Kinetics and mechanism of unimolecular heterolysis of framework compounds: XX. Solvation and steric effects in heterolysis of 2-halo-2-alkyladamantanes in sulfolane and butanol

Article abstract of DOI:10.1134/S1070428007020066

Hetrolysis rate of 2-halo-2-phenyladamantanes in BuOH is 1000 times higher than the heterolysis rate of 2-halo-2-methyladamantanes. The heterolysis rate in sulfolane does not depend on the substituent, but the phenyl group exhibits a negative steric effect.

Full text of DOI:10.1134/S1070428007020066

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2007, Vol. 43, No. 2, pp. 188−191. © Pleiades Publishing, Ltd. 2007.
Original Russian Text © G.F. Dvorko, A.I. Vasil’kevich, K.V. Mikhal’chuk, I.V. Koshchii, 2007, published in Zhurnal Organicheskoi Khimii, 2007,
Vol. 43, No. 2, pp. 196−199.
Kinetics and Mechanism of Unimolecular Heterolysis
of Framework Compounds: XX.*
Solvation and Steric Effects in Heterolysis
of 2-Halo-2-alkyladamantanes in Sulfolane and Butanol
G. F. Dvorko,A. I. Vasil’kevich, K. V. Mikhal’chuk, and I. V. Koshchii
National Technical University of the Ukraine “Kiev Politechnical Institute”, Kiev, 03056 Ukraine
e-mail: m_ ponomaryov@ukr.net
Received May 3, 2005
Abstract—Hetrolysis rate of 2-halo-2-phenyladamantanes in BuOH is 1000 times higher than the heterolysis rate
of 2-halo-2-methyladamantanes. The heterolysis rate in sulfolane does not depend on the substituent, but the
phenyl group exhibits a negative steric effect.
DOI: 10.1134/S1070428007020066
The rate of unimolecular heterolysis (SN1, Ε1,
solvolysis) is governed by the ionization of a covalent bond
occurring through successive formation of three ion pairs:
contact (I), space-separated (II), and solvent-separated
(III) [2, 3].
unimolecular heterolysis strongly depends on the solvent
nature. The ion pair arising in the heterolysis causes
structurization of the solvent around this intermediate
(electrostriction effect [6]) thus decreasing the ΔS≠ value.
In protic solvents structurized by hydrogen bonds
formation the electrostriction damages the structure of
the solvent thus increasing the ΔS≠ value. In aprotic
solvents the activation entropy is as a rule negative, and
its value indicates the additional number of solvent
molecules involved in the solvation of the transition state
as compared to that of the initial state, and the steric
hindrances caused by this solvation. In protic solvents at
prevailing electrostriction effect ΔS≠ < 0, and at
predominant damaging the solvent structure ΔS≠ > 0 [2].
Steric effects can both decrease and increase the
heterolysis rate [7]. For instance, in going from 2-methyl-
2-adamantyl tosylate to 2-tert-butyl-2-adamantyl tosylate
the solvolysis rate in 80% aqueous ethanol became 5
orders of magnitude larger [8]. Similar steric effects
should strongly affect the ΔS≠ and ΔH≠ values.
In the limiting stage ion pair I interacts with a void in
the solvent (the voids constitute approximately 10% of
the liquid volume [4]). Thus forms ion pair II that rapidly
transforms into ion pair III, and the latter quickly provides
the reaction products.
The rate of these reaction is described by a first order
kinetic equation v = k[RX]. The rate is strongly
dependent on the polarity of the solvent: For instance, in
going from hexane to water the heterolysis rate of t-BuCl
increased by a factor of 14 orders of magnitude [5]. This
is caused by a strong solvation of the polar transition state.
The information on the nature and strength of solvation
effects can be gained by comparative analysis of the
activation parameters in various heterolysis reactions.
We planned to investigate the solvent effect and the
influence of a hydrocarbon substituent in heterolysis of
2-methyl-2-chloroadamantane (IV), 2-phenyl-2-chloro-
adamantane (V), 2-bromo-2-methyladamantane (VI), and
2-bromo-2-phenyl-adamantane (VII) in sulfolane and
BuOH. The data on heterolysis kinetics in sulfolane for
bromides VI and VII are published in [9, 10]. The kinetics
of the other reactions we studied by verdazyl method [11]
In this respect the entropy of activation is especially
interesting, whose magnitude and sign in reactions of
* For Communication XIX, see [1].
188

