Lewis Acids and Bases
A compound with an available pair of electrons either unshared or in a p-orbital is a Lewis base. A Lewis acid is any species with a vacant orbital, e.g., AlCl3, BF3, SnCl4 and SO3 etc. In SnCl4, tin has a complete octet, but can accept additional pairs of electrons (e.g., in SnCl42−). Hence, it is an acid too. Lewis acidity depends on the nature of the base. The following acidity sequence of Lewis acids of the type MXn has been suggested, where X is a halogen atom or an inorganic radical.
BX3 > AlX3 > FeX3 > GaX3 > SbX5 > SnX4 > AsX5 > ZnX2 > HgX2 (Lewis acids)
Pearson, in 1963, advanced an important concept to rationalize chemical reactions. According to this, a chemical bond is made up of an acid–base combination. Thus, properties of molecules can be explained by dividing them into acid and base components. Pearson classified the Lewis acids and bases into two groups, designated as ‘hard’ and ‘soft’. The hardness of an acid or a base is generally characterized by several factors, such as small atomic radius, high effective nuclear charge and low polarizability.
Hard and Soft Acids
The acceptor atoms of hard acids are small, have high positive charge and do not contain unshared pairs in their valance shells. They have low polarizability and high electronegativity. The soft species, on the other hand, possess opposite characteristics.
Hard and Soft Bases
The donor atoms of hard bases are of high electronegativity and low polarizability and are hard to oxidize. They hold their valence electron tightly. The donor atoms of soft bases are of low electronegativity and high polarizability and are easy to oxidize. They hold their valence electrons loosely (empty low-lying orbital).
The ease with which an acid–base reaction takes place depends not only on the strength of the acid and the base, but also on the hardness or softness of the acid or base.
When a Lewis acid combines with a base to give a negative ion in which the central atom has a higher than normal valency, the resulting salt is called an ate complex. For example,
Ate complexes are analogous to the onium salts formed when a Lewis base expands its valency; e.g.,
Me3N + MeI → Me4N+I− (onium salt).
Hard and Soft Acid–Base Classification
The chemical reactivity of molecules and ions can be rationalized on the basis of hard and soft acid and base (HSAB) principle, according to which hard acids prefer to associate and react with hard bases and soft acids with soft bases. Some common examples of each kind are listed in Table 1.5.
Table 1.5 Hard and Soft Acids and Bases
| Acids (electrophiles) | Bases (nucleophiles) | |
|---|---|---|
| Hard | BF3, AlCl3, SO3, AlH3, H+, RCO+, Li+, Na+, K+, Be2+, Mg2+, Ca2+, Al3+, Fe3+, CO, Co3+, Ti4+, Zr4+, Sn(IV), Ti(IV) | NH3, RNH2, N2H4, ClO4−, CH3COO−, ROH, RO−, R2O H2O, OH−, F−, Cl−, SO42−, NO3−, O2− |
| Border line | C6H5+, R3C+. R3B, NO+, SO2, Fe2+, Co2+, Cu2+, Zn2+, Sn2+, Pb2+ | aniline, pyridine, N3−, N2, Br−, NO2−, SO32− |
| Soft | BH3, carbenes, R+, I2, Br2, GaCl3, Ag+, Cu+, Pd2+, Pt2+ Hg2+ | C6H6, C2H4, H−, R−, I− RS−, R2S, RSH, RCN, CN− |
We know that ethers are cleaved with HI to give alkyl iodides and alcohols. The soft base I− reacts readily with soft acid R+ and the hard acid H+ with hard base RO−.
The acyl cation, RCO+ is a hard acid. It combines readily with hard bases like OH− or RO− to give carboxylic acids (RCOOH) and esters (RCOOR) which are stable molecules. With soft bases it would give highly reactive and labile molecules such as with I−, and acyl iodide RCOI. The sp3 carbon is a soft electrophile whereas the proton is a hard electrophile. Thus, according to the HSAB theory, a soft anion should act primarily as a nucleophile, giving the substitution product, whereas a hard anion is more prone to abstract a proton, giving the elimination product. The property of softness correlates with high polarizability and low electronegativity. Hardness reflects a high charge density and is associated with small, highly electronegative species.
One application of the rule is found in complexes between alkenes or aromatic compounds (soft bases) and metal ions (soft acids).
Leave a Reply