Kinetics and mechanism of unimolecular heterolysis of framework compounds: XVII. Solvation effects in dehydrobromination of tert-butyl bromide, 1-bromo-1-methylcyclohexane, and 2-bromo-2-methyladamantane in dipolar aprotic solvents

Article abstract of DOI:10.1007/s11178-006-0003-2

Dehydrobromination rate of tert-butyl bromide, 1-bromo-1-methylcyclohexane, and 2-bromo-2-methyladamantane grows with increasing polarity and dipole moment of solvents. No correlation was found between rate constants of the process and electrophilicity or ionizing power of the solvents. The observed solvation effects are due mainly to dispersion interactions. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11178-006-0003-2

Russian Journal of Organic Chemistry, Vol. 41, No. 11, 2005, pp. 1594-1597. Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 11, 2005,
pp. 1628-1631.
Original Russian Text Copyright Ó 2005 by Vasil’kevich, Ponomareva, Dvorko.
Kinetics and Mechanism of Unimolecular Heterolysis
of Framework Compounds: XVII.* Solvation Effects
in Dehydrobromination of tert-Butyl Bromide,
1-Bromo-1-Methylcyclohexane, and 2-Bromo-2-methyladamantane
in Dipolar Aprotic Solvents
A.I. Vasil’kevich, E.A. Ponomareva, and G.F. Dvorko
Kiev Polytechnic Institute, Ukrainian Technical University, Kiev, 252056 Ukraine
e-mail: ponomaryov@torba.com
Received October, 4 2004
Abstract—Dehydrobromination rate of tert-butyl bromide, 1-bromo-1-methylcyclohexane, and 2-bromo-2-
methyladamantane grows with increasing polarity and dipole moment of solvents. No correlation was found
between rate constants of the process and electrophilicity or ionizing power of the solvents. The observed
solvation effects are due mainly to dispersion interactions.
The dehydrobromination of tert-butyl bromide (I) [2],
-bromo-1-methylcyclohexane (II) [3], and 2-bromo-2-
methyladamantane (III) [4] follows the E1 mechanism.
The rate of the unimolecular heterolysis is commonly
governed by the polarity and electrophilicity of solvents
[5, 6]. These parameters determine also the ionizing power
of the solvents. Therefore the decrease in the heterolysis
rate of bromide IV in the solvent series (2) is caused
mainly by the reduced electrophilic assistance from the
solvent, and in going to the cyclohexanone additionally
by the diminished polarity of the medium (the polarity of
the other three solvents does not significantly differ).
1
According to the modern data [5, 6] the unimolecular
heterolysis (E1, S 1, solvolysis) proceeds through
N
successive formation of three types ion pairs: contact,
space-separated, and solvent-separated.
5;
5 ;
5 ꢀꢀꢀꢀ;
5 _6ROY_;
5HDFWLRQꢀSURGXFWV
In the limiting stage the contact ion pair interacts with
a cavity in the solvent which results from the fluctuation
of the liquid density [7] (the cavities amount to ~10% of
the liquid volume [8]). Thus a space-separated ion pair is
formed that quickly transforms into the solvent-separated
ion pair which in its turn rapidly affords the reaction
products. The rate of these reactions is described by the
first order kinetic equation (1).
MeCN > g-butyrolactone > sulfolane > cyclohexanone (2)
It is presumed [9] that the electrophilic assistance to
the heterolysis of bromide IV originates mainly from
orientation and dipole-dipole interactions: in MeCN forms
a linear quadrupole A involving a hydrogen bond, in the
g-butyrolactone arises a cyclic quadrupole B, and in
sulfolane possessing the highest value of the Gutman
acceptor number AN [8] the molecule of the substrate
coordinates with a solvent molecule providing a charge-
transfer complex. In the least polar cyclohexanone the
v = k [RBr]
(1)
One of the previous communications from this series
9] contains data on the heterolysis kinetics of 2-bromo-
[
2
-phenyladamantane (IV) in MeCN, g-butyrolactone,
sulfolane, and cyclohexanone (S 1 reaction). In the
N
mentioned solvent series the reaction rate decreased by
three orders of magnitude because of diminishing of the
ionizing power (E [8]) and electrophilicity of the solvent.
T
No correlation with the solvent polarity was observed.
