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 the Concept of Basicity Basicity of amines is defined as the tendency of the nitrogen atom to donate its lone pair of electrons to a proton. The basicity is higher when the lone pair of electrons on nitrogen is more available for protonation. In aniline (C₆H₅NH₂), the lone pair of electrons on the nitrogen atom can interact with the benzene ring via resonance, making the nitrogen’s lone pair less available for protonation. This reduces the electron density on nitrogen, making aniline a weaker base than methylamine (CH₃NH₂), where the methyl group is an electron-donating group, increasing the electron density on nitrogen and making its lone pair more available for protonation. \bigskip Step 2: Resonance Effect in Aniline In aniline, the nitrogen's lone pair of electrons can be delocalized into the aromatic ring, creating a resonance structure where the nitrogen’s lone pair is not as available for protonation. This is called the resonance effect, and it reduces the basicity of aniline compared to methylamine. \bigskip Step 3: Methylamine and the Inductive Effect Methylamine, on the other hand, has a methyl group (–CH₃) attached to nitrogen. The methyl group is an electron-donating group via the inductive effect, which pushes electron density towards the nitrogen, making the nitrogen’s lone pair more available to accept a proton, thereby increasing its basicity. Thus, the basicity of methylamine is greater than that of aniline, and consequently, the pKₐ of aniline is more than that of methylamine.