Question

Which of the following is not a nucleophile?
(A) $$B{F}_3$$
(B) $$N{H}_3$$
(C) $$C{N}^{-}$$
(D) $$O{H}^{-}$$

Answer 1

Hint: A nucleophile is a chemical species that is electron rich in nature and attacks a group that has less electron density. A nucleophile may be neutral or anionic in nature, but it should always have unshared pairs of electrons that it can share or donate to an electron deficient site.

Complete Step By Step Answer:
A nucleophile must contain electrons that it can offer to an electron deficient species. The electrons can be present in the form of a lone pair on a neutral molecule or as a negative charge.
Ammonia is a tetrahedral molecule containing a lone pair of electrons on the nitrogen atom that remains unshared and can be easily offered to any electron deficient species, therefore ammonia is a good Lewis base as well as a nucleophile.
Cyanide ions and hydroxide ions are anionic species containing unshared pairs of electrons.
The compounds of boron are generally electron deficient as the central atom boron has a total of three electrons in its valence shell and is therefore capable of forming three bonds only. The boron atoms in boron trifluoride have six electrons around it and are short of two electrons in order to complete its octet.
$$\Rightarrow$$ Thus, $$B{F}_3$$ is a Lewis acid and an electrophile rather than a nucleophile and option (a) is correct.

Note:
It is not necessary for any base to act as a nucleophile and conversely not all nucleophiles are basic in nature. But a Lewis acid in itself is an electron deficient compound and therefore cannot be called a nucleophile.