Kinetics and mechanism of monomolecular heterolysis of cage-like compounds: XVIII. Solvent effect on the rate of heterolysis of 3-bromocyclohexene. Correlation analysis of solvation effects

Article abstract of DOI:10.1023/B:RUJO.0000036068.58478.e4

The kinetics of E1 dehydrobromination of 3-bromocyclohexene in 23 aprotic and 9 protic solvents were studied by the verdazyl technique. The reaction rate is described by the polarity, electrophilicity, and ionizing power parameters of the solvent. Nucleop

Full text of DOI:10.1023/B:RUJO.0000036068.58478.e4

Russian Journal of Organic Chemistry, Vol. 40, No. 4, 2004, pp. 489–496. Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 4, 2004,
pp. 520–527.
Original Russian Text Copyright © 2004 by Ponomarev, Stambirskii, Dvorko, Bazil’chuk.
Kinetics and Mechanism of Monomolecular Heterolysis
of Cage-Like Compounds: XVIII. Solvent Effect on the Rate
of Heterolysis of 3-Bromocyclohexene. Correlation Analysis
of Solvation Effects
N. E. Ponomarev, M. V. Stambirskii, G. F. Dvorko, and A. V. Bazil’chuk
“Kievskii politekhnicheskii institut” National Technical University of Ukraine,
pr. Peremogi 37, Kiev, 03056 Ukraine
Received April 21, 2003
Abstract—The kinetics of E1 dehydrobromination of 3-bromocyclohexene in 23 aprotic and 9 protic solvents
were studied by the verdazyl technique. The reaction rate is described by the polarity, electrophilicity, and
ionizing power parameters of the solvent. Nucleophilicity, polarizability, and cohesion parameters of the
solvent do not affect the reaction rate. The effects of equilibrium and nonequilibrium solvation of the transition
state are discussed.
Solvent effects on the rate of monomolecular
heterolysis (SN1, E1, solvolysis) are usually examined
using tertiary substrates as models, most frequently
tert-alkyl and cage-like derivatives [1–3]. Rear nucleo-
philic attack on such compounds is strongly hindered
or impossible, which essentially affects solvation and
complicates interpretation of solvation effects. Nucleo-
philic effect of solvents is a matter of continuous
discussions. Some authors believe [1, 3–5] that
heterolysis of tert-alkyl derivatives involves nucleo-
philic assistance by the solvent, while the others
adhere to the opinion that such assistance is absent
[2, 5–8]. Detailed studies of solvation effects in the
heterolysis of tert-butyl halides [2, 9–11], adamantane
derivatives substituted at the bridgehead position
[12–14], cumyl chloride [15], 2-methyl-2-(p-methoxy-
phenyl)propyl p-toluenesulfonate [16], and 2-bromo-
2-methyladamantane [17] showed that the rate of
heterolysis of tertiary derivatives decreases as the
solvent nucleophilicity rises [2, 18].
The negative effect of nucleophilic solvation
indicates nonequilibrium solvation of the transition
state [19, 20], i.e., the solvate shells of the initial and
transition states have different structures. Equilibrium
solvation of the transition state should increase the
reaction rate [21]. Kim et al. [20, 22] showed that
a satisfactory agreement (within an order of magni-
tude) between the experimental and calculated rate
constants for heterolysis of tert-butyl halides in
different solvents can be attained only with account
taken of nonequilibrium solvation. However, the time
necessary for formation of transition state (~10–13 s)
[23] is insufficient for structural reorganization of
the substrate solvate shell. This process takes 10–10 to
10–11 s [24, 25]. Therefore, nonequilibrium solvation
was interpreted in terms of orientational polarization
of the solvent [20, 22], which changes fairly quickly
(~10–16 s) and may affect the formation of transition
state. These data suggest the existence of non-
equilibrium solvation of transition state, but they do
not explain negative effect of nucleophilic solvation.
