The Gas-Phase Cannizzaro Disproportionation Reactions of Benzaldehyde and Pivaldehyde

The Cannizzaro disproportionation reaction [RCHO + HO - → RCH 2 O - + RCO 2 H (R = t -Bu and Ph)] has been studied in the gas phase (i.e. in the absence of solvent) using ion cyclotron resonance (icr), flowing afterglow (FA), and conventional reverse sector mass spectrometers. In addition, the prototypical system HO - /CH 2 O has been studied using ab initio calculations at the MP3SDQ/6-311++G(d,p)//RHF/6-311++G(d,p) level of theory. Product ions RCH 2 O - are observed in the icr spectrometer following inefficient reactions, but are not formed from thermalized reactant ions in the FA instrument. The gas-phase Cannizzaro reaction has an effective internal constraint, even though the overall reaction is exothermic. In the reverse sector mass spectrometer, competitive processes form (i) the “Cannizzaro” ions RCH(OH)O -, RCH 2 O -, and RCO 2 -, and (ii) RCH(OH)OCH(R)O -, produced by nucleophilic addition of RCH(OH)O - to RCHO. The ion RCH(OH)OCH(R)O - is not an intermediate in the gas-phase Cannizzaro reaction since it does not decompose to form either RCH 2 O - or RCO 2 - . The experimental and theoretical data are consistent with the operation of an inefficient stepwise mechanism in which the first-formed species RCH(OH)O - transfers a hydride ion to RCHO.