Abstract
The substituent effect on the single and double hydrogen atom migrations is ionized ortho-, meta-, and para-substituted isobutyl (XC 6H 4COOC 4H 9) and isopropyl (XC 6H 4COOC 3H 7) benzoates is investigated by mass-analyzed ion kinetic energy spectrometry. The observed product ion ratios [XC 6H 4COOH] + /[XC 6H 4COOH 2] + show a general tendency: the compounds with an electron-donating substituent favour the formation of [XC 6H 4COOH] + by single hydrogen atom migration (McLafferty rearrangement), while those with an electron-withdrawing substituent produce preferentially [XC 6H 4COOH 2] + through double hydrogen atom migration (McLaffery + 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 XC 6H 4COOH, i.e. the product ratios are determined by the difference of the product thermochemical stabilities which can be evaluated as IE(XC 6H 4COOH) + PA(XC 6H 4COOH).
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