Question

(b) Why pKₐ of aniline is more than that of methylamine?

Hint:

The basicity of aromatic amines like aniline is reduced due to resonance with the aromatic ring, while aliphatic amines like methylamine are more basic due to the electron-donating inductive effect of the alkyl group.

Answer 1

Step 1: Understanding Basicity
The basicity of amines refers to the ability of the nitrogen atom to donate its lone pair of electrons to a proton. The higher the availability of the nitrogen’s lone pair for protonation, the greater the basicity. In aniline (C₆H₅NH₂), the nitrogen’s lone pair can participate in resonance with the benzene ring, reducing its availability for protonation. This lowers the electron density on the nitrogen, making aniline a weaker base compared to methylamine (CH₃NH₂), where the methyl group is an electron-donating group. The methyl group increases the electron density on nitrogen, making the lone pair more available for protonation and enhancing the basicity. \bigskip Step 2: Resonance Effect in Aniline In aniline, the nitrogen’s lone pair can be delocalized into the aromatic ring through resonance, forming a structure where the lone pair is less available for protonation. This resonance effect decreases the basicity of aniline when compared to methylamine. \bigskip Step 3: Methylamine and the Inductive Effect Methylamine, in contrast, has a methyl group (–CH₃) attached to nitrogen. The methyl group donates electrons via the inductive effect, increasing the electron density on nitrogen and making its lone pair more available for protonation, which increases the basicity. As a result, methylamine has a greater basicity than aniline, leading to a higher pKₐ for methylamine compared to aniline.