One of the most remarkable chemists of the twentieth century, Gilbert Lewis (1875–1946), who died in a rather peculiar event involving cyanide (which will figure further in this account) took the story of acids and bases that I described in Reaction 2, extended its reach, and thereby captured a further huge swathe of chemical countryside. As I remarked in that section, chemists seek patterns of behaviour, partly because it systematizes their subject but also because it gives insight into the molecular events accompanying a reaction. Lewis contributed greatly to this enlargement of chemistry’s vision, as I shall unfold in this section. I explained in Reaction 2 how Lowry and Brønsted had extended Arrhenius’s vision of acids and bases by proposing that all reactions between acids and bases involve the transfer of a proton (a hydrogen ion, H+) from the acid, the proton donor, to the base, the proton acceptor. For instance, hydrochloric acid, HCl, can provide a proton that sticks to an ammonia molecule, NH3, 1, converting it into NH4+, 2. The Lowry–Brønsted account of an acid–base reaction involves a proton as an essential part of the definition: if protons aren’t around, then Lowry and Brønsted are silent on whether a substance is an acid or a base. There are, however, many reactions that resemble acid–base reactions but in which no protons are transferred. I will give what might seem to be a rather esoteric example, but it makes the point in a simple and direct way, so please bear with me; you will soon see the relevance of this presentation to everyday life. The esoteric example I have in mind is a reaction in which a boron trifluoride molecule, BF3, 3, sticks to an ammonia molecule to form NH3BF3, 4. This reaction clearly resembles the proton transfer reaction in which H+ attaches to NH3 to form NH4+, but with BF3 playing the role of H+. Lewis brought these aspects together in a very simple idea in 1923, at about the same time as Lowry and Brønsted made their proposals. A base, he proposed, is any species that can use two of its electrons to attach to an incoming species.
Complexation of 2,6-helic[6]arene and its derivatives with 1,1′-dimethyl-4,4′-bipyridinium salts and protonated 4,4'-bipyridinium salts: an acid–base controllable complexation