The Foundations of Chemistry

Theoretically, any species that contains an unshared electron pair could act as a base.
In fact, most ions and molecules that contain unshared electron pairs undergo some reac-
tions by sharing their electron pairs. Conversely, many Lewis acids contain only six
electrons in the highest occupied energy level of the central element. They react by
accepting a share in an additional pair of electrons. These species are said to have an open
sextet.Many compounds of the Group IIIA elements are Lewis acids, as illustrated by
the reaction of boron trichloride with ammonia, presented earlier.
Anhydrous aluminum chloride, AlCl 3 , is a common Lewis acid that is used to catalyze
many organic reactions. AlCl 3 acts as a Lewis acid when it dissolves in hydrochloric acid
to give a solution that contains AlCl 4 ions.

Cl

Cl

Cl Cl Al Cl

AlCl 3 (s)
Cl(aq) AlCl 4 (aq)





Cl

Cl

Cl Al

acid base product

Al is sp^3

hybridized

10-10 The Lewis Theory 389

Other ions and molecules behave as Lewis acids by expansion of the valence shell of the
central element. Anhydrous tin(IV) chloride is a colorless liquid that also is frequently
used as a Lewis acid catalyst. The tin atom (Group IVA) can expand its valence shell by
utilizing vacant dorbitals. It can accept shares in two additional electron pairs, as its reac-
tion with hydrochloric acid illustrates.

O

H

H :

base acid

+

+ O

H

H

H + O

H

O : ::

H

H ::

Bon

dformed

:

:

• Cl–

Cl

Cl

Cl

Cl

Al