KINETICS OF THE ENOLIZATION REACTIONS OF 3-ACETYL-2,5-DIMETHYLFURAN AND OF 2-ACETYLSELENOPHENE

Rate constants for the enolisation reactions of title compounds have b een measured by their rates of halogenation at 25 degrees C in water, in several buffers, in dilute hydrochloric acid, in dilute sodium hydr oxide, and in the presence of some metal ion salts. The results have b een compared with those previously obtained from the corresponding rea ctions of acetophenone and a number of other acetyl heterocycles. Elec tronegativity of the heteroatoms and the ''pi-excessive'' nature of th e heterocyclic rings appear to be the main factors determining the rel ative reactivities in the acid-catalysed reactions. Bronsted beta valu es and isotope effects, k(H)/k(D), point to a more symmetrical transit ion state for the investigated acetyl heterocycles than that for aceto phenone in the general base-catalysed reaction. Metal-activating facto rs (MAF), i.e, the catalytic constant for metal-ion (Cu2+, Zn2+, and N i2+) catalysis, k(M), relative to that for proton catalysis, k(H), are discussed as an empirical measure of the ''hard or soft'' character o f the carbonyl groups in acyl heterocycles.