Specific features of solvation effects in monomolecular and bimolecular solvolysis

Article abstract of DOI:10.1023/A:1012391403905

A method was suggested for distinguishing monomolecular and bimolecular solvolysis on the basis of reaction kinetics in water, MeOH, EtOH, i-PrOH, cyclohexanol, and t-BuOH. In solvolysis of n-PrBr, CH₂=CHCH₂Br, PhCOCl, and MeOClO₃ (SN2 reactions), a linear correlation is observed between log k and the solvent ionizing power Z, whereas in solvolysis of t-BuBr, t-BuCl, and 1-AdI (SN1, E1 reactions) this correlation is nonlinear. Deviations from linearity are due to steric hindrance decreasing the negative effect of nucleophilic solvation.

Full text of DOI:10.1023/A:1012391403905

Russian Journal of General Chemistry, Vol. 71, No. 4, 2001, pp. 591 598. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 4, 2001,
pp. 635 643.
Original Russian Text Copyright
2001 by Ponomarev, Michkov, Dvorko.
Specific Features of Solvation Effects in Monomolecular
and Bimolecular Solvolysis
N. E. Ponomarev, K. V. Michkov, and G. F. Dvorko
Kiev Polytechnic Institute, Ukrainian National Technical University, Kiev, Ukraine
Received November 9, 1999
Abstract A method was suggested for distinguishing monomolecular and bimolecular solvolysis on the
basis of reaction kinetics in water, MeOH, EtOH, i-PrOH, cyclohexanol, and t-BuOH. In solvolysis of n-PrBr,
CH2=CHCH2Br, PhCOCl, and MeOClO3 (SN2 reactions), a linear correlation is observed between log k and
the solvent ionizing power Z, whereas in solvolysis of t-BuBr, t-BuCl, and 1-AdI (SN1, E1 reactions) this
correlation is nonlinear. Deviations from linearity are due to steric hindrance decreasing the negative effect of
nucleophilic solvation.
Solvolysis (1) can occur by monomolecular (SN1,
E1) or bimolecular (SN2) mechanism [1].
ysis of t-BuX or AdX, and the second equation in-
cludes, along with Y, the nucleophilicity parameter N.
These equations often lead to ambiguous and some-
times erroneous conclusions [8 10].
RX + R OH
ROR + HX.
(1)
In the first case, the limiting stage is formation of
an active intermediate (ion pair), whereas in the sec-
ond case the reaction is controlled by interaction of
the solvent with a covalent substrate (SN2 reaction)
or with an intermediate (SN2-ion pair reaction [2]).
Under conditions of solvolysis (excess of nucleophilic
agent), these reactions are kinetically indiscernible; in
both cases the reaction rate is described by a first-
order kinetic equation (2):
logk/k0 = mY,
(3)
(4)
logk/k0 = mY + lN.
Here k0 and k are the rate constants of solvolysis
in 80% aqueous ethanol and in the solvent being
studied; m and l are the parameters of sensitivity of
the solvolysis reaction to the ionizing power and
nucleophilicity of the solvent. For t-BuX (AdX) and
MeCl (MeOTs) m and l are taken as unity.
v = k[RX].
(2)
In correlation analysis of solvation effects with
Eqs. (3) and (4), as a rule, data for binary solvent mix-
tures are used. The use of a mixed solvent severely
complicates interpretation of the solvation effects. In
this case, major deviations are possible, because the
structures of the solvation shells of the initial reactants
and transition state can vary differently with changing
solvent composition. This fact is probably one of the
causes of large dispersion of the logk Y correlations.
The deviation should be the larger, the stronger the
difference between the mixed solvent components in
properties. Indeed, the largest deviations are observed
with mixtures of prot3ic and aprotic solvents [11,
12]. For example, the dependence of logk for solvol-
ysis of 2-AdOClO3 on YOTs in methanol acetone
mixtures passes through a maximum at a 50% content
of MeOH [13]. Kinetic study of the solvolysis of
PhCCl3 in water acetonitrile mixtures showed that
with increasing water content the reaction mechanism
abruptly switches from SN2 to SN1 [14]; this fact
In some cases, comparison of the solvent effects on
rate of solvolysis of various substrates allows conclu-
sions about the reaction mechanism [3]. As a rule,
various binary mixtures of protic solvent with pro-
tic or aprotic solvents are used. Under such condi-
tions, it is often impossible to distinguish the effects
of the polarity and nucleophilicity of the solvents
[4, 5]. This problem can be overcome by using fluori-
nated alcohols whose nucleophilicity is low but the
ionizing power and electrophilicity are high [3, 6].
