Chapter 5. ACIDITY AND BASICITY OF ORGANIC COMPOUNDS

At present, there are two main concepts of acids and bases in organic chemistry. In the first one, independently proposed by the Danish physico-chemist J.N. Branstedt and the English chemist T.M. Lowry (in the 1920's), it is stated that:

An acid is a neutral molecule or an ion that can donate a proton, and a base is a neutral molecule or an ion that can accept a proton.

For example, when gaseous hydrogen chloride dissolves in water, the latter accepts a proton from hydrogen chloride:

Here hydrogen chloride acts as a proton donor (an acid), and water acts as a proton acceptor (a base). The products of this reaction are the hydronium ion, H₃O⁺ (the conjugate acid of water), and the chloride ion, Cl^- (the conjugate base of hydrogen chloride). Note that the term conjugate is used in another sense different from that in Chapter 3.

In a general sense, acid-base reaction can be expressed in the following way: Other examples of acid-base interaction are presented below:

In the above examples water acts either as an acid or as a base, as well as it represents either a conjugate acid or conjugate base.

Another definition of acids and bases was proposed by G.N. Lewis (1926).

A Lewis acid is any substance that can accept an electron pair, and a Lewis base is any substance that can donate an electron pair in forming a covalent bond.

According to this definition, a Lewis acid must have a vacant orbital for bonding. The simplest Lewis acid is a proton because it can accept an electron pair from a Lewis base to fill its 1 s shell, for example:

Not only proton donors belong to Lewis acids but also many other species with an atom whose valence shell is unfilled, such as various metal cations or compounds of Group 3A elements (BF₃, AlCl₃). Thus, boron trifluoride (a gaseous Lewis acid) reacts with diethyl ether (a liquid Lewis base, boiling point 36 °C) to form a stable addition product:

The Lewis definition of acids is much broader than the Bronstedt definition, whereas bases are defined very similar in both theories. The Lewis conception finds a use for a big variety of organic reactions. The only, but substantial, disadvantage of the Lewis theory consists in the absence of quantitative acidity and basicity scales. The Bronstedt concept is therefore more widely used in organic chemistry.