Ring Substituent Effects on Acetophenone Dimethyl Acetal Formation. 2. Dual-Parameter Treatment of Kinetic Data for Acid-Catalyzed Acetal Formation and Hydrolysis in Methanol Containing Small Amounts of Water

Article abstract of DOI:10.1021/jo00224a056

The rates of formation and hydrolysis of para- and meta-substituted acetophenone dimethyl acetals have been measured, by a water-concentration method, in methanol containing HBr and small amounts of water.The rate coefficients for pure anhydrous methanol have been extrapolated .Acid and water-concentration effects are consistent with the usual mechanism for acetal hydrolysis in water, but the rate-limiting step in the hydrolysis direction corresponds to water addition to the hemiacetal-yielding oxocarbenium ion.Substituent effects on both extrapolated rate coefficients fit the Young-Jencks-modified Yukawa-Tsuno equation.The validity of this equation and its usefulness for separating polar and conjugative effects are discussed.