However, in this case, another problem arises: distin-
guishing the effects of electrophilic and nucleophilic
solvation [6, 7].
To determine the solvation mechanism and reveal
the role of solvation effects, most authors use a one-
(3) or two-parameter (4) Grunwald Winstein equation
[2, 3]. In the first case, the parameter is the ionizing
power of the solvent Y calculated from data on solvol-
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592
PONOMAREV et al.
was explained by changes in the structure of the sol-
vation shell of the substrate.
cavities formed by fluctuations of the liquid density
[35] and occupying about 10% of the liquid volume
[36]. The resulting loose ion pair rapidly transforms
into a solvent-separated ion pair, which (also rapidly)
transforms into reaction products.
Equations (3) and (4) are hardly suitable for eluci-
dating the solvolysis mechanism and revealing the
nature of solvation effects also because of the fact that
parameters Y and N are not independent [2] and can-
not be related to a single specific property of a solvent
[15]. However, these equations are used for this pur-
It is believed [10, 21, 25] that the negative effect
of nucleophilic solvation in monomolecular solvolysis
is due to formation of a cyclic solvation complex of
pose for already 50 years [3, 16, 17], especially ac- a contact ion pair A:
tively in recent studies [3, 18 20].
R
O
H
The role of the nucleophilic effect of a solvent in
R+
O R
monomolecular solvolysis is the subject of especial
controversy [3, 7, 10]. Some authors believe that these
reactions occur without nucleophilic assistance of the
solvent [7, 10, 21, 22]; others have the opposite opin-
ion [3, 23, 24], including the case of solvolysis of
tert-butyl halides.
X
H
A
This process stabilizes the ion pair and hinders
separation of ions. At high temperatures, complex A
is unstable, and the reaction rate becomes independent
of the solvent nucleophilicity [10]; this was demon-
strated for t-BuCl at 120 C [27]. The negative effect
of nucleophilic solvation should decrease as the steric
hindrance to nucleophilic solvation increases [10].
Such hindrance increases in going from water to pri-
mary, secondary, and then tertiary alcohols [37].
Indeed, the greatest steric hindrance arises in nucleo-
philic solvation in t-BuOH [10].
Study by the verdazyl procedure [25] of solvation
effects in monomolecular heterolysis (SN1, E1, sol-
volysis) of various substrates in a wide range of indi-
vidual protic and aprotic solvents revealed no nu-
cleophilic assistance of the solvent [7, 10, 21, 25 29].
This conclusion was based on correlation analysis of
solvation effects for 15 substrates (t-BuX, 1-AdX,
2-bromo-2-methyladamantane, Ph2CHBr, p-MeO
C6H4CCl3, PhCHClMe, Ph2CCl2, PhCMe2Cl, 7 -
bromocholesterol benzoate, p-methoxyneophyl tosy-
late) using Koppel Palm [28] and Kamlet Taft [29]
multiparameter equations including the parameters of
polarity, polarizability, nucleophilicity, electrophilic-
ity, and cohesion density of the solvents. The heterol-
ysis rate is mainly governed by the polarity and elec-
trophilicity parameters. In the case of Ph2CHBr,
PhCHClMe, and 7 -bromocholesterol benzoate, the
rate is independent of the solvent nucleophilicity [30,
31], and in the other cases it decreases with increasing
solvent nucleophilicity [7, 10, 21]. The negative effect
of the nucleophilic solvation is also revealed by other
methods of analysis of solvation effects. This was
demonstrated for solvolysis of p-methoxyneophyl
tosylate [32] and 2-phenyl-2-adamantyl p-nitrobenzo-
ate [33] and for heterolysis of 2-bromo-2-methylpen-
tane in DMF [34].
Thus, the rate of monomolecular solvolysis should
either be independent of the solvent nucleophilicity
or decrease with increasing nucleophilicity, whereas
the rate of bimolecular solvolysis should increase with
increasing nucleophilicity, with the steric effect taking
place in both cases.
