Mechanisms of acid catalysis

Survey of mechanisms of acid—base catalyzed reactions 5.1 MECHANISMS OF ACID CATALYSIS 

Acid—base catalysis is caused by the formation of a reactive intermediate from substrate and catalyst which opens a low free energy pathway for the reaction. Consequently, the phenomenon of catalysis cannot be separated from problems of reaction mechanism. In this section, various possibilities of reaction mechanisms involving acid—base catalysis are discussed from a deductive point of view, with respect to structure and reactivity of substrates and intermediates [3, 4,14, 83]. 

In the mechanism of an acid catalyzed reaction, protonation of a substrate may occur at an unshared electron pair of an O (S) or N atom. This leads to the formation of intermediates of the types R2OH+ or R3 NH+ which may undergo subsequent SN 2 or SN1 reactions, as outlined in the examples (Nu = nucleophile) 

Protonation of a carbonyl oxygen atom facilitates addition of a nucleophile to the carbonyl carbon in the next step, viz. 

There are many examples of acid catalyzed carbonyl addition reactions, such as formation of hydrates (R2C(OH)2), hemiacetals, hemiketals, cyanohydrins, bisulfite compounds, azomethines, oximes, hydrazones, etc. These important reactions are discussed in Vol. 11. 

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C. M. Regan and J. Allen Mechanism of Acid Catalysis. The Kinetics... 

Traditionally, the same overall mechanisms of acid catalysis invoking carben-ium ions have been assumed to prevail both in heterogeneous (2) and in liquid homogeneous (3) systems. But these mechanisms do not adequately take into account the fact that adsorbed, rather than free, carbenium ions are formed in the pores of solid catalysts. Consequently, a quantum-chemical model that demonstrates how the interaction of carbenium ions with the sites of their adsorption can influence the reaction mechanism has been formulated by Kazansky (4), taking double-bond-shift reactions in olefins as a particular example. According to this view, adsorbed carbenium ions are best regarded as transition states rather than reaction intermediates, a notion that had also been proposed earlier by Zhidomirov and one of us (5). 

Explain the mechanism of acid catalysis of nucleophilic additions to the carbonyl group. 

It was generally believed that the characteristic non-linear dependence of the rate constant on n was associated with the existence of the preequilibrium (Bell, 1941). For the mutarotation of glucose a linear variation of rate constant with n had been reported (Hamill and La Mer, 1936c) and cited as an example of a mechanism of acid catalysis without pre-equilibrium. 

Equations (38) to (40) represent the formation of the transition state for the three most commonly considered mechanisms of acid catalysis for hydrogen ions in aqueous solution. They are designated A-l, A-2 and A-S 2 mechanisms, and the appropriate equilibrium constants for the formation of the respective transition states are expressed by equations (41) to (43). Equations for other mechanisms—the existence of which we certainly do not wish to exclude by implication—can be developed in an analogous fashion. 

The arguments given on p. 115 on the basis of which mechanisms of acid catalysis can be distinguished are not restricted to water, and it should be possible to distinguish mechanisms in other solvents. There are at present few results that test this prediction. The volume of activation of the acid-catalyzed hydrolysis of ethylene oxide is about 3 3 cm mole more negative in 50% v/v dioxane-water than in water (Withey et al., unpublished results) and that for the acid-catalyzed hydrolysis of methyl acetate is more negative in 25% v/v and 50% v/v acetone-water than in water by 1-6 and 4-5 cm mole respectively (Osborn and Whalley, quoted by Koskikallio, et al., 1959 Withey et al., unpublished results). Measurements on other reactions, particularly on A-1 reactions, have not yet been performed, and the usefulness of volumes of activation of acid-catalyzed reactions in solvents other than water is not yet confirmed. 

Abstract. Alkoxide ion departure from Meisenheimer complexes of the 1,l-dialkoxy-2,6-dinitro-4-X-cyclohexadienate type is catalyzed by pyridinium ions and by the hydronium ion. Bronsted a values which vary between 0.35 and 0.65 decrease when the alkoxy group becomes more electron withdrawing and increase when the X-substituent is made more electron withdrawing. This is inconsistent with a mechanism where proton transfer is rate limiting but in complete agreement with a concerted mechanism of acid catalysis as shown with the help of More O Ferrall-Jencks energy diagrams. 

The kinetics and mechanisms of acid catalysis of intramolecular cyclization of 1,3-diaryl-3-(2-aminophenylsulfenyl)propan-l-ones (14) to yield cyclic imines in mixtures of methanol and various acids have been described. The X substituent significantly... 

In the case of a mechanism of acid catalysis involving a fast pre-equilibrium... 

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