What Is the Geometry at Trigonal Nitrogen?

K. P. Sudlow and A. A. Woolf

University of the West of England, Faculty of Applied Sciences, Frenchay Campus, Coldhorbour Lane, Bristol BS16 1QY, UNITED KINGDOM    

Abstract

Organic texts invariably depict aliphatic amines as pyramidal but anilines as planar presumably because of contributions from resonance forms with double bonds from carbon to nitrogen. Other evidence relies on comparisons made from aqueous basicities and dipole moments. Subsequent developments using data from gas-phase acidities, computational modeling and X-ray diffraction contradict these old premises. Ab intio calculations pyramidal anilines to be more stable than planar in the gas phase. Single crystal structures show both planar and pyramidal anilines exist. The latter, including aniline itself and halo or methyl substituted anilines, have C-N lengths near single bonds. the former contain nitro-substituents which help delocalize the nitrogen lone pair density to produce C-N bond orders between 1.5 and 2. Planarity is also induced by attaching very electronegative groups directly to nitrogen as well as by sufficient intermolecular hydrogen bonding between amino and nitro groups to overcome energy differences between planar and pyramidal forms.

Journal of Chemical Education, 1998, 75, 108            Full text (PDF)

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