Pause for thought

BrØnsted and Lowry independently put forward their acid-base theory based on proton transfer in 1923. In the same year Gilbert Lewis from the United States of America. proposed an even more encompassing theory by extending it to the movement of pairs of electrons. Lewis bases are able to donate a non-bonding pair of electrons to form a coordinate bond with a Lewis acid which is itself deficient in electrons. It is worth emphasising that organic reaction mechanisms, such as nucleophilic substitution S_N2 reactions, are in fact Lewis acid-base reactions where 'curly arrows' are used to indicate the movement of pairs of electrons. Lewis acids and bases do not have to involve hydrogen or hydrogen ions and classic Lewis acids include transition metal ions in ligand replacement reactions.

Although that is the limit of IB definitions there are later even more all-encompassing definitions of acids and bases. One of the better known is that of Mikhail Usanovich (1894 – 1981) (see right) who in 1939 designated an acid as any electropositive or electrophilic substance and a base as any nucleophilic substance, i.e. an acid is anything that accepts negative species (including a single electron) or donates positive species. Usanovich worked in the USSR and his work was only truly recognised in the West much later. The essential difference between the Gilbert Lewis and Mikhail Usanovich theories is that the Lewis definition is concerned with the making and/or breaking of chemical bonds whereas the Usanovich definition is more concerned with the physical movement of electrons and thus strays very much more into the concept of oxidation and reduction. The property of acids reacting with reactive metals such as zinc to produce hydrogen or the reaction of chlorine with sodium to produce sodium chloride is covered by this definition whereas it is not covered by the Arrhenius, BrØnsted-Lowry or Lewis definitions.