Thermodynamics of gas phase proton transfer reactions involving substituted benzaldehydes

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 metafluorobenzaldehyde 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.