Fragmentation reactions of some deprotonated acetophenones

Abstract The collision‐induced dissociation reactions of the enolate ions, [XC 6 H 4 COCD 2 ] − , derived from labelled acetophe‐none and the fluoro‐. chloro‐ and nitro‐substituted acetophenones, were studied as a function of collision energy over the range 5‐100 eV on the laboratory scale, enabling breakdown graphs to be established. For the acetophenone and fluoroacetophenone enolates the major primary fragmentation reaction forms [XC 6 H 4 ] − + D 2 CCO, although the formation of XC 6 H 4 D + [ DCCO] − is favoured thermocbemically. This result implies that there is a significant energy barrier for D + abstraction in the [XC 6 H 4 ] − −D 2 CCO ion‐dipole complex formed initially on collisional activation of the enolate. The ortho ‐ and meta ‐fluorophenyl anions fragment further by elimination of HF. By contrast, chlorophenyl anions show no loss of HCl but may fragment to form Cl − . In addition, Cl − is formed from the chloroacetophenone enolate ions at low collision energies, possibly by a displacement reaction of [DCCO] − on the chlorobenzene before the ClC 6 H 4 D−[DCCO] − ion‐dipole complex separates. The lowest energy fragmentation channel for the nitro‐substituted enolate ions is elimination of NO. At higher collision energies the nitrophenyl anion is formed and itself fragments to form NO 2 − ; this reaction appears to be particularly facile for the ortho isomer. In a number of cases exchange of hydrogen between the ring and the enolic position is observed, particularly at low collision energies. A pronounced effect of collision gas identity on the breakdown graph for acetophenone enolate is reported and possible origins are discussed.