- Nitrogen inversion
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Nitrogen inversion in ammonia
Inversion of an amine. The pair of dots represents the lone electron pair on the nitrogen atom.In chemistry, a nitrogen compound like ammonia in a trigonal pyramid geometry undergoes rapid nitrogen inversion whereby the molecule turns inside out. This interconversion is a room temperature process because the energy barrier (24.2 kJ/mol) is relatively small. Contrast this to phosphine which does not show inversion at room temperature (energy barrier: 132 kJ/mol) [1]. Even if all three substituents on the nitrogen in an amine are different, rapid inversion would prevent the nitrogen atom from becoming a permanent chiral center, since such inversion becomes effectively like a conformational change.
However, if the nitrogen is a bridgehead atom in a bicyclo or a similar compound where it cannot invert around the lone electron pair, then the nitrogen atom could be a chiral center if all three substituents on it are effectively different. An example of such a compound is Tröger's base.
In one study the inversion in an aziridine was slowed down by a factor of 50 by placing the nitrogen atom in the vicinity of a phenolic alcohol group compared to the oxidized hydroquinone [2] :
The system interconverts by oxidation by oxygen and reduction by sodium dithionite.
References
- ^ Kölmel, C.; Ochsenfeld, C.; Ahlrichs, R. An ab initio investigation of structure and inversion barrier of triisopropylamine and related amines and phosphines. Theor. Chim. Acta. 1991, 82, 271-284. doi:10.1007/BF01113258
- ^ Control of Pyramidal Inversion Rates by Redox Switching Mark W. Davies, Michael Shipman, James H. R. Tucker, and Tiffany R. Walsh J. Am. Chem. Soc.; 2006; 128(44) pp 14260 - 14261; (Communication) doi:10.1021/ja065325f
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