Substituent effect and rearrangements in the electron‐impact spectra of long chain esters of m ‐and p ‐methoxybenzoic acid

The mass spectra of eighteen n‐alkyl estars of p‐methoxybenzoic acid are examined. Few differences exist in the spectra of the corresponding para and meta isomers. We describe a new rearrangement, loss of OH from the molecular ion, whichis observed only in the para substituted isomkers. Oxygen‐18 labeling indicates that the oxygen lost in the [MOH]+ ion derives from the carbonyl. No indication is found of hydrogen abstration from the chain by the methoxy group competing withthe benzoate ester rearrangement. We find a substituent effect on that rearrangement, with electron‐donating groups decreasing the values of [ArCO 2 H 2 ] + /[ArCO 2 H] + · ion ratios. this ratio decreases as the ester chain length increases.