Reduction of the rate of heterolysis with rise in
solvent nucleophilicity may be understood on the as-
sumption that nucleophilic solvation involves an inter-
mediate which is formed prior to the rate-determining
stage [2]. Such intermediate may be a contact ion pair.
Nucleophilic solvation stabilizes the intermediate and
hampers nucleofuge departure according to the SN1
mechanism [26, 27]. If this is the case, a question
arises so as to which stage of the heterolysis process is
rate-determining? It is known that product formation
does not affect the reaction rate and that the latter is
described by the first-order kinetic equation v = k[RX]
[2, 28]. It is commonly assumed that the rate-
determining stage is formation of solvent-separated
ion pair from the substrate [29–31]. Therefore,
the rate of heterolysis should increase with rise in
solvent nucleophilicity (nucleophilic assistance by the
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PONOMAREV et al.
490
solvent). However, detailed analysis of solvation
effects using multiparameter equations based on the
linear free energy relationship showed that heterolysis
of tertiary substrates is not accompanied by nucleo-
philic solvent assistance [2, 6–18]. A conclusion was
drawn that solvent-separated ion pair is formed after
the rate-determining stage [2]. This means that the
rate-determining stage should occupy an intermediate
place between contact ion pair and solvent-separated
ion pair on the reaction coordinate. The transformation
of contact ion pair into solvent-separated ion pair is
believed to occur in two steps: in the first step, ions in
a contact ion pair move apart, and in the second step,
solvent molecule occupies the interionic space
[2, 18, 32]. Quantum-chemical analysis of ion separa-
tion in liquid showed that one more intermediate is
present on the reaction coordinate between contact ion
pair and solvent-separated ion pair. It was termed
a contact ion pair which begins to divide [33, 34].
The nature of solvation effects in the heterolysis of
tertiary substrates has been well documented [2, 9–18].
Here, the polarity and electrophilicity (or ionizing
power) of a solvent, as well as cohesion, act to increase
the reaction rate, whereas nucleophilicity and polariz-
ability reduce it. The effects of polarizability and
cohesion are related to the negative nucleophilic
solvation effect: the greater the polarizability, the
stronger the nucleophilic solvation of contact ion pair;
the greater the energy of solvent self-association, the
more difficult is to extract a solvent molecule for
nucleophilic solvation of intermediate [2]. Negative
nucleophilic solvation effects are seen most clearly
in protic solvents, presumably due to formation of
hydrogen bonds between molecules which are respons-
ible for nucleophilic solvation of carbocation and
electrophilic solvation of nucleofuge.
Solvation effects in the heterolysis of secondary
substrates have been studied poorly; even less
information is available for primary derivatives. There
are detailed data on solvent effects on the rate of
heterolysis of Ph2CHBr [13] and limited data for
the heterolysis of PhCHClMe [40] and 7α-bromo-
cholesterol benzoate (I) [41].
Taking into account the existence of voids in
liquids [35, 36] (which occupy up to ~10% by volume
[37]), monomolecular heterolytic dissociation of a co-
valent bond [2, 18] may be represented as successive
formation of three types of ion pairs: contact (A), loose
(B), and solvent-separated (C).
Me
Me
RX
R+ X–
R+··· X–
Me
Me
A
B
Me
R+|Solv| X–
Products
C
BzO
Br
I
The reaction begins with formation of contact ion
pair which interacts with a solvent cavity in the rate-
determining stage. As a result, loose ion pair is formed
and is quickly converted into solvent-separated ion
pair, and the latter is transformed (also quickly) into
the products.
ax
ax'
eq
eq'
eq'
eq
ax'
ax
The above mechanistic considerations are based on
the results of studies performed on tertiary substrates
which cannot undergo substitution by the SN2
mechanism, for rear nucleophilic attack on such sub-
strates is impossible [1, 28]. The SN1 (E1) mechanism
implies that the initial molecule occur in equilibrium
with the corresponding ion pair and that the lifetime of
the latter be sufficient (>10–10–10–11 s) to react with
nucleophile. Therefore, not only tertiary but also
activated secondary (Ph2CHX [38]) and even primary
(p-MeOC6H4CH2Cl [39]) derivatives can react accord-
ing to the SN1 pattern.
