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Can anyone explain this trend (the amines are listed in order of increasing basicity)?

Actually, I thought of understanding this by thinking of inductive effect of the alkyl groups attached to the amine group but since they are cyclic I am confused how to apply the inductive effect here or I want to know whether there are other ways of dealing with this problem.

  • $\begingroup$ Well I would love to know the answer aswell! Well the the option with methyl group has to be obviously the most basic as it is a 3° amine. But what about the others? $\endgroup$ – Prakhar Dec 9 '16 at 14:54

Inductive effect in this sequence in the reason of the increased basicity only in the case of N-methyl piperidine compared to piperidine. Increased basicity in the sequence of azetidine < pyrrolidine < piperidine is connected with the ring strain. Upon the decrease in the C-N-C angle, sp3-hybridized N atom "rehybridize" to increase p-character of orbitals involved in the C-N bonds formation, leading to the increased s-character of orbital involved in the N-H bond and the lone pair of electrons. Since s-electrons are closer to nucleus, this rehybridization leads to the increased stability of the N-H forming orbital and the lone pair in more strained cyclic amines making the lone pair being 'less exposed' to protonation (less basic).

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  • $\begingroup$ An alternative explanation is hyperconjugation between the lone electron pair on nitrogen and the sigma antibonding N-C orbital. The electron accepting ability of the orbital increases as the ring size decreases because the p character increases in the same manner (the energy drops). $\endgroup$ – RBW Dec 19 '16 at 15:39

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