We decided to use this difference between the sol-
vation effects in monomolecular and bimolecular sol-
volysis for distinguishing these reactions. It was nec-
essary to compare the rates of SN2 and SN1 (E1)
reactions in water, methanol, ethanol, 2-propanol,
cyclohexanol, and tert-butanol, i.e., in a series of
solvents with increasing steric parameter of substitu-
ent ES [37].
Data on solvolysis in these solvents at 25 C are
available for t-BuBr [38] and 1-AdI [7]; also, there are
incomplete data for t-BuCl [38] and PhCHClMe [31]
(SN1, E1 reactions), and also for PhCOCl [39] and
MeOClO3 [40]. The solvation mechanism for the
latter two substrates is insufficiently clear. Solvolysis
of PhCOCl is believed to occur by the SN2-ion pair
or SN2-intermediate mechanism [2, 10, 41]. In the
first case, the limiting stage involves the nucleophilic
attack of the solvent at the contact ion pair, and in
the second case the contact ion pair is formed in the
limiting stage by the nucleophilic attack of the solvent
According to modern views [10, 21, 25], heterol-
ysis of a covalent bond occurs via successive forma-
tion of three ion pairs: contact, loose, and solvent-
separated:
RX
R+X
R+ 3X
R+ Solv X
Reaction products.
(5)
In the limiting stage, a contact ion pair transforms
into a loose ion pair through interaction with solvent
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SPECIFIC FEATURES OF SOLVATION EFFECTS IN MONOMOLECULAR
593
at the covalent substrate. Also, solvolysis of PhCOCl
can start with addition of the solvent to the C=O
group [42]. The C O bond in MeOClO3 is very polar;
in this case the SN2-intermediate reaction is also
possible [43].
and n-PrBr (SN2 reaction). The kinetics of hydrolysis
of these substrates were studied previously [44, 45].
The kinetic experiments were performed by the
verdazyl method [25] using as internal references
1,3,5-triphenylverdazyl (II) and 1-phenyl-3,5-di(4-
methoxyphenyl)verdazyl (III). The reactions follow
stoichiometric equations (1) and (6).
In this connection, we studied the kinetics of alco-
holysis of allyl bromide (I) (SN2-ion pair reaction [41])
C6H4Z-p
C6H4Z-p
C6H4Z-p
N
N
N
N
N
NH
N
N
N
N
N
HBr + 2
Ph
+
(6)
N+ Br
C6H4Z-p
C6H4Z-p
C6H4Z-p
Ph
Ph
Z = H (II), OMe (III).
The hydrogen bromide released in solvolysis rapid-
ly and quantitatively reacts with verdazyl [25]. The re-
The verdazyl method is usually used for studying
the SN1 and E1 reaction kinetics [25]. In this case,
action rate was monitored by spectrophotometric by the verdazyl rapidly and quantitatively reacts after the
decrease in the indicator concentration (
720 nm).
limiting stage with the intermediate solvent-sepa-
rated ion pair of the substrate. In solvolysis by the
SN2 mechanism, verdazyl should react with the re-
leased acid. To check our method for monitoring the
reaction rate, we determined the rate of hydrolysis of
bromide I in 40% aqueous dioxane at 30 C. The rate
constant determined by this method, (3.94 0.54)
max
The reaction rate is independent of the concentration
and nature of the indicator and is satisfactorily de-
scribed by Eq. (2). The conversion of the substrate in
the kinetic experiments was 0.01 0.002%. The con-
ditions and results of the kinetic experiments are listed
in Tables 1 and 2.
Table 1. Kinetics of solvolysis of allyl bromide in alcohols
1
1
Solvent
Temperature,
C
[II] 104, M
[I] 102, M
k 107, s
kav 107, s
40% aqueous
dioxane
30.0
30.0
30.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
31.0
35.6
40.0
44.2
3.25
2.23
1.26
2.38
2.03
1.48
1.57
1.44
3.72
1.57
1.79
2.24a
1.63a
1.87a
1.03
1.19
1.02
0.968
1.34
1.07
1.18
3.07
7.81
5.22
31.5
22.9
20.7
33.5
24.5
24.4
8.14
8.60
6.13
7.86
2.11
4.51
6.42
6.67
6.83
5.65
5.43
4.90
4.75
3.48
3.59
0.143
0.152
0.256
0.234
0.360
0.411
1.15
1.14
5.10
5.83
4.44
1.80
1.82
1.84
4.65
5.80
7.10
10.8
3.94 0.54
t-BuOH
Cyclohexanol
2-PrOH
EtOH
0.148 0.005
0.245 0.011
0.386 0.026
1.15 0.01
MeOH
5.13 0.19
1.82 0.01
PhCH2OH
a
Experiments were performed with indicator III.
