Acid-base catalysis molecular mechanism

III. The Molecular Mechanism of Acid-Base Catalysis 1. The General Nature of Acid-Base Catalysis... 

Clearly, a detailed discussion of the equilibrium and kinetic properties of proton-transfer reactions is necessary in order to establish a molecular mechanism of acid-base catalysis. Only a few specific aspects of acid-base catalysis are discussed here more detailed accounts are available elsewhere [21,22]. 

Edwards [25] investigated the hydrolysis of aspirin in aqueous solution at 17A° and demonstrated that the aspirin is hydrolyzed by general acid-base catalysis and water molecules for ionic and nonionic aspirin, comprising six simultaneous reactions involving H O, O H, and H2O for ionic and nonionic aspirin. The intrinsic hydrolysis rate constant in the heated mixture was comparable with the hydrolysis rate constants of the two-element reactions of nonionic aspirin and H2O or ionic aspirin and H2O in aqueous solution. As aspirin molecules would be adsorbed onto the pore surface of PCC in the molecular state, as a possible mechanism of aspirin hydrolysis in the mixture with PCC it was suggested that the aspirin is dispersed monomolecularly in the heated mixture and reacts with water molecules rather than by acid-base catalysis. 

In originally considering the 5 3 mechanism, involving base catalysis, Bennett, Brand, James, Saunders and Williams were trying to account for the small increase in nitrating power which accompanies the addition of water, up to about 10%, to sulphuric acid. The dilution increases the concentration of the bisulphate ion, which was believed to be the base involved (along with molecular sulphuric acid itself). The correct explanation of the effect has already been given ( 2.3.2). 

Since the products are the same chemical species as the reactants, the over-all reaction is substantially thermoneutral except for activation energy, the problem of energetics is thus side-stepped. The apparent ter-molecular reaction required by Eq. (43) is also no problem, as the dissolved molecules are essentially in constant collision with water molecules. Wilmarth, Dayton, and Flournoy questioned the adequacy of this concerted attack mechanism, however, as they believed that it would predict acid catalysis of the exchange reactions, as well as base catalysis. 

There has been a resurgence of interest in proton-coupled redox reactions because of their importance in catalysis, molecular electronics and biological systems. For example, thin films of materials that undergo coupled electron and proton transfer reactions are attractive model systems for developing catalysts that function by hydrogen atom and hydride transfer mechanisms [4]. In the field of molecular electronics, protonation provides the possibility that electrons may be trapped in a particular redox site, thus giving rise to molecular switches [5]. In biological systems, the kinetics and thermodynamics of redox reactions are often controlled by enzyme-mediated acid-base reactions. 

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