Structure-Reactivity Effects in the Hydration of Benzaldehydes

Article abstract of DOI:10.1021/ja00347a033

Rate constants have been measured for the approach to hydration equilibrium for seven substituted benzaldehydes <4-Cl, 3-Cl, 4-CF3, 3-NO2, 4-NO2, 3-NO2-4-Cl, 3,5-(NO2)2>.The kinetic method involves the perturbing of the equilibrium position by forming hydrate anion in concentrated sodium hydroxide; addition of acid and buffer results in solutions containing excess hydrate.Hydration equilibrium constants have been calculated by knowing the rate constants in the two directions for the hydroxide ion catalyzed reaction, and a value for the 3,5-(NO2)2 system has been obtained by using a trapping technique.Literature values are also summarized; hydration has a ρ value of +1.7 in the hydration direction.General base catalysis is associated with Broensted β values around 0.4 and ρ values near zero in the dehydration direction.The β values increase with decreased electron withdrawal, while the ρ values decrease with increased base strength (δβ/δ? = -0.06 = δρ/δpK_a).A three-dimensional reaction coordinate diagram is used to show that this behavior is consistent with a class n mechanism - in the dehydration direction, equilibrium deprotonation of the hydrate followed by acid-catalyzed expulsion of OH.The water rates fit the general base correlations, although rate constants at the diffusion limit are required in one step.Two distinct Broensted plots are observed for general acids, one for RCOOH and a second for RPO3H(-), including H2PO4(-).The line for the latter is about 1 log unit above that for the former; this is attributed to an electrostatic interaction and not bifunctional catalysis.A significant cross correlation is also found here, with δα/δ? = -0.12 = δρ/-δpK_a.This is shown to be consistent with a class e mechanism - in the dehydration direction, equilibrium protonation of the hydrate followed by base-catalyzed expulsion of water.