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.