*
For communication XXII, see [1].
1070-4280/05/4111-1594Ó2005 Pleiades Publishing, Inc.

KINETICS AND MECHANISM OF UNIMOLECULAR HETEROLYSIS OF FRAMEWORK COMPOUNDS: XVII. 1595
nucleofuge solvation in the transition state occurs by
forming a cyclic solvate C.
in a table. The reaction rate decreased in the series of
solvents (4):
These conceptions are based on comparison of the
activation parameters of the reaction. The quadrupole
sulfolane > g-butyrolactone > MeCN > cyclohexanone (4)
¹
formation is accompanied by a decrease in DS value by
-
1
-1
In this series the polarity of solvents (dielectric
constant e) decreases, and electrophilicity (E) and ionizing
~
80 J mol
K
[9, 10], and a coordination of a single
-1
-1
monodentate logand result in a loss of ~45 J mol K
11, 12]. The formation of solvate C is in agreement with
power of the solvents (E ) first grow and then diminish
[
T
a sharp decrease both in the enthalpy and entropy of
activation for the reaction in cyclohexanone [9].
in going to cyclohexanone. As in the case of bromide IV
heterolysis, the reaction rate is the least in cyclohexanone
for all the three substrates under study. However the
effects of the first three solvents for substances I–III
were opposite to those observed for bromide IV. In the
solvent series sulfolane–g-butyrolactone–MeCN the
heterolysis rate of bromide IV grew nearly by an order
of magnitude, and the heterolysis rate of bromides I–III
decreased 5.2, 3.0, and 2.2 times respectively. This
difference is due to dissimilar sensitivity of the substrates
to the electrophilicity and polarity of the solvents. Whereas
the electrophilicity has the decisive importance for bromide
IV, with the other substrates polarity is the most
significant.
In continuation of the research on the nature of the
solvation effects in the unimolecular heterolysis we
planned to study the influence of the substrate structure
on these effects in dipolar aprotic solvents. To this end
¹
¹
we compared the values of log k, DH , DS for the
heterolysis of bromides I–III in the solvents of the
series (2).
Kinetic pareameters were published on heterolysis of
bromide I in MeCN [2] and g-butyrolactone [13], of
bromide II in MeCN [3], and of bromide II in MeCN [4]
and g-butyrolactone [13]. With the use of verdazyl method
[
5, 9] we studied the kinetics of dehydrobromination of
bromides I–III in sulfolane and cyclohexanone, and also
of bromide II in g-butyrolactone at various temperatures.
As an internal indicator we used in the kinetic experi-
The observed solvent effect on the heterolysis rate
of the four substrates is understandable under assumption
that in the heterolysis of bromides I–III the solvation
originates mainly fron the dispersion forces, and the
orientation and dipolar interactions that dominate in the
heterolysis of bromide IV are less important. The
prevalence of the dispersion interaction is apparently due
to the presnce of a methyl group at the a-carbon
hampering the dipolar solvation. The dispersion forces to
a great extent depend on the polarizability of the solvent,
therefore the dehydrobromination rate of the substrates
under study grows with the growing dipolarity (polarity–
polarizability, p* [8]) and the dipole moment (m) of
solvents. Usually the dispersion forces dominate in the
.
ments 1,3,5-triphenylverdazyl (Vd ) that quickly and
quantitatively reacted with the sovent-separated ion pair
of the substrate to give olefins V–VII, verdazyl salt
+
–
(
Vd Br ), and leucoverdazyl (VdH). The reaction
proceeds in keeping with a stoichiometric equation [5].
The reaction rate was measured by the decrease in
·
the verdazyl concentration Vd . The rate is satisfactorily
described with the kinetic equation (3).
·
v = –d [Vd ]/2dt = k [RBr]
(3)
Kinetic parameters of the reactions of the substrates
under study and some data on the solvents are presented
RUSSIAN JOURNALOF ORGANIC CHEMISTRY Vol. 41 No. 11 2005

1596
VASIL’KEVICH et al.
a
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The values in parentheses are evaluated from the temperature dependence.
solvents of low polarity, but they may give the main
contribution into the interaction of polar molecules with a
constant dipole moment [14].
