Ring Substituent Effects on Acetophenone Dimethyl Acetal Formation. 1. Dual-Parameter Treatment of Equilibrium Data in Methanol, Water, and Dodecane

Article abstract of DOI:10.1021/jo00173a006

For substituted acetophenones, the ketone to acetal equilibrium constants, K_x, measured by a water concentration jump method, are reported.Ketone and acetal partition coefficients, P, for the water-dodecane and methanol-dodecane solvent systems provide Gibbs free energies of transfer from methanol to dodecane and from methanol to water.Both sets of results make it possible to calculate K_x values in water and dodecane; these values are compared with those directly measured in methanol and with equilibrium constants reported for other reactions involving the trigonal to tetrahedral conversion of the carbonyl carbon atom.Substituent effects tor the three solvents are examined by means of the usual dual-parameter Gibbs free energy relationships.The Young-Jencks modified Yukawa-Tsuno equation, log K_x = ρⁿ?ⁿ + ρ^r(?⁺-?ⁿ) + i, gives ρⁿ = 1.73, 1.81, and 0.99 for methanol, dodecane, and water, respectively, but ρ^r ca. 1 whatever the solvent.It is concluded that the through-ring conjugation effects are solvent independent.In contrast, the Taft-Lewis equation, log K_x = ρ_I?_I + ρ⁺_R?⁺_R + i, gives ρ_I and ρ⁺_R values that are both solvent dependent.The substituent effects on relative solubilities in methanol, dodecane, and water are examined by means of the equation Π_X(Y) = log P_X(Y) - log P_H(Y) where Y is the carbonyl or the acetal group and X is the substituent.It is shown that log P_X(Y) deviates from additivity (Π_X(Y) <*> Π_X(H) and is Y dependent) because of the effects of X on carbonyl and acetal solvation.These effects are particularly large for solvation of acetals by water molecules.Therefore, substituent effects on K_x are interpreted as being mainly due to (a) an inductive effects of the sustituted ring on ketone stability, (b) a solvent-independent through-ring conjugation effect within the ketone, and (c) a specific inhibition of acetal solvation by water molecules.