Abstract: | The sources of the migrant hydrogen atom(s) in reactions (a) and (b) in the electron impact mass spectrum of n-propyl benzoate have been investigated: (a) [C6H5CO2C3H7]+ →[C6H5CO2H]+ + C3H6; (b) [C6H5CO2C3H7]+ → [C6H5CO2H2]+ + C3H5sdot;. Deuterium labelling of the propyl group showed that, for reaction (a) at 70 eV ionizing energy 3 ± 1% of the hydrogen originates from C-1 of the propyl group, 86 ± 4% from C-2 and 11 ± 3% from C-3. The specificity of the transfer from C-2 increases as the internal energy of the fragmenting ions decreases, indicating that the results cannot be rationalized in terms of H/D interchanges between positions in the propyl group, but rather that the reaction involves specific, competing, H transfer reactions from each propyl position, in contrast to the high site specificity characteristic of the McLafferty rearrangement. Reaction (b) involves, almost exclusively, transfer of one hydrogen from C-2 and one from C-3 with only very minor participation of C-1 hydrogens. The [C6H5COOH]+ ion produced in reaction (a) fragments further to [C6H5CO]+ + OH. and the labelling results indicate some interchange of the carboxylic hydrogen with (ortho) ring hydrogens for those ions fragmenting in the first drift region. The extent of interchange is less than that observed for fragmentation of the same ion produced by direct ionization of benzoic acid or by reaction (a) in ethyl benzoate. |