KINETICS AND MECHANISM OF UNIMOLECULAR HETEROLYSIS OF FRAMEWORK COMPOUNDS: XX. 189
X = H Cl, Br; R = H Me, Ph.
in the temperature range 20–50°C. We used as internal
indicator 1,3,5-triphenylverdazyl (VIII) that quickly
and quantitatively reacted with the solvent-separated ion
pair of the substrate giving verdazylium salt IX and
product of verdazyl alkylation (reaction SN1) [11]. In
the hetero-lysis of methyl derivatives the alkylation
product decomposed in sulfolane into an olefin and
leucoverd-azyl X (reaction Ε1). In butanol formed
verdazylium salt IX, solvolysis product, and compound
X. The reaction proceeded in keeping with the stoichio-
metric equation.
respectively [5, 12]. The higher activity of chlorides in
alcohol is usually attributed to the strong electrophilic
assistance of the solvent by formation of a strong hydrogen
bond with the nucleofuge, and the higher activity of
bromides in sulfolane is ascribed to higher polarizability
of the C–Br bond facilitating intensive dipolar solvation
[13].
In our event the difference in heterolysis rates of
halides IV–VII in sulfolane and butanol is considerably
greater, especially for phenyl derivatives V and VII that
react in BuOH ~ 100 times faster, whereas methyl
derivatives IV and VI are more active in sulfolane. It is
known that at replacing a methyl group by a phenyl one,
e.g., in going from t-BuCl to PhCMe2Cl, the heterolysis
rate in protic and aprotic solvents is 3–4 orders of
magnitude higher [5, 14]; therewith the acceleration is
caused by a decrease in the ΔH≠ value due to enhanced
stability of the carbocation intermediate resulting from
the conjugation between the p-orbitals of the carbocation
and the phenyl group. In heterolysis of halides IV–VII
these trends are observed only in butanol whereas in
sulfolane the heterolysis rate both of chlorides and
bromides is virtually insensitive to the nature of the
hydrocarbon substituent, and the ΔH≠ values are smaller
for the methyl derivatives.
Irrespective of the heterolysis type (SN1, Ε1, solvo-
lysis) one mol of the substrate reacts with two mols of
radical VIII. The reaction rate was measured spectro-
photometrically by consumption of radical VIII, and it is
fairly described by the first order kinetic equation: v =
–d[VIII]/2dt = k[RX].
In the table are compiled the log k25°C values and
activation parameters of heterolysis of halides IV–VII
in sulfolane and butanol. The ionizing power Eò of butanol
is considerably higher than that of sulfolane (210 and
184 kJ mol−1 respectively), but the heterolysis rates of
tertiary chlorides and bromides in these solvents are
commonly similar. For instance, the values log k25°C for
t-BuCl are 7.91 and 7.28, and for t-BuBr, 5.18 and 5.61
Kinetic parameters of heterolysis of 2-halo-2-methyl- and 2-halo-2-phenyladamantanes IV–VII in BuOH and sulfolane
Substrate
–log k25°C
ΔG≠ , kJ mol−1
ΔH≠, kJ mol−1
ΔS≠, J mol−1K−1
ΒuOН
ІV
V
6.95
3.93
113 10
117
101
5
3
13
17
5
9
95
6
VI
4.94
2.15
101 10
85 12
87.2 5.4
56.1 6.3
–47 16
– 98 19
VII
Sulfolane
ІV
V
VI
VII
6.33a
6.39a
3.78a
3.87a
109 11
89.9
104
48.9b
93.3b
5
–68 18
109
94.5b
95b
3
2
–18
5
–153b
–13b
a Calculated from log k values obtained at higher temperature. bFrom [9].
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 2 2007

