Question 79: Amongst the given options which of the following molecules / ion acts as a Lewis acid?
(1) \mathrm{BF}_{3}
(2) \mathrm{OH}^{-}
(3) \mathrm{NH}_{3}
(4) \mathrm{H}_{2} \mathrm{O}
Answer: Option (1)
Explanation:
A Lewis acid is defined as a species that can accept an electron pair.
\mathrm{BF}_{3} has an electron-deficient boron atom with only six electrons in
its valence shell and an empty orbital, so it can accept a pair of electrons.
\mathrm{OH}^{-} has excess electron density and donates an electron pair,
so it acts as a Lewis base.
\mathrm{NH}_{3} has a lone pair of electrons on nitrogen and donates it,
so it is a Lewis base.
\mathrm{H}_{2}\mathrm{O} also has lone pairs of electrons on oxygen and donates them, acting as a Lewis base.
Therefore, among the given options, only \mathrm{BF}_{3} acts as a Lewis acid,
and the correct answer is option (1).