II
The rate of heterolysis of Ph2CHBr depends on the
solvent polarity and electrophilicity and is independent
of the solvent nucleophilicity, which is typical of SN1
reactions [13]. However, in keeping with published
data, the substitution in benzhydryl halides [42, 43]
and 1-phenylethyl halides [44, 45] can follow SN2
mechanism. Hunziker and Mullner [46] presumed that
dehydrobromination of compound I occurs according
to the E2 mechanism. The rate of heterolysis of
Ph2CCl2 [47] and p-methoxybenzotrichloride [48], for
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 4 2004

KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS ... XVIII.
491
which rear nucleophilic solvation is possible, decreases
as the solvent nucleophilicity rises.
parameters necessary for analysis of solvation effects.
In the correlation analysis we used Koppel’–Pal’m
equation (3) [6, 56] with the cohesion energy density
δ2 as an additional parameter, Kamlet–Taft equation
(4) [7, 57], and Eq. (5).
The above stated prompted us to study in detail
solvation effects in the heterolysis of compounds
which are capable of reacting according to the SN1
(E1) mechanism and are readily accessible for rear
nucleophilic attack. An example of such substrates is
3-bromocyclohexene. It is known that the most stable
cyclohexene conformer is half-chair (II), where the
bonds in the allylic positions, pseudoaxial (ax') and
pseudoequatorial (eq'), form a torsion angle of 59°
[49]. Therefore, the corresponding carbon atoms are
readily accessible for rear nucleophilic attack.
Heterolysis of 3-bromocyclohexene can be regarded as
a model of dehydrobromination of compound I [50],
i.e., a reaction included in the large-scale synthesis of
vitamin D3 [51].
ε – 1
logk = a0 + a1─── + a2──── + a3E + a4B + a5── ; (3)
ε + 1 100
n2 + 1
n2 – 1
δ2
δ2
100
logk = a6 + a1π* + a2α + a3β + a4── ;
(4)
(5)
n2 – 1
n2 + 1
δ2
100
logk = a0 + a1ET (or Z)+ a2──── + a3B + a4── .
Here, ε is the dielectric constant, n is the refractive
index, E, α and V, β are, respectively, the empirical
electrophilicity and nucleophilicity parameters, π* is
the dipolarity parameter (polarity + polarizability), ET
and Z are the solvatochromic parameters characterizing
ionizing power, and δ2 = (∆Hm – RT)/Vm is the
parameter characterizing the energy of self-association
[58] (where ∆Hm is the molar vaporization energy and
Vm is the molar volume). The solvent parameters were
taken from [56, 59, 60].
There are published data on the kinetics of
heterolysis of 3-bromocyclohexene in acetonitrile [52]
and nitrobenzene [53, 54] at 25°C. We performed
kinetic experiments by the verdazyl technique [55]
which employs 1,3,5-triphenylverdazyl (Vd ·) as
internal indicator. Verdazyl radical quickly and quan-
titatively reacts with the substrate solvent-separated
ion pair to give cyclohexadiene, verdazylium bromide
(Vd+ Br–), and leuco verdazyl (VdH) according to
scheme (1).
Application of Eqs. (3)–(5) to the whole set of
solvents gave regression equations (6)–(9):
logk = –(9.64±0.32) + (2.72±0.43)f(ε)
+ (0.0700±0.0040)E;
R = 0.972, S = 0.38, F = 246 (1.85), n = 32;
Br
Ph
·
N
N
N
(6)
+
2
N
logk = –(9.84±0.37) + (2.70±0.34)α + (0.572±0.082)δ2;
Ph
Ph
R = 0.970, S = 0.38, F = 482 (1.87), n = 32;
(7)
Vd
·
(1)
logk = –(17.7±0.5) + (0.0635±0.0030)ET;
R = 0.970, S = 0.38, F = 482 (1.87), n = 32;
Ph
Ph
(8)
(9)
N
N
N
N
N
N
NH
N
Br–
Ph
+
+
logk = –(19.5±0.4) + (0.0467±0.0020)Z;
R = 0.984, S = 0.28, F = 897 (1.87), n = 32.
