Among the following compounds, the increasing order of their basic strength is : (I) aniline (II) pyrrole (III) N-methylpiperidine (IV) cyclohexylamine
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Pyrrole's lone pair is part of the aromatic sextet (least basic), aniline's is delocalised into the ring, and the aliphatic amines are the most basic; compare the primary and tertiary aliphatic ones by solvation and +I effects.
The compounds are (I) aniline, (II) pyrrole, (III) N-methylpiperidine (a tertiary amine) and (IV) cyclohexylamine (a primary amine). (II) Pyrrole: the nitrogen lone pair is part of the 6 \(\pi\)-electron aromatic sextet, so it is not available for protonation. Weakest base (conjugate acid \(\mathrm{pK_a}\) about \(-3.8\)). (I) Aniline: the lone pair is delocalised into the benzene ring (\(\mathrm{pK_a}\) of \(\mathrm{C_6H_5NH_3^+}\) = 4.6), so it is a weak base, but stronger than pyrrole. (III) and (IV): aliphatic amines, lone pair localised and enhanced by the +I effect of alkyl groups, so both are strong bases. In aqueous solution the tertiary amine is less stabilised by hydrogen bonding of its conjugate ion, so N-methylpiperidine (\(\mathrm{pK_a}\) about 10.1) is slightly weaker than cyclohexylamine (\(\mathrm{pK_a}\) about 10.6). Order: (II) < (I) < (III) < (IV). Answer: (4) (II) < (I) < (III) < (IV)