In our case the adamantyl substrate, bromide III, is
more active. Its heterolysis rate grows with increasing
electrophilicity of the solvent, and the heterolysis rate of
bromide I decreases. The latter fact results from the de-
creasing dipolarity and dipole moment in the series of
solvents (4) that proves to be decisive for bromide I. As
we already mentioned, in the heterolysis of bromide IV
the dipolar solvation led to a linear quadrupole A; with
bromide I this solvation apparently also provided a similar
The relative activity of bromides I–III strongly
depends on the nature of solvent. For instance, in going
from bromide I to bromide IV the heterolysis rate in
MeCN grew by ~3 orders of magnitude, in g-butyro-
lactone by ~2 orders of magnitude, in sulfolane by ~1
order of magnitude, i.e., the difference decreased with
the diminishing electrophilicity of the solvent [15, 16].
¹
quadrupole (this assumption agreed with the value of DS
in MeCN), but this effect had however a secondary
importance.
The difference in activity of tert-butyl and adamantyl
substrates is known to decrease with the growing
electrophilicity of the solvent. For instance, in the
propylene carbonate (E 20) tert-butyl iodide is by ~4
orders of magnitude more active than 1-adamantyl iodide
The hetrolysis rate of bromide II is nearly the same
as that of bromide I but in contrast to the latter whose
decrease in the heterolysis rate in the solvents series (4)
is due to the alterations in both activation parameters, the
decrease in the heterolysis rate of bromide II is caused
[2, 17], in 80% aqueous ethanol tert-butyl halides are by
¹
¹
~
3 orders of magnitude more active than 1-adamantyl
by the growth of the DH value, whereas the DS values
in all solvents are virtually constant and ~2 times less
that those of bromide I. The special behavior of the
activation parameters in the heterolysis of bromide II
originates apparently from the conformational effects
weakly sensitive to the nature of solvent. The cyclohexyl
substrate in the ground state exists in the chair form, and
halides [2, 15, 18], in 2,2,2-trifluoro-1-ethanol (E 81) this
difference for chlorides and bromides equals to 20 [5,
1
5, 18], and in 1,1,1,3,3-hexafluoro-2-propanol (E 88) it
amounts to ~2 [5, 15, 18]. The changes in relative rates
of heterolysis of the tert-butyl halides and 1-adamantyl
halides is due to the greater sensitivity of the less active
adamantyl substrates to the electrophilicity of solvents.
Thus in going from propylene carbonate to hexafluoro-2-
propanol the heterolysis rate of tert-butyl bromide grows
by ~4 orders of magnitude, and that of 1-adamantyl
bromide by ~8 orders of magnitude.
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RUSSIAN JOURNALOF ORGANIC CHEMISTRY Vol. 41 No. 11 2005

KINETICS AND MECHANISM OF UNIMOLECULAR HETEROLYSIS OF FRAMEWORK COMPOUNDS: XVII. 1597
the carbocation arising in the transition state has a twist-
conformation D [3, 19] thus governing the spatial
requirements to the formation of the transition state.
20.0, 0.917 ± 0.021; 25.0, 1.56 ± 0.14; 30.0, 2.27 ± 0.07;
35.0, 3.62 ± 0.23; sulfolane: 30.0, 4.60 ± 0.40; 32.5,
6.10 ± 0.20; 34.5, 8.00 ± 0.10; 40.5, 11.3 ± 1.3; 45.0,
1
0
0
9.0 ± 1.0; cyclohexanone: 13.0, 0.0191 ± 0.0002; 20.0,
.0408 ± 0.0012; 25.0, 0.0802 ± 0.0030; 29.5, 0.117 ±
.008; 35.0, 0.206 ± 0.010.
Under these conditions the reaction rate is determined
by the stability of carbocations that grows with the
growing dielectric constant of the solvent.
Bromide III – sulfolane: 30.0, 234 ± 12; 32.5, 279 ±
5; 35.0, 324 ± 8; 39.0, 423 ± 20; cyclohexanone: 16.0,
0.136 ± 0.007; 20.5, 0.261 ± 0.003; 25.0, 0.601 ± 0.016;
0.0, 0.926 ± 0.012; 36.0, 2.16 ± 0.06.
