Please consider 2-acetyl-1-pyrroline: http://en.wikipedia.org/wiki/2-Acetyl-1-pyrroline


What is the molecular geometry of the nitrogen lone pair? Is the lone pair oriented in-plane with the ring? How is this distinct from the orientation carbon-acetyl bond, which should presumably point out-of-plane with the ring?


The nitrogen lone pair is in plane with the ring because it is in one of the three $sp^2$ hybrid orbitals of N. The other two hybrid orbitals form $\sigma$ bonds to the adjacent carbons, and all three are in plane, with an angle of 120° between them. The $p_z$ orbital of N which is perpendicular to the ring plain forms the $\pi$ bond of the C=N double bond. The acetyl substituent should also lie in plane with the ring because this would allow energetically favorable conjugation between the two double bonds (C=N and C=O).

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  • $\begingroup$ Thank you for your answer! Would the carbon substituent group still lie in-plane with the ring if we replaced the acetyl with something like a methane? $\endgroup$ – user5463 May 11 '14 at 15:06
  • $\begingroup$ There would be no conjugation, but the methyl group would still lie in plane because it is smaller, and it does not contain more substituents (only 3 H) which can rotate out of plane and thus significantly affect conformation of the molecule (like in the case of C=O, where the orientation is significant due to the possible conjugation). $\endgroup$ – Jannis Andreska May 11 '14 at 15:11

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