Journal of the American Chemical Society p. 6466 - 6472 (1980)
Update date:2022-08-12
Topics:
Palmer, John L.
Jencks, William P.
At pH>6 the formation of formaldehyde thiosemicarbazone proceeds with rate-limiting dehydration of the carbinolamine intermediate, which is at equilibrium with formaldehyde hydrate and thiosemicarbazide (K = 550 M-1).At higher concentrations of formaldehyde a bis(formaldehyde) addition compound is formed, which undergoes dehydration more slowly.The dehydration step is subject to general-acid catalysis by phosphate and phosphonate buffers with α = 0.83.A solvent deuterium isotope effect of kHA/kDA = 2.6 for catalysis by ethylphosphonate monoanion and published evidence support a concerted mechanism of catalysis.The calculated rate constant for formation of the O-protonated carbinolamine is > 104 faster than the observed rate constant for dehydration and the rate constant for expulsion of water from this species is < 3 x 107 s-1.Thus, it appears that a concerted mechanism can exist when it is not enforced by the nonexistence of the O-protonated species.The secondary α-deuterium isotope effect of KH/kD = 1.06 (1.03 /D) for catalysis by phosphate monoanion suggests an early transition state but other criteria suggest a central or late transition state for C-O cleavage.
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