Ph
Ph
Ph
Vd+ Br–
VdH
Hereinafter, f(ε) = (ε – 1)/(ε + 1), and F is the
apparent and critical (in parentheses) Fisher consist-
ency [61] (a model is considered to be reliable when
the former exceeds the latter).
The reaction rate was monitored by spectro-
photometry, following decrease in the concentration of
Vd· (λmax ~720 nm). The reaction rate is satisfactorily
described by first-order kinetic equation (2).
The correlations drawn on the basis of Eqs. (3) and
(5) show that the reaction rate depends on the ionizing
power of the solvent or on its polarity and electro-
philicity, which is the same (for the ionizing power is
satisfactorily described by the polarity and electro-
philicity parameters) [6, 59].
[Vd]
v = –
= k[C6H9Br].
(2)
2 τ
Table contains our experimental and published rate
constants measured at 25°C in 32 solvents and solvent
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 4 2004

PONOMAREV et al.
Solvent effect on the rate of heterolysis of 3-bromocyclohexene and solvent parameters
492
Z,
ET,
E,
B,
δ2/100,
No.
Solvent
k25, s–1a
–logk25 ε20
n2D0
π*
α
β
kJ/mol kJ/mol kJ/mol kJ/mol kJ/l
1 MeOH
(5.37±0.33)×10–4
3.27 32.70 1.329 350
232
217
212
206
210
197
183
196
172
189
195
62.0
49.0
44.0
39.0
43.0
30.0
21.0
29.0
19.0
14.0
21.0
2.61
2.81
2.67
2.82
2.76
2.82
2.95
2.89
3.03
4.33
2.18
8.58 0.60 0.98 0.66
6.78 0.54 0.86 0.75
5.99 0.52 0.84 0.90
5.56 0.48 0.76 0.84
5.41 0.47 0.84 0.84
5.19 0.40 0.69 0.80
4.60 0.41 0.42 0.93
5.15 0.45 0.66 0.84
4.60 0.40 0.28 0.93
7.08 1.00 0.00 0.76
7.40 0.83 0.00 0.40
2 EtOH
(1.41±0.05) ×10–4 3.85 24.30 1.361 333
(5.25±0.16) ×10–5 4.28 20.20 1.385 328
(2.51±0.12) ×10–5 4.60 18.30 1.377 319
(5.01±0.18) ×10–5 4.30 17.10 1.399 325
(1.29±0.08) ×10–5 4.89 16.60 1.398 316
(6.92±0.21) ×10–6 5.16 10.90 1.385 298
3 PrOH
4 i-PrOH
5 BuOH
6 s-BuOH
7 t-BuOH
8 Cyclohexanol (1.15±0.03) ×10–5 4.94 15.00 1.467 314
9 t-AmOH
(2.09±0.01) ×10–6 5.68 05.80 1.386 296
(2.34±0.05) ×10–6 5.63 46.70 1.479 294
(4.47±0.17) ×10–6 5.35 62.90 1.421 303
10 DMSO
11 Propylene
carbonate
12 γ-Butyro-
(2.82±0.12) ×10–6 5.55 41.00 1.437 290
185
12.0
2.48
6.95 0.87 0.00 0.49
lactone
13 Acetonitrile
5.59b 35.90 1.344 298
191
173
173
176
173
21.0
0.0
0.0
8.5
5.4
1.91
0.80
1.85
2.68
2.50
5.86 0.75 0.19 0.40
5.11 0.01 0.00 0.30
5.15 0.90 0.00 0.37
3.88 0.71 0.08 0.43
3.61 0.67 0.06 0.48
14 Nitrobenzene (1.02±0.02) ×10–7 c 6.99 36.10 1.551 274
15 Benzonitrile (1.10±0.03) ×10–7 6.96 25.20 1.528 272
