Basicity and structure of α,Β-unsaturated heterocyclic ketones

Abstract

The constants of the protolytic equilibrium and the displacement of the frequency of the stretching vibrations of the hydroxy group of phenol taking place under the influence of the formation of a hydrogen bond with the compounds studied have been measured for dibenzylideneacetone and dibenzylidenecycloalkanones (with five-, six-, and seven-membered rings) and their furan and selenophene analogs. It has been shown that the screening of the carbonyl group of the aliphatic ring creates considerable steric hindrance for the solvation of the carbenium ions formed in an acid medium, as the result of which the constants of the protolytic equilibrium fall on passing from compounds with an open chain to the cycloalkanone derivatives. The inclusion of a carbonyl group in a five-membered aliphatic ring creates more favorable conditions for conjugation than exist in analogous compounds with an open chain and in those with six- and seven-membered rings, in consequence of which the capacity for forming a hydrogen bond changes in the corresponding sequence.