Lewis acid properties of benzaldehydes and substituent effects

Abstract

Benzaldehydes, which do not exist in the hydrated form at lower pH values, have been found to add hydroxide ions in a reversible, rapidly established acid–base reaction in strongly alkaline media. In the reaction in aqueous solutions, an anion of the geminal diol is formed in reaction (1). Measurement of the decrease of absorbance in the 250–280 nm region as a function of sodium hydroxide concentration made it possible to determine equilibrium constants of reaction (1) for substituted benzaldehydes. The pK values follow a Hammett ρσ plot with ρ= 2·76. Deviations from the linear plot observed for m-hydroxybenzaldehyde and isophthalaldehyde are discussed.