Lewis acid-base theory
The Lewis acid-base theory, devised by G. N. Lewis in 1923, is the most comprehensive acid-base theory that covers all acid-base reactions as well as the formation of complexes. Lewis acids are often used to catalyze reactions, and anhydrous aluminium trichloride, AlCl3, is an especially important Lewis acid in many industrial chemical processes. Many Lewis acids are pyrophoric and will spontaneously burn when exposed to the moisture present in air.
Definitions
In the Lewis theory, a Lewis acid is any species that can accept a share of an electron pair and a Lewis base is any species that can donate a share of an electron pair. In this theory, neutralization occurs when the Lewis acid and Lewis base form a coordinate covalent bond.
Examples
In the reaction of boron trifluoride, BF3, with ammonia, NH3, boron trifluoride is the Lewis acid and ammonia is the Lewis base. The two chemicals form a complex in which BF3 accepts a share of the lone electron pair of NH3.
BF3 + :NH3 --> F3B:NH3
When anhydrous aluminium trichloride in dissolved in hydrochloric acid, aluminium tetracloride ions are formed.
AlCl3 + HCl --> AlCl4 + H+
Anhydrous aluminium trichloride is used in many organic reactions in which the aluminium atom catalyses a reaction by accepting electrons from carbonyl oxygen atoms, nitrogen atoms in amide groups, or ether-like oxygen or nitrogen atoms, often within cyclic compounds. Al3 is often used as a catylists in commercial polymerization reactions.
Valence expansion
Compounds of the elements of group IIIA (B, Al) tend to have an unfilled valence shell, and hence are often good Lewis acids. Other elements are Lewis acids because of their ability to expand their valence shell. Anhydrous tin tetrachloride (tin[IV]chloride) SnCl4, also called stannic chloride, is a typical example. Although the valence shell of the tin atom is filled with eight electrons, the tin atom has two empty d-orbitals which can each accept a share of an electron pair, as shown below.
SnCl4 (liquid) + 2 Cl- (aq) -->SnCl6 (aq)
Because stannic chloride can accept two additional electron pairs, it is a useful reagent for chelation controlled, stereo-specific reactions in which the formation of a complex locks the organic reactant in one particular conformation for a subsequent reaction.
Comparison to Brønstad-Lowry acid-base theory
The Brønstad-Lowry acid-base theory normally thought of regarding the presence of hydronium and hydroxide ions is included within the Lewis acid-base theory. Thus, two water molecules react in which one of the water molecules acts as the Lewis acid and the other water molecule acts as the Lewis base, as shown in the reaction below, in which the water molecule on the left acts as the Lewis base and the other water molecule is the Lewis acid.
H2O: + H:OH --> H2O:H+ + :OH