Question

Amines behave as:
A. Lewis acids
B. Lewis Base
C. Aprotic acid
D. neutral compound

Answer 1

Hint: Amines are derivatives of ammonia in which one or all three hydrogen atoms are replaced by alkyl groups.
\[N{H_3}\xrightarrow[{ - H}]{{ + R}}RN{H_2}\xrightarrow[{ - H}]{{ + R}}{R_2}NH\xrightarrow[{ - H}]{{ + R}}{R_3}N\]
                      \[1^\circ \]amine \[{2^\circ }\]amine \[{3^\circ }\]amine

Complete step by step answer:
We will explain basic nature of amine using primary amines \[ \leftarrow R - N{H_{^2}}\]
In amines, lone pairs of electrons are present on nitrogen atoms. They donate electrons during reaction and form bonds with other atoms.
The molecule which donates a lone pair of electrons is called Lewis base.
The molecule which accepts lone pairs of electrons is called Lewis acid.
\[\therefore \] Amines are Lewis Base
Hence option (B) is correct.

Additional information:
Basicity of amines increases from primary amines to tertiary amines.
Aliphatic amines are stronger bases than ammonia. Alkyl groups are electron releasing groups, it increases electron density on nitrogen atoms and therefore, they donate electrons easily.

Number of alkyl groups increases from primary to tertiary amines. Alkyl group has a $ + I$ effect and it will increase electron density on nitrogen and therefore its basic nature increases. As a result its basic nature should be in order, Tertiary amine \[ > \] Secondary amine \[ > \] Primary amine.
This trend follows in a gaseous state. But in aqueous state basic character changes as,
Secondary amine \[ > \]Primary amine \[ > \] Tertiary amine
This is due to steric factors and solvation of ions. Aryl amines are less basic than alkyl amines due to delocalization of electron pairs in benzene rings.

Note:
Strength of Lewis base depends upon the availability of electron pairs present on nitrogen atoms. If it is easily available for bonding, then the compound is a strong base.