85152-57-4Relevant articles and documents
THE PROTON ACTIVATING FACTOR AND ITS APPLICATION TO ACID AND BASE CATALYSIS OF ALDEHYDE HYDRATION
Stewart, Ross
, p. 907 - 910 (2007/10/02)
The data of McClelland and Coe for the general acid and general base catalyzed hydration of a series of substituted benzaldehydes has been analyzed in terms of the proton activating factor, paf, and concepts related thereto.Although the latter were derived for use with prototropic systems, the present work shows that they have application to nucleophilic addition as well.Three conclusions emerge from the analysis.1.Alternative means of calculating paf from buffer catalytic data that were previously derived on theoretical grounds can be used in practice to give essentially the same results.2.The mechanism of the general acid catalyzed pathway for aldehyde hydration is that favoured by McLelland and Coe and by Jencks, to wit the class (e) mechanism, rather than the class (n) mechanism.3.Electron-withdrawing groups in the benzaldehyde ring cause the disparity between the contributions of alternative, kinetically equivalent, pathways for the uncatalyzed reaction to become greater.
Structure-Reactivity Effects in the Hydration of Benzaldehydes
McClelland, Robert A.,Coe, Margaret
, p. 2718 - 2725 (2007/10/02)
Rate constants have been measured for the approach to hydration equilibrium for seven substituted benzaldehydes .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/δpKa).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/-δpKa.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.
