Thiamin (vitamin B1) combines with benzaldehyde in alkaline solutions to form 2-(1-hydroxybenzyl)thiamin (HBzT), a reactive intermediate in the thiamin-catalyzed benzoin condensation. In neutral solutions, HBzT fragments into pyrimidine and thiazole constituents by cleavage of the bridging methylene-thiazale bond. The fragmentation is promoted by protonation of the pyrimidine moiety of HBzT. The N1'-benzyl derivative of HBzT (4 in Scheme 4, BHT) also undergoes fragmentation in neutral and alkaline solutions, consistent with fragmentation being driven by positive charge on the pyrimidine derived from thiamin. Anionic Bronsted bases catalyze the reaction (beta = 0.5, for a series of substituted acetates). The dependence of the observed first-order rate coefficient for fragmentation of BHT on buffer concentration is nonlinear, becoming buffer-independent at concentrations above 0.05 M. This is consistent with a change in rate-determining step with buffer concentration from proton removal to subsequent. fragmentation of the conjugate base of BHT. The solvent isotope effect is inverse, also consistent with reversible formation of the conjugate base. Analysis of the kinetic data reveals that the fragmentation step is very fast (k(f) = 1.2 x 10(5) s(-1) at 40 degrees C). Such a low barrier is consistent with electron-shift mechanisms for the fragmentation step.