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594
PONOMAREV et al.
Table 2. Kinetics of solvolysis of n-propyl bromide in alcohols
1
1
Solvent
t-BuOH
Temperature,
C
[II] 104, M
[n-PrBr] 102, M
k 109, s
kav 109, s
25.0
28.6
32.5
35.0
38.0
38.0
42.5
42.5
45.5
45.5
25.0
25.0
25.0a
25.0
24.5
28.0
30.0
33.5
35.5
38.0
41.5
41.5
41.5
43.0
25.0`
25.0a
25.0a
25.0a
25.0
25.0
1.91
2.35
2.43
1.38
1.92
1.56
1.13
2.44
1.77
1.69
0.957
1.24
1.45
8.63
11.2
11.5
11.2
6.40
11.5
11.3
11.4
11.3
11.3
10.7
10.9
10.7
0.302
0.372
0.623
0.867
1.40
1.36
1.71
2.07
2.58
2.40
0.619
0.521
0.705
0.752b
0.672
1.41
1.66
2.21
3.03
4.45
5.34
7.31
6.25
7.83
1.46
2.05
1.96
2.43
1.84
1.96
1.38 0.02
1.89 0.18
2.49 0.09
Cyclohexanol
2-PrOH
0.615 0.059
0.648
1.79
1.69
1.28
1.61
1.42
1.50
1.57
1.51
1.30
1.01
0.962
1.57
1.98
1.68
0.992
11.3
11.5
11.4
11.5
10.7
11.4
10.6
10.6
10.4
11.2
9.29
9.26
9.31
9.15
12.4
11.6
6.30 0.67
EtOH
1.82 0.22
2.08 0.24
MeOH
a
b
Experiments were performed with indicator III.
Calculated from the temperature dependence.
7
1
10 s , reasonably agrees with the value obtained
increases in order (7), but the nucleophilicity (B)
decreases.
7
1
titrimetrically, 4.32 10
s
[46].
In Table 3 are given the logk25 values for the se-
lected substrates in water and alcohols, analogous
published data for PhCOCl, t-BuBr, 1-AdI, and
MeOClO3 [6, 38, 39, 40], and some solvent param-
eters [28, 36, 47].
Figure 1 shows the correlations between logk25 of
solvolysis of I in water and alcohols and the similar
quantities for PhCOCl, MeOClO3, n-PrBr, 1-AdI, and
t-BuBr. For the substrates reacting by the SN2 mech-
anism, these correlations are linear (r 0.995); in the
case of 1-AdI and t-BuBr, which react by the SN1
(E1) mechanism, the correlations are nonlinear.
The rate of solvolysis of all the substrates increases
with increasing ionizing power of the solvent:
Thus, comparison of the solvolysis rates in H2O
and primary, secondary, and tertiary alcohols allows
us to distinguish monomolecular and bimolecular
solvolysis.
t-BuOH < cyclohexanol < 2-PrOH < EtOH
< MeOH < H2O.
(7)
In this order both solvent polarity ( ) and electro-
philicity (E), determining its ionizing power Z, in-
crease. The steric parameter of substituents ( ES) also
In all solvolysis reactions, the transition state is
more polar than the initial state [1, 10, 41], as indi-
cated by the growth of the reaction rate with increas-
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SPECIFIC FEATURES OF SOLVATION EFFECTS IN MONOMOLECULAR
595
Table 3. Solvent effect on solvolysis of n-PrBr, CH2=CHCH2Br (I), t-BuBr, 1-AdI, PhCOCl, and MeOClO3; solvent
parameters
logk25
Run
no.
