Structural effects on the gas phase reactivity of organic salt complexes: Substitution versus Hofmann elimination

Salt complexes between a dianion and a tetraalkylammonium cation were formed during electrospray ionization (ESI) and their reactions were studied under collision-activated dissociation (CAD) conditions in a quadrupole ion trap mass spectrometer. Products corresponding to S(N)2 and E2 reactions within the complexes were observed. The competition between substitution and elimination was explored by varying the nature of the alkyl group on the ammonium. In addition, primary and secondary deuterium kinetic isotope effects were determined for the E2 (k(H)/k(D) = 3.5 +/- 0.2) and S(N)2 reactions (k(H)/k(D) = 0.94 +/- 0.05), respectively. When compared with the analogous bimolecular reactions of alkyl bromides, the data indicate that Hofmann (ammonium salts) and Saytzev (alkyl bromides) reactivity patterns are operative in the gas phase.