Site of alkylation of N ‐methyl‐ and N ‐ethylaniline in the gas phase: a tandem mass spectrometric study

N ‐Methylaniline (NMA) was ethylated and N ‐ethylaniline (NEA) was methylated under chemical ionization conditions using C 2 H 5 I and CH 3 I, respectively, as reagent gases. The structures of the resulting m / z 136 adduct ions have been probed using metastable ion and collision‐induced dissociation (CID) methods. From the similarity of the spectra obtained and from the presence of structure‐diagnostic ions at m / z 59 (CH 3 NHC 2 H 5 +• ) and m / z 44 (CH 3 NHCH 2 + ), it is concluded that predominantly N ‐alkylation occurs in both systems. This interpretation was aided by the use of C 2 D 5 I and CD 3 I as reagents. Adduct ions of m / z 136 were also formed by ethylation of the isomeric toluidines and by methylation of the ring‐ethylanilines. The resulting CID mass spectra were distinctly different from those obtained for the m / z 136 ions obtained by alkylation of NMA and NEA. Protonation of N ‐ethyl‐ N ‐methylaniline using CH 3 C(O)CH 3 as Brønsted acid reagent produced an m / z 136 species whose CID mass spectrum also featured intense ion signals at m / z 59 and 44. This observation led to the conclusion that protonation with acetone as reagent results, in this case, in dominant N ‐protonation. However, the CID mass spectrum of the m / z 136 ion formed when CH 3 OH was the protonating agent featured a weak signal at m / z 44 and no signal at m / z 59. Hence it was concluded that the latter m / z 136 ion contains a larger contribution from the ring‐protonated adduct. Copyright © 2004 John Wiley & Sons, Ltd.