Phenylacetaldehyde and its cis- and trans-enols and enolate ions. Determination of the cis : trans ratio under equilibrium and kinetic control

Abstract

A method of determining individual keto–enol equilibrium and acid dissociation constants for systems in which unstable enols can exist in cis and trans isomeric forms is developed and is applied to phenylacetaldehyde in aqueous solution; the results give equilibrium cis : trans ratios of 35 : 65 in acidic and neutral solutions and 20 : 80 in a basic solution (where the enols are converted to enolate ions), but show considerably by less cis : trans differentiation for enols formed under kinetic control.