16 Acetone
(1.70±0.08) ×10–7 6.77 21.40 1.359 275
17 Methyl ethyl (4.37±0.06) ×10–8 7.36 18.90 1.379 268
ketone
18 Acetophenone (5.13±0.02) ×10–8 7.29 18.20 1.534 270
170
166
0.0
0.0
2.42
2.89
4.33 0.90 0.04 0.49
4.08 0.76 0.00 0.53
19 Cyclo-
hexanone
(3.80±0.04) ×10–8 7.42 16.00 1.451 265
20 Ethyl acetate (6.03±0.10) ×10–8 7.22 06.00 1.372 258
159
156
6.7
0.0
2.17
3.43
3.39 0.55 0.00 0.45
3.61 0.58 0.00 0.55
21 Tetrahydro- (1.62±0.03) ×10–8 7.79 07.39 1.408 246
furan
22 1,4-Dioxane (1.55±0.09) ×10–8 7.81 02.27 1.422 270
150
153
148
0.0
0.0
0.0
2.84
1.89
1.06
4.20 0.55 0.00 0.37
3.80 0.69 0.00 0.30
4.04 0.66 0.00 0.13
23 Phenetole
(4.37±0.12) ×10–9 8.36 04.22 1.507 246
(3.72±0.04) ×10–9 8.43 03.69 1.581 241
24 Diphenyl
ether
25 1,2-Dichloro- (8.91±0.03) ×10–8 7.05 10.40 1.551 265
ethane
173
154
159
9.6
0.0
0.0
0.48
0.45
0.33
4.12 0.81 0.00 0.10
3.76 0.71 0.00 0.07
4.20 0.80 0.00 0.03
26 Chloroben-
zene
(9.55±0.05) ×10–9 8.02 05.74 1.524 249
27 1,2-Dichloro- (1.95±0.04) ×10–9 7.71 10.40 1.551 251
benzene
28 Chloroform
(1.70±0.01) ×10–7 6.77 04.89 1.446 264
(1.74±0.03) ×10–7 6.76 09.02 1.424 269
163
170
14.0
11.0
0.17
0.28
3.57 0.58 0.20 0.10
4.12 0.82 0.13 0.10
29 Methylene
chloride
30 Benzene
31 Toluene
32 p-Xylene
(2.01±0.22) ×10–9 8.70 02.27 1.501 226
(5.01±0.22) ×10–9 8.30 02.43 1.497 233
(9.55±0.40) ×10–10 9.02 02.37 1.497 227
143
142
138
0.0
0.0
0.0
0.57
0.69
0.81
3.53 0.59 0.00 0.10
3.31 0.54 0.00 0.11
3.24 0.43 0.00 0.12
a
Average value from 2–5 measurements.
Data of [53].
b
c
k = (1.00±0.04)×10–7 s–1 [54].
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS ... XVIII.
493
(17)
(18)
logk = –(17.6±0.7) + (0.0612±0.0040)ET;
R = 0.956, S = 0.31, F = 221 (2.08), n = 23;
For 32 solvents, we obtained Eq. (10):
ET = (1118±3) + (0.988±0.039)E + (54.8±3.8)f(ε);
R = 0.991, S = 3.35, F = 763 (1.85), n = 32.
logk = –(19.4±0.7) + (0.0464±0.0030)Z;
R = 0.970, S = 0.26, F = 318 (2.10), n = 22.
(10)
The polarizability, nucleophilicity, and cohesion
energy density parameters are insignificant. The cor-
relation factor R for five-parameter Eq. (3) is 0.979,
while for four-parameter Eq. (5), R = 0.975 (0.986).
Figure 1 shows the correlations between logk and ET
and between logk and Z. Using Eq. (4), we obtained
a two-parameter correlation with the electrophilicity
and cohesion energy density. This may be due to com-
plex character of the parameter π*, which sometimes
leads to artefacts [10, 11].