Z,
kJmol
E,
kJmol
B,
kJmol
Solvent
(20)
ES
1
1
1
n-PrBr
I
t-BuBr 1-AdI PhCOCl MeOClO3
1
2
3
4
5
6
H2O
MeOH
EtOH
2-PrOH
Cyclohexanol
t-BuOH
6.57
8.62
8.74
9.12
9.21
9.41
4.80 0.119 2.00
0.15
2.4
3.1
4.3
2.75
3.20
3.50
3.70
400
350
333
319
314
298
80.4
32.7
24.3
18.3
16.0
10.9
91.2
62.3
48.5
33.6
28.9
21.8
1.87
2.61
2.81
2.82
2.89
2.95
1.24
0.00
0.07
0.47
0.71
1.54
6.32 3.90
6.94 5.36
7.41 6.06
7.61 6.11
7.83 6.28
7.08
8.43
9.16
9.59
9.86
4.9
3.90
ing solvent polarity. The sensitivity of the reaction to
solvent polarity should strongly depend on the
solvolysis mechanism. The more polar the transition
state, the stronger should be the effect of the solvent
ionizing power on the reaction rate. Below are given
the differences between the logarithms of the solvol-
ysis rates in water and t-BuOH at 25 C for the chosen
substrates:
mechanism, in which the reaction rate is limited by
nucleophilic attack of the solvent at the solvent-
separated ion pair.
Of particular interest is the fact that the rates of
solvolysis of bromide I and n-PrBr depend on the
solvent polarity to a similar extent. This fact suggests
that solvolysis of n-PrBr occurs by the SN2-interme-
diate mechanism (the limiting stage involves forma-
tion of a contact ion pair by nucleophilic attack of the
solvent at the covalent substrate [41, 50], rather than
by the classical single-stage SN2 mechanism (continu-
ous displacement of the leaving group by the nucleo-
phile [41]). In this case, the polarity of the transition
state should be close to that observed in SN2-ion pair
reactions, which is the case.
1-AdI > PhCHClMe > t-BuCl > t-BuBr
7.9
7.1
6.8
6.2
> PhCOCl > I > n-PrBr > MeOClO3.
5.0 3.0 2.8 1.1
(8)
In monomolecular solvolysis (the first four sub-
strates), the most polar transition state is formed,
involving separation of ions in the contact ion pair.
Indeed, in this case the dependence of the reaction rate
on the solvent polarity is the strongest. The reaction
rate also significantly depends of the natures of the
substrate and leaving group. In solvolysis by the SN2-
ion pair mechanism, at the nucleophilic attack of the
solvent molecule at the contact ion pair, a less polar
transition state is formed. A typical example of such
reactions is solvolysis of bromide I, whose rate
changes by only three orders of magnitude in going
from H2O to t-BuOH [for SN1(E1) reactions, the ef-
fect is by 3 5 orders of magnitude more pronounced].
It is believed that solvolysis of PhCOCl also follows
this mechanism [41, 48]. A strong effect of the sol-
vent on the rate of this reaction, on the one hand,
suggests that reaction follows a pathway other than
addition of the solvent across the C=O bond and, on
the other hand, shows that the polarity of the transi-
tion state in this case is close to that in SN1 reactions.
However, it is known that the rate of solvolysis of
PhCOCl increases with increasing nucleophilicity of
the solvent [48, 49], i.e., an SN2 reaction takes place.
Probably, solvolysis of PhCOCl follows the SN2(C+)
The activation parameters of solvolysis of n-PrBr
and I are listed in Table 4. The increase in the reaction
rate by approximately two orders of magnitude in
going from n-PrBr to I is due to the lower activation
enthalpy, with the S values being similar. This fact
is consistent with the assumptions that the transition
log kI
1
5
4
3
2
1
5
6
2
3
4
5
6
7
8
10
8
6
4
2
0
log kRX
Fig. 1. Correlations between logk of solvolysis of allyl
bromide I and logk of solvolysis of (1) PhCOCl, (2)
t-BuBr, (3) 1-AdI, (4) MeOClO , and (5) n-PrBr. The
3
solvent numbering is the same as in Table 3.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

596
PONOMAREV et al.
Table 4. Activation parameters of solvolysis of n-PrBr and allyl bromide I
n-PrBr
I
Solvent
1
1
1
1
H , kJ mol
S , J mol 1 K
H , kJ mol
93
S , J mol 1 K
25
H2O
102
28
2-PrOH
t-BuOH
95.9 0.5
83.5 0.5
98 2
147 2
69.4 0.5a
141 2a
a
In PhCH OH.
