THE EFFECT OF STRUCTURE ON REACTIVITY IN SEMICARBAZONE FORMATION OF SOME SUBSTITUTED 4-THIANONES AND 4-THIANONE-1,1-DIOXIDES

Abstract

The rates of semicarbazone formation of twenty-five heterocyclic ketones (4-thianones and 4-thianone-1,1-dioxides) have been measured at 10°C in 80% ethanol/20% water (v/v) at pH = 6.45 ± 0.05. The data indicate an overall second-order process, first-order each in ketone and semicarbazide. The addition of the semicarbazide is subject to general acid catalysis. In all the cases investigated, the reaction appears to be irreversible under the experimental conditions employed. Increased steric retardation is observed as the steric crowding increases around the carbonyl function, suggesting the rate-determining step is the attack of the semicarbazide on the carbonyl carbon. From the second-order rate constants for the semicarbazone formation from r−2, cis-6-diphenyl-4-thianone, 2,2-dimethyl-6-phenyl-4-thianone, r-2, cis−6-di-p-chlorophenyl-4-thianone and 2,2-dimethyl-6-p-chlorophenyl-4-thianone, the syn-axial interaction energy for CH₃[sbnd]OH has been calculated to be 2.41 kcal mol^−1.