Proton donor-acceptor terminology

For the sake of consistency of terminology, triads of molecules in which the central unit acts simultaneously as both proton donor and acceptor will be termed sequential to distinguish such configurations from those in which the central molecule acts as double proton donor or double acceptor. A perhaps more quantitative expression of cooperativity is referred to in the literature as nonadditivity. The latter term is commonly taken as the difference between the total interaction energy of an aggregation of molecules on one hand and the sum of all the pairwise interactions on the other. 

In the Bronsted-Lowry description, an acid is a proton donor and a base is a proton acceptor. Review the terminology of conjugate acid-base pairs in Section 10-4. 

HCl + H2O HsO + Clin view of the disadvantages, it can be questioned whether it is worth tr dng to extend the ideas of acid and base in the way that Lowry and Bronsted do. Chemistry might be better served if the classification of molecules and ions according to their ability to accept or donate protons - which is undoubtedly a very useful classification - were to be divorced Ifom the classification of substances into acids and bases, and be given a terminology of its own. In fact, such terminology lies readily to hand, the terms proton donor and proton acceptor being entirely suitable for this purpose. 

A subset of Lewis acids and Lewis bases is represented by Brpnsted acids and Bronsted bases. Whilst Lewis acids are electron accepters and Lewis bases are electron-donors, Bronsted acids are defined as hydrogen (or proton) donors and Bronsted acids are defined as hydrogen (or proton) acceptors. The occurrence of Bronsted acid-base interactions thus is less general than that of Lewis acid-base interactions. Because of its wider application and to avoid confusion, the Lewis terminology is the preferred one in this work. 

This level of ionization is particularly relevant in some enzymic reactions where histidine residues play an important role (see Section 13.4.1). This means that the imidazole ring of a histidine residue can act as a base, assisting in the removal of protons, or, alternatively, that the imidazolium cation can act as an acid, donating protons as required. The terminology used for such donors and acceptors of protons is general acid catalyst and general base catalyst respectively. 

Rather than introduce new words into an already confusing terminology, Bell (64) is of the opinion that the words acid and base should be confined to proton acids, and the Sidgwick classification of molecules as electron donor and electron acceptor (which is essentially equivalent to the Lewis classification of acids and bases) be employed, together with the categories of nucleophilic and electrophilic reagents as defined by Ingold (65). 

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