A comparison of the effects of phenyl, pyridyl, and pyridinio substituents upon keto-enol tautomerisation and enol ionisation equilibria of acetophenone shows that polar effects upon bond hybridisation and resonance interaction with 'neutral' double bonds are important influences upon enol stability.In the past five years the availablity of measurements of keto-enol tautomerisation constants of a number of simple aldehydes and ketones, including acetaldehyde, acetone, and acetophenone ,1--4 has made possible an examination of substituent effects upon these equilibria. 135.6 Steric effects have been discussed in detail by Rappoport,s,6 and the purpose of this communication is to consider electronic effects. These are interesting because keto-enol equilibria involve only neutral molecules whereas electronic effects normally reflect resonance or inductive stabilisation of ionic charges.Enols can be substituted at aor P-carbon atoms [see (2)]. At either position the main effect of the substituent is upon the double bond of the enol. At the a-position however the effect is complicated by a compensating interaction with the carbonyl group of the keto tautomer,176 and for aryl and alkyl groups this leads to opposite effects of substituents upon the equilibrium at the the two positions. Thus the enol content of acetaldehyde (pKE = 6.23 in aqueous solution)4 is greater than that of acetophenone (pKE = 7.96) but much less than that of diphenylacetaldehyde7 (pKE = 0.98) or 9-formyl-fluorene8 (4) (pKE = -1.7).
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