THE GAS-PHASE CANNIZZARO DISPROPORTIONATION REACTIONS OF BENZALDEHYDEAND PIVALDEHYDE

The Cannizzaro disproportionation reaction [RCHO + HO- --> RCH2O- + RC O2H (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 after glow (FA), and conventional reverse sector mass spectrometers. In addi tion, the prototypical system HO-/CH2O has been studied using ab initi o calculations at the MP3SDQ/6-311++G(d,p)//RHF/6-311++ G(d,p) level o f theory. Product ions RCH2O- are observed in the icr spectrometer fol lowing inefficient reactions, but are not formed from thermalized reac tant 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 proce sses form (i) the ''Cannizzaro'' ions RCH(OH)O- RCH2O-, and RCO2-, 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-pha se Cannizzaro reaction since it does not decompose to form either RCH2 O- or RCO2-. 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.