The origin of the [M – 1] + ion in the mass spectra of N , N ‐dialkylbenzamides. A reinvestigation of the mechanism

A reinvestigation of the mechanism of formation of the [M – 1] + ion in a series of N,N ‐dialkylbenzamides suggests that previous mechanisms put forward to account for the formation of the [M – 1] + ion are deficient. A new mechanism is proposed which accounts for the data observed previously, as well as our results for a series of N,N ‐dialkyl‐2‐chlorobenzamides, 4‐substituted N,N ‐dimethylbenzamides and some related compounds. For the N,N ‐dialkyl‐2‐chlorobenzamides, comparison of the abundances of the [M – 1] + ion with the [M – 35] + ion suggests that a concurrent reaction is occurring, besides loss of the ortho aromatic hydrogen atom. A study of substituent effects on the intensity ratio [M – 1] + /[M] + shows an upward concave plot of this against σ + , suggesting that two competing mechanisms occur for the formation of the [M – 1] + ion.