SINGLE AND DOUBLE HYDROGEN-ATOM MIGRATIONS IN SUBSTITUTED ALKYL BENZOATES - A STUDY ON THE SUBSTITUENT EFFECT USING MIKE SPECTROMETRY

The substituent effect on the single and double hydrogen atom migrations in ionized ortho-, meta-, and para-substituted isobutyl (XC6H4COOC4H9) and isopropyl (XC6H4COOC3H7) benzoates is investigated by mass-analyzed ion kinetic energy spectrometry. The observed product ion ratios [XC6H4COOH].+/[XC6H4COOH2]+ show a general tendency: the compounds with an electron-donating substituent favour the formation of [XC6H4COOH].+ by single hydrogen atom migration (McLafferty rearrangement), while those with an electron-withdrawing substituent produce preferentially [XC6H4COOH2]+ through double hydrogen atom migration (McLafferty + 1 rearrangement). The thermochemical considerations combined with MO calculations show that the substituent effects observed are rationalized by the effects of substituent on the ionization energy (IE) and proton affinity (PA) of XC6H4COOH, i.e. the product ratios are determined by the difference of the product thermochemical stabilities which can be evaluated as IE(XC6H4COOH) + PA(XC6H4COOH).