Hyperconjugation
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Hyperconjugation in organic chemistry is the stabilizing interaction that results from the interaction of the electrons in a sigma bond (usually C-H or C-C) with an adjacent empty (or partially filled) non-bonding p-orbital or antibonding π orbital or filled π orbital to give an extended molecular orbital that increases the stability of the system [1]. Only electrons in bonds that are β to the positively charged carbon can stabilize a carbocation by hyperconjugation.
The term was introduced in 1939 by Robert S. Mulliken [2] in the course of his work on UV spectroscopy of conjugated molecules. Mulliken observed that on adding alkyl groups to alkenes the spectra shifted to longer wavelengths. This bathochromic shift is well known in regular conjugated compounds such as butadiene. He was also the first to attribute the lower heat of hydrogenation for these substituted compounds (compared to those without substitution) to hyperconjugation. An effect predating the 1939 hyperconjugation concept is the Baker-Nathan effect of 1935.
Hyperconjugation can be used for rationalizing a variety of other chemical phenomena, including the anomeric effect, the gauche effect, the rotational barrier of ethane, the beta-silicon effect, the vibrational frequency of exocyclic carbonyl groups, and the relative stability of substituted carbocations. Hyperconjugation is proposed by quantum mechanical modeling to be the correct explanation for the preference of the staggered conformation rather than the old textbook notion of steric hindrance. [3] [4].
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[edit] References
- ^ Organic chemistry John McMurry 2nd edition ISBN 0534079687
- ^ Intensities of Electronic Transitions in Molecular Spectra IV. Cyclic Dienes and Hyperconjugation Robert S. Mulliken J. Chem. Phys. -- May 1939 -- Volume 7, Issue 5, pp. 339-352 doi:10.1063/1.1750446
- ^ Hyperconjugation not steric repulsion leads to the staggered structure of ethane Pophristic, V. & Goodman, L. Nature 411, 565–568 (2001)Abstract
- ^ Chemistry: A new twist on molecular shape Frank Weinhold Nature 411, 539-541 (31 May 2001) Abstract
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