Question

why is aniline less basic than methylamine

Answer 1

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Aniline or Phenylamine is a amine group attached to a phenyl group, or C6H5NH2. The ring structure possesses resonance. Electrons are delocalised from individual atoms and their position varies around the structure. The source of the basicity of the amine group is the lone pair on the nitrogen atom. This lone pair can form a dative covalent bond with a H+ ion. But in Aniline, the lone pair on the nitrogen is also delocalised. It loses the lone pair to resonance, and so cannot donate an electron to a proton, and therefore loses a lot of its basicity. Methylamine, CH3NH2, on the other hand does not possess resonance, and the lone pair on the nitrogen is in place, so it can accept protons, making it basic.

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Answer 2

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The strength of a base or an alkaline solution is determined by the pK$_b$ value. The lower the pKb value, the stronger the base. In the case of methylamine and aniline, aniline has more pK$_b$ value. Hence, aniline is less basic than methylamine. This happens because :

In aniline, the lone pair of electrons on N atom is delocalized over the benzene ring. As a result, electron density on the nitrogen decreases.

On the other hand, in $CH_3NH_2$, + I effect of $CH_3$ group increases the electron density on N atom.

Therefore, aniline is less basic than methylamine and hence pK$_b$ of aniline is higher than that of methylamine and aniline is less basic than methylamine.