Question

Explain why a carbocation is a Lewis acid

Answer 1

According to Lewis concept an acid is defined as a pair of electron acceptor and a base is defined as a pair of electron donor (including lone pair). In this way a carbocation is capable of accepting a pair of electrons.

Just realize whats going on during SN1 reactions. The first step is loss of nucleophile by heterolytic bond dissociation (we call it as leaving group, LG), then a carbocation is formed and rearranged to a stable one, which on further attacked by a nucleophile (we call it as simply Nu-). A nucleophile is nothing but a pair of electron donor, hence a carbocation is an acceptor of pair of electrons,as per lewis this is Lewis acid.

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