Which of the following species is
unlikely to act as a Lewis base? (A) OH−, (B) NH3, (C) Al3+, (D) F−,
or (E) H2O.
To answer this question, we must
understand the definition of a Lewis base. A Lewis base is a substance that
can donate a lone pair of electrons forming a bond. The bond that is formed is called a
coordinate covalent bond and results from a Lewis acid–base reaction. A coordinate covalent bond is a
covalent bond where the pair of electrons forming the bond is donated from one atom
only. The species that is acting as the
Lewis base donates the pair of electrons to form the coordinate covalent bond and
can be considered the species that is electron rich, while the species receiving the
electron pair, or the Lewis acid, can be considered electron deficient. The product which is formed in a
Lewis acid–base reaction is called an adduct. The adduct is formed by the
coordinate covalent bond between a Lewis acid and a Lewis base.
With this information in mind,
let’s have a look at our answer choices. Answer choice (A) OH− or the
hydroxide ion is an electron-rich species with three lone pairs of electrons and
thus would act as a Lewis base. Since the question is asking us to
identify the species that is unlikely to act as a Lewis base, we can eliminate
answer choice (A). Answer choice (B) NH3 or ammonia is
also considered electron rich with one lone pair of electrons and can behave as a
Lewis base. We can eliminate answer choice
Answer choice (C) Al3+ or the
aluminum cation would be considered electron deficient with no lone pairs in its
valence shell and would therefore be more likely to act as a Lewis acid. Answer choice (C) is the correct
answer. Let’s rule out answer choices (D)
and (E) just to be sure. Answer choice (D) F− or the
fluoride ion is electron rich with four lone pairs of electrons and would behave as
a Lewis base. Finally, answer choice (E) H2O or
water is an electron-rich species with two lone pairs on the oxygen atom and can
behave as a Lewis base.
Therefore the species that is
unlikely to act as a Lewis base is answer choice (C), Al3+.