The Proton Transfer Concept

The claim of Arrhenius that the rate of acid-catalyzed reactions is proportional to the hydrogen ion concentration was soon found to require amendments as catalytic effects were discovered where the hydrogen ion concentration was negligible. In view of this predicament, T. M. Lowry [9] created a generalized proton transfer theory. For the most simple case of a mere rearrangement (isomerization) of a molecule, this theory can be outlined as follows  

Any acid-catalyzed reaction consists of three steps  

The species adding the proton is called proton donator , and the species withdrawing the proton is called proton acceptor . 

Lowry s ideas extended the notion of an acid to that of a substance capable of acting as a proton donator, so that, in his terminology, even pure water is an acid. On the other hand, the notion of a base was extended to that of a substance capable of acting as a proton acceptor, so that, in Lowry s terminology, water is a base as well. Hence, water turns out to be an amphoteric substance of central importance for catalytic processes. 

When the catalysis is supported not only by hydrogen ions but also by other spec-cies Xj, the reaction rate is expressed as 

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Our principal interest in acid-base chemistry is in aqueous solutions, where the Br0nsted-Lowry theory prevails. The balance of this chapter is limited to the proton-transfer concept of acids and bases. 

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