Hyperconjugation resonance description

HYPERCONJUGATION. The description of the properties of a molecule m terms of resonance structures in which an atom or group is not joined by any sort of bond to the atom to which it is ordinarily considered linked. Also called no-hond resonance. The hypothesis of hyperconjugaiion has been advanced lo interpret some properties of substances containing but I double bond by analogy wilh those of substances containing conjugated double bonds. Consider a substance with a terminal... 

Hyperconjugation refers to the delocalization of electrons in a bonds. Its application to carbocations such as CH3CH2 can be described in terms of resonance between contributing structures, and valence bond and molecular orbital models as well. According to the resonance description, delocalization of the electron pair in a C—H bond of the methyl group is represented by a contributing structure containing a C=C double bond. 

Free radicals, like carbocations, have an unfilled 2p orbital and are stabilized by substituents such as alkyl groups that can donate electrons by hyperconjugation. According to the resonance description of hyperconjugation, the unpaired electron, plus those in radical site, are delocalized. 

A methyl group releases electrons to an attached double bond in much the same way that it releases electrons to an 5p -hybridized carbon of a carbocation or free radical—by an inductive effect and by hyperconjugation. The resonance description of hyperconjugation in an alkene is consistent with a flow of electrons from the alkyl group to the carbons of the double bond. 

The results of a valence bond treatment of the rotational barrier in ethane lie between the extremes of the NBO and EDA analyses and seem to reconcile this dispute by suggesting that both Pauli repulsion and hyperconjugation are important. This is probably closest to the truth (remember that Pauli repulsion dominates in the higher alkanes) but the VB approach is still imperfect and also is mostly a very powerful expert method [43]. VB methods construct the total wave function from linear combinations of covalent resonance and an array of ionic structures as the covalent structure is typically much lower in energy, the ionic contributions are included by using highly delocalised (and polarisable) so-called Coulson-Fischer orbitals. Needless to say, this is not error free and the brief description of this rather old but valuable approach indicates the expert nature of this type of analysis. 

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