DVORKO et al.
190
These relationships are difficult to understand, the
transition state formation. The latter is likely due to the
strong dipolar solvation of the covalent substrates (initial
states) of the phenyl derivatives which prevents the
conjugation of the phenyl group with the arising
carbocation. As a result the heterolysis rate of the phenyl
derivatives in sulfolane is ~ 2 orders of magnitude smaller
than in butanol, whereas the methyl derivatives
characterized by no steric hindrances are more active in
sulfolane.
solvation effects only are not enough for their interpreta-
tion. The different behavior in butanol and sulfolane
apparently originates from various manifestations of steric
effects of methyl and phenyl groups in these solvents.
The phenyl is considerably more bulky than methyl. Spatial
parameters calculated from the average van der Waals
radii of these groups are 1.66 and 0.52 respectively [15].
However not only the volume of the phenyl group is
important, but also its ability to be in conjugation with the
arising carbocation. If the steric hindrance prevents the
conjugation, the rate of the reaction may become over
two orders of magnitude smaller: The steric constants
–Es’ calculated from the kinetic effects are for phenyl
and methyl 2.31 and 0.0 respectively [7].
EXPERIMENTAL
2-Halo-2-alkyladamantanes IV–VII were prepared
reacting the corresponding alcohols with PCl3 (PBr3)
[10, 17, 18]. To a dry alcohol was added dropwise a three-
fold excess of PCl3 (PBr3), the product was extracted
with petroleum ether of pentane, the extract was washed
with water solution of potassium carbonate and water till
neutral washings, dried with Na2SO4, evaporated to
dryness, and the residue was thrice recrystallized from
MeCN. Yields 60–70%. Compound IV, mp 174°C (publ.:
mp 176°C [17]), compound V, mp 131°C (publ.: mp 133°C
[17]), compound VI, mp 132°C (publ.: mp 132–135°C
[19]), compound VII, mp 125°C (publ.: mp 126°C [20]).
In butanol where dominates the electrophilic solvation
resulting from hydrogen bonds with nucleofuge, the phenyl
group is conjugated with the arising carbocation. This is
manifested by the heterolysis rate 3 orders of magnitude
lower for phenyl derivatives compared to methyl deriva-
tives, At the lack of this conjugation the heterolysis rates
of methyl and phenyl derivatives should not be very
E
different since the σp values of Me and Ph are similar:
–0.30 and –0.18 respectively [7]. The ΔS≠ values show
that in solvolysis of chlorides IV and V prevails the effect
of the damaging solvent structure, and in solvolysis of
bromides VI and VII, electrostriction effect. Chlorine
forms a stronger hydrogen bond with the alcohol
molecules and therefore stronger damages the solvent
structure. The coordination of a single monodentate ligand
is known to reduce the ΔS≠ by ~ 45 J mol−1K−1 [16].
Consequently, in solvolysis of bromide VI during the
formation of the transition state at least one solvent
molecule is involved, and in solvolysis of bromide VII,
two molecules.
1,3,5-Triphenylverdazyl was synthesized and purified
as described in [21]. UV spectrum, λmax (ε): 710 (4000)
in BuOH; 720 (3910) in sulfolane. The solvents were
dried over CaO and subjected to rectification.
Kinetic experiments were performed in a tempera-
ture-controlled cell of a spectrophotometer SF-26. Sub-
strate concentration in the kinetic runs was 0.01–
0.6 mol l−1, that of verdazyl indicator, (1–3)×10–4 mol l−1.
The conversion of the substrate in a run was 0.1–0.01%.
The average of three runs was evaluated, the error was
~ 5%. The rate constants at various temperatures are
available from the authors.
The pattern in sulfolane is quite different: The hetero-
lysis rate does not change in going from methyl to phenyl
derivatives, and the ΔH≠ value is higher for the phenyl
derivatives. In this dipolar aprotic solvent (μ 4.8 D)
electrostatic dipole-dipole solvation occurs that con-
siderably stronger affects the formation of the transition
state in the heterolysis of the methyl than the phenyl
derivatives. This results apparently from the steric
hindrances originating from the phenyl group. The ΔS≠
values show that in formation of the transition state in
the heterolysis of chloride IV one solvent molecule is
involved, in the case of bromide VII three molecules,
whereas in the heterolysis of the phenyl derivatives
practically no additional solvation occurs during the
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