Theoretical calculations of the reaction of formyl cation (CHO+) with toluene, cumene and p-cresol showed that proton transfer is thermodynamically preferred over acylation. For toluene, acylation of the aromatic ring, to form the Wheland complex, is 11.7 kcal mol-1 higher in energy (ΔH) than protonation, at MP4(SDTQ)/6-31++G(d,p)//MP2(full)/6-31G(d,p) level of theory. This difference reduces upon introduction of a hydroxy group in the ring (p-cresol) or replacing the methyl group by an isopropyl group (cumene). Protonation of toluene by H3+ and isoformyl cation is 88.6 and 84.5 kcal mol-1, respectively, more exothermic than acylation, at MP4(SDTQ)/6-31++G(d,p)//MP2(full)/6-31G(d,p). The proton affinity of p-cresol was calculated to be 195.4 kcal mol-1.
formyl cation; Friedel-Crafts acylation; gas phase ion reaction; proton transfer
