Thermodynamics of gas phase proton transfer reactions involving substituted benzaldehydes
Abstract
Gas phase proton transfer equilibria involving
ortho-, meta- and
para-isomers of tolualdehyde, fluorobenzaldehyde and
chlorobenzaldehyde have been studied experimentally for the first time
as a function of temperature. Proton affinities have been determined for
ortho- and meta-tolualdehyde,
ortho-fluorobenzaldehyde and ortho-
and meta-chlorobenzaldehyde. The values found for the
other isomers are in good agreement with previous reports, except for
para-chlorobenzaldehyde. Complementary values have
been obtained using semi-empirical MO theory which are in excellent
agreement.Whereas the tolualdehydes all have proton affinities greater than
that of benzaldehyde, the values for the halobenzaldehydes are lower.
The order of proton affinities is readily rationalised in terms of
classical aromatic substituent effects. In addition, the data are shown
to correlate well with solution based linear free energy substituent
constants. This correlation with substituent protonated species is
new.Observed deviations from ideal temperature behaviour indicate
that
protonated meta-fluorobenzaldehyde probably isomerises
at high temperatures, to the energetically preferred
ortho- or para-isomer, but only when
reacting with mesitylene. There is also evidence that during the
reaction of benzaldehyde and protonated
ortho-fluorobenzaldehyde, isomerisation to the
energetically less favourable meta-isomer may occur.
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