2
states in solvolysis of these substrates are structurally
similar.
t-BuOH is toward larger logk; it increases with in-
creasing nucleophilicity of the solvent. This trend
might be due to nucleophilic assistance of the solvent
to heterolysis [3]. However, such effect could be
expected for bimolecular solvolysis, rather than for
monomolecular solvolysis realized in this case. The
nucleophilic solvation of 1-AdI from the rear side is
impossible, and in the case of t-BuBr it is severely
hindered sterically. The nonlinearity of the logk Z
correlations for monomolecular solvolysis is due to
the negative effect of nucleophilic solvation, decreas-
ing in going from H2O to t-BuOH owing to growing
steric hindrance [7, 10, 21]. This conclusion is con-
firmed by the fact that additional inclusion into the
logk Z correlation of the nucleophilicity parameter
(B) or steric parameter ( ES) leads to excellent or
good two-parameter correlations:
It is interesting that the rate of solvolysis of
MeOClO3, which occurs by the classical SN2 mechan-
ism according to [40] and by the SN2-ion pair mech-
anism according to [43], depends on the solvent polar-
ity to the least extent. This is probably due to the high
polarity of the initial state [40].
Figure 2 shows the dependence of logk of solvol-
ysis of the substrates in water and alcohols on the
parameter Z. The substrates reacting by the SN2
mechanism give excellent linear correlations (r 0.99,
s 0.1). In the case of substrates reacting by the SN1
(E1) mechanism, the dependences are nonlinear, and
the correlation logk Z is only satisfactory (r 0.97,
s
0.6).
logkt-BuBr = 4.18 + 0.0259Z 3.50B; r 0.987, s 0.286,
logk1-AdI = 0.279 + 0.0213Z 5.47B; r 0.997, s 0.178,
In solvolysis of the second group of substrates, the
deviation from the linearity in going from water to
1
1
log k
log k
2
2
2
1
3
2
3
4
4
0
2
4
6
6
5
0
2
6
3
1
4
5
4
6
6
8
8
1
10
300 320 340 360 380 400
Z, kJ mol
1
300 320 340 360 380 400
Z, kJ mol
Fig. 2. Correlation between logk of solvolysis of
(1) PhCOCl, (2) t-BuBr, (3) 1-AdI, (4) MeOClO ,
Fig. 3. Correlation between the rate ( logk) of solvolysis
of t-BuCl at (1) 25 and (2) 120 C and the ionizing power
Z of the solvent (water, alcohols). The solvent number-
ing is the same as in Table 3.
3
(5) bromide I, and (6) n-PrBr at 25 C and the ionizing
power Z of the solvent (water, alcohols). The solvent
numbering is the same as in Table 3.
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SPECIFIC FEATURES OF SOLVATION EFFECTS IN MONOMOLECULAR
logkt-BuBr = 4.02 + 0.105Z + 1.84ES; r 0.979, s 0.434, Chem., 1977, vol. 14, pp. 1 67.
597
3. Bentley, T.W. and Llewellyn, G., Prog. Phys. Org.
Chem., 1990, vol. 17, pp. 121 158.
logk1-AdI = 52.2 + 0.131Z + 2.21ES; r 0.987, s 0.364.
4. Kevill, D.N., Kamil, A., and Anderson, S.W., Tetra-
hedron Lett., 1982, vol. 23, no. 45, pp. 4635 4638.
The fact that the parameter B appears in the corre-
lation equation with the minus sign is consistent with
the decelerating effect of nucleophilic solvation on
solvolysis of t-BuBr and 1-AdI, and the influence of
ES reflects the fact that the negative effect of nucleo-
philic solvation decreases in going from water to
t-BuOH.
5. Doherty, R.M., Abraham, M.H., Harris, J.M.,
Taft, R.W., and Kamlet, R.W., J. Org. Chem., 1986,
vol. 51, no. 25, pp. 4872 4874.
6. Raber, D.J., Neal, W.C., Dukes, M.D., Harris, J.M.,
and Mount, D.L., J. Am. Chem. Soc., 1978, vol. 100,
no. 12, pp. 8137 8142.
A similar pattern of the solvent effect is observed
with monomolecular solvolysis of t-BuCl in water and
alcohols at 25 C [38]. However, at 120 C the logk Z
correlation for t-BuCl becomes linear (Fig. 3; data of
[51]). This is due to instability of solvation complex
A at high temperature, so that the negative effect of
nucleophilic solvation does not take place.
