Structure, reactivity and thermochemical properties of protonated lactic acid.

Abstract : The protonation energetics of lactic acid (LA) were experimentally determined by the kinetic method including the entropy effect. The values (proton affinity, PA(LA) = 817.4 +/- 4.3 kJ mol(-1); protonation entropy, DeltaS degrees (p)(LA) = -2 +/- 5 J K(-1) mol(-1); gas-phase basicity, GB(LA) = 784.5 +/- 4.5 kJ mol(-1)) agree satisfactorily with computed G2(MP2) expectations (PA(LA) = 811.8 kJ mol(-1); DeltaS degrees (p)(LA) = -7.1 J K(-1) mol(-1); GB(LA) = 777.4 kJ mol(-1)). The fragmentation behaviour of protonated lactic acid (LAH(+)) is dominated by carbon monoxide loss followed by elimination of a water molecule. Direct dehydration of LAH(+) is only a high-energy process hardly competitive with the CO loss. A complete mechanistic scheme, based on MP2/6-31G* calculations, is proposed; it involves isomerization of the various protonated forms of LA and the passage through the ion-neutral complex between the 2-hydroxypropyl acylium cation and a water molecule. Copyright 2004 John Wiley & Sons, Ltd.