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Nitration and aromatic reactivity - Hoggett J.G.

Hoggett J.G., Moodie R.B., Penton J.R. Nitration and aromatic reactivity - Cambridge, 1971. - 252 p.
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are unknown and we must enquire which of the experimentally available
quantities is most appropriately used for this purpose.
In transition state theory, the rate constant, k, is given by the
following expression: " (tm)
& = ^-exp (-AG4RT).
Admitting the impossibility of calculating absolute rates, we can concern
ourselves with the effect of a structural modification to a particular
reactant which we take as a point of reference; if the rate constant for
the reaction involving the modified compound is k, and that for the
reference substance k0, then:
2-303 RTlog10k/k0 = AG^-AG\
Arguments have been presented9 that this difference in changes in Gibbs'
function, rather than the similar difference in enthalpies of activation,
AU - AI*, better represents the quantity with which
122
Introduction
theoretical treatments of structural effects on reactivity are
concerned, namely differences in the zero energy* of activation. Rate
constants of reaction are, then, the data to use in testing theoretical
predictions, but small changes in rate must be treated cautiously.
The behaviour of benzene is the datum from which any discussion of
aromatic compounds must start: the reactivity of an aromatic compound is
its rate of reaction relative to that of benzene when both are taking
part in reactions occurring under the same conditions and proceeding by
the same mechanism.
7.1.2 Limits to the meaning of aromatic reactivity The above
definition implies that the reactivity of an aromatic compound depends
upon the reaction which is used to measure it, for the rate of reaction
of an aromatic compound relative to that for benzene varies from reaction
to reaction (table 7.1). However, whilst a compound's reactivity can be
given no unique value, different substitution reactions do generally set
aromatic compounds in the same sequence of relative reactivities.
As a means of studying the reactivities of aromatic compounds towards
electrophiles, nitration has one major advantage compared with other
substitution processes; over a wide range of experimental conditions it
involves the same electrophile, the nitronium ion, and so is applicable
to a large range of compounds, differing widely not only in reactivity
but in such practically important properties as solubility. In very
varied conditions of nitration an aromatic compound shows surprisingly
similar reactivities (see the data for toluene; table 4.1, columns b-g,
k-o, q). Nitration in aqueous sulphuric acid can provide data for
compounds covering a very large span of reactivities for, as has been
seen (2.4.2), the second-order rate coefficient decreases by a factor of
about io4 for each decrease of 10 % in the concentration of the sulphuric
acid.
Generally the determination of the reactivity of a particular compound
depends upon comparison of its rate of nitration with that of benzene at
the same acidity and temperature. Because of the spread of rates this may
not be practically possible and, in any case, is usually not necessary
because of the parallelism existing among rate profiles (fig. 2.4).
Reactivities in aqueous sulphuric acid are, in fact, very nearly
independent of acidity, and stepwise comparison of data for a compound
with those of benzene determined at different acidities is possible.
* Zero energy is the energy which a species would have at absolute zero
in the absence
of zero-point vibrational energy.
123
Aromatic reactivity: A. Theoretical background
table 7.1 Partial rate factors for some electrophilic
substitutions of toluene
Reaction Reagent fo fm /,
Nitration* HNOJAcOH/25 C 49 24 70
HN03/MeN02/2S C 49 2'5 56
HN03/Ac20/0 c 39 3-0 5i
Chlorination13 Cl,/aq. AcOH 534 552
ClJAcOH 617 5 820
CIJMeCN 1830 9-1 6250
HC10/H20/H+ (i.e. C1+) 134 4-0 82
Bromination13 Br2/aq. AcOH 600 5'5 2420
HBr0/H20/H+ (i.e. Br+) 76 25 59
* See table 4.2.
There are certain limitations to the usefulness of nitration in
aqueous sulphuric acid. Because of the behaviour of the rate profile for
benzene, comparisons should strictly be made below 68% sulphuric acid
(2.5; fig. 2.5); rates relative to benzene vary in the range 68-80%
sulphuric acid, and at the higher end of this range are not entirely
measures of relative reactivity.10(r) For deactivated compounds this
limitation is not very important,11 but for activated compounds it is
linked with a fundamental limit to the significance of the concept of
aromatic reactivity; as already discussed (2.5), nitration in sulphuric
acid cannot differentiate amongst compounds not less than about 38 times
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