What Is the Geometry at Trigonal Nitrogen?
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K. P. Sudlow [1] ; A. A. Woolf [1]
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[1] University of the West of England
University of the West of England
Reino Unido
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- Localización: Journal of chemical education, ISSN 0021-9584, Vol. 75, Nº 1 (January), 1998, págs. 108-110
- Idioma: inglés
- Texto completo no disponible (Saber más ...)
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Resumen
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.
