Question

How can I identify a Lewis base?

Answer 1

Hint: By the theory of Lewis acid and base, the Lewis acid is defined as the species which are electron pair acceptors and the Lewis base are defined as the species which are electron pair donors.

Complete step by step answer:
The Lewis acid and Lewis bases are named after the American chemist Gilbert Newton Lewis.
The Lewis bases are defined as the electron rich species which have the ability to donate their electron pair to the neighboring atom. The Lewis base has highly localized HOMO (The highest occupied molecular orbital). The Lewis base forms an adduct with Lewis acid by donating its electron pairs. The Lewis base is considered as an electron donor.
The Lewis base are anionic in nature. The base strength of Lewis base is dependent upon the $p{K_a}$ of the parent acid.
The reaction between Lewis acid and Lewis base is shown below.
${A^ + } + {B^ - } \to A - B$
Where,
${A^ + }$ act as Lewis acid
${B^ - }$ act as Lewis base
The Lewis base can be identified by the presence of unbonded electrons pairs present in the central atom.
Example of Lewis bases: Pyridine and the derivative of pyridine act as a Lewis base as they contain an electron pair which they can donate to other atoms.
Benzene, ethyne, ethene can donate pairs of electrons.

Note:
As the Lewis base can donate the lone pairs of electrons, they are considered as nucleophiles and the Lewis acid which accepts the lone pairs of electrons are considered as electrophile. The benzene, ethene and ethyne have a $\pi$-system which are rich in electrons.