Question

Methyl m- and o-methylbenzoates give nearly identical Electron Impact mass spectra with the exception of the ion at m/z 118. Which isomer would be expected to give a strong ion at m/z 118 and why? Show the structure of the ion at m/z 118

Answer 1

Ortho isomer give a strong ion at m/z 118.

Peak at m/z = 118 corresponds to loss of neutral methanol from the base peak to give the cation-radical as shown below. Loss of methanol requires a cyclic mechanism in which the methyl group donates a hydrogen to the methoxyl group, concurrent with the cleavage of the carbon-oxygen bond. This process has a higher activation energy due to the entropic requirement of attaining the correct geometry for the reaction to occur, hence the process is less probable and the intensity of the fragment is less.