Bordwell et al.4 observed a linear relation between the differ-
ence in the electronegativities of the atoms forming the bond
that is cleaved and the ρ value. The latter represents the slope
from a linear plot of ∆Ed and the Brown substituent constants
σϩ for a set of substituted compounds. The ρ values for phenols,
anilines, anisols and thiophenols are clearly positive and range
from ϩ7 to ϩ2. For benzyl bromides a negative value (Ϫ5) has
been used in the correlation, while with the enthalpy data from
our study the ρ-value for benzyl bromides appears to be close to
zero.
blanket of argon was maintained above the solution during the
whole experiment. The instrument was calibrated using o-
hydroxybenzophenone in the same mixtures, but without the
peroxide.
Chemicals
All chemicals, except p-methoxybenzyl bromide and p-CN-tert-
butylbenzene, were obtained from commercial sources. For
thermolysis experiments they were used as received, for photo-
acoustic experiments they were purified using appropriate
methods such as distillation and sublimation.
When a heteroatom (oxygen, nitrogen or sulfur) is attached
directly to the aromatic ring, the interaction of its lone pair
with the ring system in the parent compound can be quite sub-
stantial. Recently26 this has been shown for substituted anisols
by means of theoretical density functional theory calculations.
For para-donor groups the stabilization of the radical is more
important while for para-withdrawing substituents the lone pair
interaction causes an additional decrease in the heat of form-
ation of the parent compound, resulting in an increase of the
bond energy. In benzyl bromides such a substantial effect in the
closed shell compound is not obvious. In the benzylic com-
pounds, any additional interaction between, for example, the
C᎐Br bond and the remote substituent can only occur through
a partial charge on the carbon attached to the aromatic ring.
The electron densities on the benzylic carbon in benzyl brom-
ides (C᎐Br) and tert-butylbenzenes (C᎐C) are indeed expected
to be different, but since no influence of substitution was
observed for either compound, the magnitude of any (de-)-
stabilizing effect of a substituent can only be within our
The synthetic procedure for p-methoxybenzyl bromide as
described by Offerman and Vögtle28 based on conversion of the
toluene using N-bromosuccinimide in methyl formate, CH2Cl2
or CCl4, proved unsuccessful in our experiments, as mainly
bromination of the benzene ring occurred. The compound was
therefore synthesized from p-methoxybenzyl alcohol and PBr3
cooled in an ice–salt bath. The reaction mixture was stirred for
a few hours and at 70% conversion, as determined by GC, the
reaction was stopped by adding diethyl ether and water. Allow-
ing the reaction to proceed further led to formation of poly-
mers. The ether layer was washed with 10% NaHCO3 and water,
dried over MgSO4 and the ether was carefully evaporated. The
benzyl bromide could then be distilled in vacuo (bp 115 ЊC, 1.5
mmHg), but was found to be unstable in concentrated form.
Therefore a few ml of benzene were added to the ether sol-
ution before the ether was removed. The resulting 3% solution
of p-MeO-benzyl bromide (of ca. 90% purity) in benzene was
stored at ϩ4 ЊC.
experimental error, i.e. less than 4 kJ molϪ1
.
p-CN-tert-butylbenzene of 98% purity was synthesized from
p-tert-butylbromobenzene following a procedure described by
Friedman and Shechter29 with an overall yield of 58%.
Experimental
Gas-phase thermolysis
Acknowledgements
Using p-fluorotoluene as a carrier, the thermolyses of benzyl
bromide and (substituted) tert-butylbenzenes were performed
in the gas phase with an on-line GC-analysis instrument. Com-
petition experiments with several substituted benzyl bromides
were performed using either p-fluorotoluene or cumene as a
carrier in a set-up with off-line GC-analysis. Both types of
apparatus and the experimental procedures have been described
before.27
The authors thank M. de Boo, I. Höld, J. Joosting-Bunk and
S. Miralles Nieves for experimental assistance and the Nether-
lands Foundation for Chemical Research (SON) for financial
support.
References
On-line analysis. The instrument with on-line analysis con-
sisted of a heated reactor (5.2 ml quartz tube) in which a con-
tinuous stream of reactant, diluted with p-fluorotoluene and
nitrogen was thermolyzed. The product mixture was analyzed
by GC. The complete apparatus could operate automatically.
Samples were taken at a reference reactor temperature of 548 K
(×4) (blank run) and at the reaction temperature between 750–
950 K (×6).
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Off-line analysis. The experiments with the apparatus with
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(substituted) benzyl bromides in p-fluorotoluene or cumene as
the solvent (and carrier) together with o-dichlorobenzene and
either o-dicyanobenzene or α-bromonaphthalene as internal
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ml, at atmospheric pressure over a temperature range of 750–
850 K. Liquids were introduced into the reactor by a motorized
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Photoacoustic calorimetry
The photoacoustic apparatus and procedure has been described
in detail before.14,23 The benzyl bromide experiments were per-
formed in a non-flow fashion, using a standard fluorescence
cuvette (Hellma 221) containing solutions of di-tert-butyl
peroxide and benzyl bromide in an appropriate solvent mixture,
deoxygenated by purging with argon through a capillary. A
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13 S. J. Ashcroft, J. Chem. Eng. Data, 1976, 21, 397.
J. Chem. Soc., Perkin Trans. 2, 1997
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