
reveals decreased values of logkt-BuX in a weakly
nucleophilic (strongly electrophilic) medium, and this
trend is attributed to nucleophilic assistance by the
solvent in heterolysis of t-BuX in strongly nucleo-
philic solvents [59 61].
EXPERIMENTAL
3-Chloro-3-methylbut-1-ene was prepared by the
reaction of isoprene with HCl [72] and was purified
The erroneous conclusion about nucleophilic assist-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 7 2007

1212
PONOMAREV et al.
by double distillation, bp 42 44 C/230 mm Hg, nD20
1.4192. 1,3,5-Triphenylverdazyl was prepared and
purified as described in [73]. The majority of alcohols
were dried by prolonged refluxing over calcined CaO,
distilled, and fractionated from sodium metal. Allyl
alcohol was dried over calcined K2CO3. Viscous hex-
anol and octanol were kept at 100 C over sodium
metal grains and fractionated in a vacuum. Aprotic
solvents were dried and fractionated.
42.6, 0.0518; 47.3, 0.0784; 53.0, 0.100; 60.0, 0.180.
PhBr. 25.5, 0.0127; 30.4, 0.0248; 33.3, 0.0292; 37.9,
0.0342; 40.9, 0.0410; 52.5, 0.137. PhCl. 29.4, 0.0844;
33.4, 0.0114; 38.0, 0.0198; 50.4, 0.0477; 55.5, 0.0710.
Dioxane. 28.8, 0.00424; 34.0, 0.00613; 40.1, 0.0118;
46.5, 0.0184; 50.9, 0.0262. THF. 34.0, 0.00548; 37.2,
0.00801; 45.2, 0.0161; 54.1, 0.0349. AcOEt. 40.8,
0.00321; 44.4, 0.00474; 52.2, 0.0135; 56.0, 0.0179;
60.2, 0.0233.
Kinetic experiments were performed in a tempera-
ture-controlled cell of an SF-26 spectrophotometer.
The substrate concentration in kinetic experiments
was 0.01 0.5 M, and the verdazyl indicator concen-
Calculation by Eqs. (6) (8) was performed using the
EXCEL 97 program package, confidence level 95%.
REFERENCES
4
tration, (1 3) 10 M. The substrate conversion in
kinetic experiments was 0.001 1%, and the indicator
conversion, 5 50%. The rate constants k were deter-
mined with an accuracy of 3%.
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1
k
107 (s ). MeOH. 11.6, 283; 16.7, 605; 21.2,
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2840; 46.2, 4800. EtOH. 18.5, 146; 22.0, 228; 25.0,
328; 26.7, 464; 32.6, 779; 39.8, 1380. BuOH. 22.0,
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41.5, 312. i-BuOH. 23.5, 53.3; 25.0, 57.4; 28.5, 71.0;
38.5, 215; 46.2, 445; 51.2, 595. PentOH. 22.0, 23.9;
25.0, 35.1; 27.2, 48.3; 33.7, 93.3; 40.8, 201; 46.33,
383. i-PrOH. 25.0, 30.5; 27.0, 39.1; 29.1, 39.3; 35.5,
65.8; 39.0, 87.7; 47.5, 138. HexOH. 19.3, 13.4; 22.0,
19.8; 25.0, 29.7; 28.7, 46.4; 34.8, 123; 41.3, 214.
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152; 48.4, 458. OctOH. 22.4, 16.0; 25.0, 22.4; 27.5,
31.2; 32.5, 54.4; 38.3, 120.0; 42.3, 170.0. 2-BuOH.
22.0, 13.6; 25.0, 18.8; 29.2, 28.7; 38.1, 80.5; 42.5,
113. t-BuOH. 22.0, 2.50; 25.0, 3.57; 27.5, 4.77; 31.0,
7.18; 34.8, 11.1; 39.7, 20.8. t-PentOH. 22.0, 1.15;
25.0, 1.67; 28.4, 2.38; 35.4, 4.71; 43.0, 7.87; 48.6,
11.1. Sulfolane. 25.3, 8.22; 29.9, 18.3; 32.7, 23.3;
36.7, 34.9; 40.9, 55.8. Propylene carbonate. 22.4,
5.26; 27.1, 8.99; 32.4, 18.6; 36.2, 27.3; 40.8, 56.9.
-Butyrolactone. 22.0, 2.21; 30.6, 8.02; 37.0, 17.7;
43.5, 33.5; 52.0, 61.8. MeCN. 22.0, 1.80; 25.0, 2.75;
26.3, 3.72; 32.6, 7.58; 39.8, 19.7; 47.0, 42.8. PhNO2.
18.0, 0.0758; 21.6, 0.104; 25.0, 0.174; 27.2, 0.249;
33.8, 0.515; 39.4, 0.808. PhCN. 21.4, 0.0775; 24.4,
0.134; 30.6, 0.261; 35.3, 0.417; 39.8, 0.839. PhCOMe.
20.5, 0.0589; 25.0, 0.109; 25.2, 0.118; 29.3, 0.193;
34.6, 0.315. 1,2-Dichloroethane. 17.0, 0.100; 31.2,
0.167; 40.4, 0.377; 48.5, 0.866; 56.3, 0.945. Acetone.
25.0, 0.0497; 29.5, 0.0686; 38.4, 0.137; 44.0, 0.179;
50.4, 0.299. MeCOEt. 23.1, 0.0379; 27.5, 0.0598;
32.6, 0.0996; 39.2, 0.155; 53.5, 0.642. Cyclohexan-
one. 22.3, 0.0341; 28.4, 0.0674; 33.7, 0.0973; 41.2,
0.200; 47.9, 0.342. 1,2-Dichlorobenzene. 34.8, 0.0349;
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