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).
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).
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).More
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).
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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