138372-91-5Relevant articles and documents
Kinetics of breakdown of aryl hemiacetals of α-bromoacetophenone. Effect of phenol leaving groups on the lifetime of tetrahedral intermediates
MacClelland, Robert A.,Kanagasabapathy, V. M.,Mathivanan, N.
, p. 2084 - 2093 (2007/10/02)
The kinetics of breakdown of seven aryl hemiacetals of α-bromoacetophenone (TOH) have been examined, with aryl substituents 4-MeO, 4-Me, 3-Me, H, 3-MeO, 4-Cl, 3-Cl.The hemiacetals were generated as intermediates in the aqueous bromination of α-aryloxystyrenes, where the breakdown of the hemiacetal is the rate-limiting stage in the overall reaction.Base catalysis of the breakdown shows large Broenstedt β values (0.8-1.0) and similarly large βlg values (-0.8 to -1.0).These are argued to arise because of a preassociation mechanism in which the hydrogen-bonded intermediate TO-...HA loses aryloxide faster than diffusional separation of the acid-base pair.This mechanism is enforced by lifetimes of the deprotonated hemiacetals TO- in the 1010 - 1012 s-1 range.Such lifetimes were estimated in two independent ways, by extrapolation of data previously obtained for alkyl hemiacetals of α-bromoacetophenone, and through a calculation based upon the experimental catalytic coefficients, and estimates of the acidity constants for TOH and the equilibrium constants for formation of hydrogen-bonded complexes.The pH-independent breakdown has βlg = -1.03, and the point lies near the Broenstedt line for the carboxylate catalysis.This, therefore, also occurs by a preassociation mechanism with water as the catalyst forming the intermediate TP-...H3O+, which loses aryloxide faster than diffusional separation.Catalysis by hydroxide ion occurs with rate constants of 8*109 M-1 s-1 independent of aryl substituent.This represents rate-limiting diffusional encounter of the hemiacetal and hydroxide ion, the hydrogen-bonded intermediate TOH...OH-1 rapidly proceeding on to products.Acid catalysis of the hemiacetal breakdown occurs by a class "n" mechanism in which the acid assists departure of the substituted phenol.Alkyl hemiacetals of α-bromoacetophenone break down by a class "e" mechanism where the leaving group is fully protonated before bond cleavage.The class "e" mechanism can be excluded for the aryl derivatives, since rate constants much greater than diffusion are required to explain the observed catalytic coefficients.The different mechanism is followed in order to avoid the fully protonated hemiacetal, which, being of an O-protonated anisole structure, is of very high energy. Key words: hemiacetal kinetics, preassociation, intermediate lifetime