The heterolysis rate of bromide III is nearly by two
1
orders of magnitude greater that the reaction rates of
¹
¹
bromides I and II. The values of DH and DS in the
heterolysis of bromide III depend strongly on the solvent
character and grow in the series (4). The relatively low
3
The study was carried out under financial support of
the Ukrainian State Foundation for Fundamental
Research.
¹
¹
DH and DS values in sulfolane may result from the
involvement into formation of the transition state of two
solvent molecules (hexapole formation) or from reaction
via cyclic solvate E.
REFERENCES
Close values of the hetrolysis rate in sulfolane for
1
. Dvorko, G.F., Pervishko, T.L., Leunov, D.I., and Ponomare-
va, E.A., Zh. Org. Khim., 1999, vol. 35, p. 1643.
bromides III and IV (log k –3.78 and –3.87 respectively)
2
5
¹
¹
and strong difference in the values of DH and DS [for
bromide III 43 kJ mol and -153 J mol K , for bromide
IV 87 kJ mol and -13 J mol K respectively] confirm
the dissimilar nature of the solvation effects in the
heterolysis of these substrates.
2. Dvorko, G.F., Ponomareva, E.A., and Kulik, N.I., Usp. Khim.,
-1
-1 -1
1984, vol. 43, p. 948.
-1
-1 -1
3. Vasil’kevich, O.I., Pan’kiv, I.O., and Ponomar’ova, E.O., Dop.
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Dvorko, G.F., Zh. Org. Khim., 1988, vol. 24, p. 549.
5
In cyclohexanone the heterolysis of all substrates is
the slowest. Unlike the reaction in sulfolane the values
. Dvorko, G.F. and Ponomareva, E.A., Usp. Khim., 1991,
vol. 60, p. 2089.
¹
¹
DH and DS in the heterolysis of bromide III are
considerably greater than those of bromide IV [for
6. Dvorko, G.F. and Ponomar’ova, E.O., Ukr. Khim. Zh., 1993,
vol. 59, p. 1190.
-
1
-1 -1
bromide III 100 kJ mol and –30 J mol K , for bromide
7
8
. Moura-Ramos, J.J., J. Solut. Chem., 1989, vol. 18, p. 957.
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Ponomareva, E.A., Zh. Org. Khim., 1999, vol. 35, p. 549.
-1
-1 -1
IV 63 kJ mol and –151 J mol K respectively]. These
data once more confirm the difference in the solvation
effects in the heterolysis of these substrates and also
show that in cyclohexanone the decisive role belongs to
dispersion interactions.
9
10. Khoffman, R.V., Mekhanizmy khimicheskikh reaktsii
Mechanisms of Chemical Reactions), Moscow: Khimiya,
979, 300 p.
1. Yakhimovich, R.I., Ponomarev, N.E., and Dvorko, G.F., Zh.
(
1
EXPERIMENTAL
1
The substances I–III were synthesized and purified
as described in [3, 4, 9]. The solvents were purified by
procedures from [9]. The kinetic measurements were
carried out in a cell of spectrophotometer SF-26 equipped
with the temperature control; concentration of substrates
Obshch. Khim., 1988, vol. 58, p. 881.
1
2. Classon, S., Lundcen, B., and Szwarc, M., Trans. Faraday,
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1
-
4
I–III 0.03–0.00, of verdazyl indicator (0.80–3.3)´10
1
1
–
1
mol l ; conversion of substrates in all cases was below
.01%.
Further are reported: substrate – solvent, temperature
0
1
1
1
6
–1
(
°C), k´10 (s ).
Bromide I – sulfolane: 30.5, 12.1 ± 0.9; 35.0, 22.0 ±
0
1
0
3
.5; 39.0, 34.8 ± 0.7; 46.0, 67.5 ± 0.1; cyclohexanone:
7.0, 0.00467 ± 0.00032; 20.0, 0.00739 ± 0.00052; 25.0,
.0162±0.011; 30.0, 0.0307±0.0002; 36.0, 0.0363± 0.0023;
9.5, 0.0917 ± 0.0009.
1984, vol. 49, 5183.
19. Ranganayakulu, K., Devi, M.V., Rao, R.B., and Rajeswri, K.,
Can. J. Chem., 1980, vol. 58, p. 1484.
RUSSIAN JOURNALOF ORGANIC CHEMISTRY Vol. 41 No. 11 2005