7. Dvorko, G.F., Tarasenko, P.V., Ponomareva, E.A.,
and Kulik, N.I., Zh. Org. Khim., 1989, vol. 25, no. 5,
pp. 922 937.
8. Roberts, D.D., J. Org. Chem., 1984, vol. 45, no. 13,
pp. 2521 2527.
9. Allen, A.D., Ambridge, I.Ch., Michael, H., Nuir, R.J.,
and Tidwell, Th.T., J. Am. Chem. Soc., 1983, vol. 105,
no. 8, pp. 2343 2348.
Thus, a question arises: Why is the logk Z correla-
tion for SN2 reactions linear if the reaction rate also
depends on the solvent nucleophilicity and the Z B
correlation is nonlinear (r 0.969, s 8.9)? However, if
we additionally take into account the steric constant,
we obtain an excellent two-parameter correlation (9):
10. Dvorko, G.F., Ponomarev, N.E., and Ponomare-
va, E.A., Zh. Obshch. Khim., 1999, vol. 69, no. 11,
pp. 1835 1851.
11. Kevill, D.N., Bahari, M.S., and Anderson, S.W.,
J. Am. Chem. Soc., 1984, vol. 106, no. 10, pp. 2895
2901.
Z = 468
47.7B + 20.1ES; r 0.990, s 3.8. (9)
12. Luton, P.R. and Whiting, M.C., J. Chem. Soc., Perkin
Trans. 2, 1979, no. 11, pp. 1507 1511.
The effects of the parameters B and ES are mutu-
ally compensating. This fact explains why the logk Z
correlation is linear and shows that in SN2 reactions
the steric hindrance to nucleophilic attack at the co-
valent substrate is also significant.
13. Kevill, D.N. and Bahari, M.S., Chem. Commun., 1972,
no. 11, pp. 572 574.
14. Dvorko, G.F. and Zhovtyak, V.N., Zh. Obshch. Khim.,
1990, vol. 60, no. 4, pp. 880 891.
Thus, the logk Z correlations for solvolysis reac-
tions in water and primary, secondary, and tertiary
alcohols give insight into the reaction mechanism and
into the nature of solvation effects.
15. Abraham, M.H., Doherty, M.R., Kamlet, M.J., Har-
ris, J.M., and Taft, R.W., J. Chem. Soc., Perkin Trans.
2, 1987, no. 6, pp. 1097 1101.
16. Grunwald, E. and Winstein, S., J. Am. Chem. Soc.,
EXPERIMENTAL
1948, vol. 70, no. 3, pp. 846 854.
17. Kevill, D.N., Anderson, S.W., and Ismail, N.H.J.,
Allyl bromide and n-PrBr were dried over K2CO3
and purified by fractional distillation in a 45-cm
packed column (8 TP). Solvents of chemically pure
(methanol, 2-propanol) or analytically pure (cyclohex-
anol, tert-butanol) grade were refluxed for 8 10 h
over CaO, fractionated, and rectified. Rectified alco-
hol was dried over anhydrous CuSO4, refluxed over
CaO for 10 h, fractionated, and rectified. Kinetic ex-
periments were performed in a temperature-controlled
cell of an SF-26 spectrophotometer.
J. Org. Chem., 1996, vol. 61, no. 18, pp. 7256 7262.
18. Kevill, D.N. and Upadhyay, Y., J. Phys. Org. Chem.,
1997, vol. 10, no. 8, pp. 600 606.
19. Huang, X.C., Tanaka, K.S.-E., and Bennet, A.I.,
J. Am. Chem. Soc., 1998, vol. 120, no. 7, pp. 1405
1409.
20. Liu, K.T., Liu, Y.S., and Tsao, M.L., J. Phys. Org.
Chem., 1998, vol. 11, no. 3, pp. 223 229.
21. Dvorko, G.F. and Ponomar’ova, E.O., Ukr. Khim. Zh.,
1993, vol. 59, no. 11, pp. 1190 1201.
22. Farcasin, O., Jahme, J., and Ruchardt, Ch., J. Am.
Chem. Soc., 1985, vol. 107, no. 20, pp. 5717 5722.
23. Fawcett, W.F. and Krygowsky, T.M., Aust. J. Chem.,
1975, vol. 28, no. 10, pp. 2115 2124.
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