The quality of correlations (15)–(18) is appreciably
lower, presumably due to greater diversity of solvation
effects including dipolar solvation. Satisfactory cor-
relations (16) and (17) were obtained only when two
(1,2-dichloroethane and ethyl acetate) and one (di-
oxane) solvents, respectively, were excluded.
A characteristic feature of heterolysis of tertiary
substrates is that the correlations logk—ET(Z) consist
of two straight lines: one for protic solvents, and the
other, for aprotic. This is explained by the fact that
the negative nucleophilic solvation effect is most
pronounced in protic solvents; therefore, the reaction
rate is lower than might be expected from the linear
dependence for aprotic solvents [2]. In the case of
3-bromocyclohexene, where solvent nucleophilicity
does not affect the reaction rate, all points for protic
and aprotic solvents satisfactorily fall onto a single
straight line (Fig. 1). An analogous pattern was ob-
served in the heterolysis of benzhydryl bromide [14].
Correlations like (6)–(9) are also typical of the
heterolysis of tertiary substrates in aprotic solvents or
in a set consisting of protic and aprotic solvents, the
latter prevailing (as in our case). However, correlations
for heterolysis of tertiary substrates in protic solvents
usually include the nucleophilicity and polarizability
parameters taken with the minus sign. Therefore, we
performed correlation analysis of solvation effects
separately for protic solvents. On the basis of Eqs. (3)–
(5) we obtained correlations (11)–(14).
Figure 2 illustrates the difference between secon-
dary and tertiary substrates. The logk(C6H9Br) values
logk = –(7.65±0.75) + (1.70±1.02)f(ε)
+ (0.0444±0.0050)E;
R = 0.990, S = 0.12, F = 142 (4.15), n = 9;
(11)
–logk(C6H9Br)
logk = –(8.37±0.52) + (5.81±1.61)π* + (1.51±0.49)α;
R = 0.978, S = 0.18, F = 65 (4.15), n = 9;
(12)
logk = –(12.5±0.6) + (0.0393±0.0030)ET;
R = 0.983, S = 0.14, F = 202 (3.73), n = 9;
(13)
(14)
logk = –(18.0±1.1) + (0.0419±0.0030)Z;
R = 0.977, S = 0.17, F = 146 (3.73), n = 9.
The rate of heterolysis of 3-bromocyclohexene in
protic solvents is excellently or well described by the
polarity (dipolarity) and electrophilicity parameters.
Nucleophilicity of solvents does not affect the reaction
rate. As applied to aprotic solvents, we obtained the
same correlations [Eqs. (15)–(18)] as for the whole set
of solvents:
logk = –(9.63±0.29) + (2.53±0.41)f(ε)
+ (0.0870±0.0120)E;
R = 0.949, S = 0.34, F = 91.4 (2.10), n = 23;
logk = –(10.8±0.3) + (7.50±1.07)α + (0.662±0.63)δ2;
R = 0.954, S = 0.34, F = 90.1 (2.28), n = 21; (16)
(15)
Fig. 1. Correlations between the ionizing power ET (Z) of
solvents and the rate of heterolysis of 3-bromocyclohexene
at 25°C. For solvent numbering, see table.
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PONOMAREV et al.
494
–logk(C6H9Br)
of a covalent tertiary substrate is impossible because of
steric factor; it appears at the stage of formation of
contact ion pair, which stabilizes the intermediate and
hampers departure of nucleofuge according to the SN1
mechanism. This is clearly seen in protic solvents due
to participation of hydrogen bonds. The negative effect
of nucleophilic solvation may be regarded as a result
of nonequilibrium solvation of transition state.
Secondary substrates, especially those like 3-bromo-
cyclohexane, are readily accessible for nucleophilic
solvation from the rear. There are stereochemical
proofs for rear nucleophilic solvation of secondary
substrates which react according to the SN1 mechanism
[44, 62, 63]. This sort of solvation hampers nucleo-
philic attack from the rear, and solvolysis of optically
active compounds gives products with the same con-
figuration. An example is the hydrolysis of 1-chloro-1-
phenylethane in the presence of nitriles [44]. Due to
nucleophilic solvation, phenolysis of optically active
substrates occurs with partial or complete retention of
configuration [44, 62].
