Author:
Richards KE,Wilkinson AL,Wright GJ
Abstract
The rates of detritiation at the ring positions in the
polymethylbenzenes have been obtained for exchange at 70�in
anhydrous trifluoroacetic acid. The first-order rate constants are analysed in
terms of the additivity principle, which is shown not to hold for this system;
derived partial rate factors, describing the effect of ortho,
meta, and para methyl groups on reactivity, decrease as the reactivity of
the ring site increases. Much better agreement between observed and calculated
results is obtained if the demand for stabilization made on the methyl groups
in the transition state is assumed proportional to the activation free energy.
This approach requires no more parameters than the additivity principle, but
corrections for ortho-meta and meta-para buttressed methyl groups can
give further improvement where required. The application
of this "demand-dependent free energy relationship" to other systems,
and the origin of the buttressing effects, are discussed.
Cited by
17 articles.
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