–logk(Ph2CHBr) or –logk(2-Br-2-MeAd)
The fact that the rate of heterolysis of secondary
substrates does not depend on the solvent nucleo-
philicity is difficult to explain, though it is consistent
with the up-to-date interpretation of the SN1 (E1)
mechanism [64, 65]. According to Ingold, the rate-
determining stage of these reactions is formation of
carbocation which quickly undergoes nucleophilic
attack; here, the reaction rate should depend on the
energy of solvation of ions thus formed [28]. There-
fore, increase in the solvent nucleophilicity should
favor the process. Our data indicate that increase in the
solvent nucleophilicity either reduces the reaction rate
or does not affect it.
Fig. 2. Correlations logk(C6H9Br)—logk(Ph2CHBr) (light
circles) and logk(C6H9Br)—logk(2-Br-2-MeAd) (dark
circles). For solvent numbering, see table.
correlate well [Eq. (19)] with logk(Ph2CHBr) values
for protic and aprotic solvents.
log k(C6H9Br) = (1.7±0.15) + (0.719±0.023)log k(Ph2CHBr);
R = 0.989, S = 0.24, F = 962 (2.08), n = 23.
(19)
By contrast, no correlation was found between
logk(C6H9Br) and the corresponding data for 2-bromo-
2-methyladamantane [17] for the whole set of solvents
(R = 0.83); treatment of the data for protic and aprotic
solvents separately gave satisfactory correlations (20)
and (21), respectively.
It is reasonable to presume that nucleophilic solva-
tion involves a covalent substrate (initial state) and
that this solvation does not change upon formation of
transition state [2], i.e., we have equilibrium solvation
of transition state. Therefore, no solvent nucleophilic-
ity effect is observed in cases when rear nucleophilic
solvation is possible.
logk(C6H9Br) = (0.740±0.399)
+ (0.733±0.082)logk(2-Br-2-MeAd);
R = 0.973, S = 0.17, F = 87.9 (4.95), n = 7;
(20)
The importance of nucleophilic solvation is much
more general than it may seem at first glance. One
of the most widely used methods for quantitative
estimation of solvent effects on the rate of heterolytic
reactions (Grünwald–Winstein equations) utilizes tert-
butyl chloride as reference substrate (later on, ada-
mantyl derivatives were also used) [1, 4]. However,
recent studies have shown that increase in the sol-
vent nucleophilicity leads to decrease in the rate of
logk(C6H9Br) = (3.18±0.29)
+ (0.685±0.047)logk(2-Br-2-MeAd);
R = 0.962, S = 0.30, F = 208 (2.28), n = 19.
(21)
Thus, in the heterolysis of tertiary substrates,
increase in the solvent nucleophilicity reduces the
reaction rate, while in the heterolysis of secondary
substrates this parameter does not affect the rate of the
process. The reason is that rear nucleophilic solvation
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KINETICS AND MECHANISM OF MONOMOLECULAR HETEROLYSIS ... XVIII.
495
heterolysis of tertiary substrates, which inevitably
distorts comparative pattern of solvation effects. In
fact, in many cases application of the Grünwald–
Winstein equations gave unsatisfactory results
[1, 66, 67]. More appropriate reference substrates are
those for which solvent nucleophilicity does not affect
the rate of heterolysis and the logk—ET(Z) dependence
is linear. Such compounds are, e.g., benzhydryl
bromide and especially 3-bromocyclohexene which
is more accessible for nucleophilic solvation from
the rear.
cold carbon tetrachloride, the filtrate was evaporated
under reduced pressure (18 mm), and the residue was
twice distilled in a vacuum. Yield 18.6 g (62%),
bp 37–38°C (4 mm), n2D0 = 1.5292; published data [70]:
bp 51°C (10 mm), n2D0 = 1.5290.
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