Hyperconjugation • Inductive effect of alkyl group follows following order (CH3)3C- > (CH3)2CH- > CH3CH2- > CH3- when they are attached to saturated carbon. But when they are attach to unsaturated carbon i.e. C=C or aromatic system then order is reversed. This abnormal behavior of electron release can be explained by a new concept other than resonance and inductive effect which is called as Hyperconjugation. • Here electron releasing effect can be explained by assuming that ó orbital of á C-H bond overlap with the adjacent ð orbital. This displacement of electron pairs of á C-H bond cause a partial positive charge on the H atom without the actual proton release. • ability of sigma bonded electrons of an alpha C-H bond to undergo conjugation with the adjacent pi electron is called as hyperconjugation. • Concept of hyperconjugation is generally applied to carbocation , free radical & C=C double bonded system such as alkene and aromatic compounds. www.OrganicChemistry.co.in
Hyperconjugation • In case of carbocations & free radicals, the carbon atom having positive charge or unpaired electron respectively is á carbon. Therefore in that case only the electrons in the C–H ó bond that are â to the positively charged carbon or free radical show hyperconjugation & can stabilize them. • In case of alkenes the carbon adjacent to double bonded carbon is á carbon. Here the electrons in the C–H ó bond that are á to the double bonded carbon atom stabilize it by hyperconjugation. • Hyperconjugation is also permanent effect similar to Resonance & Inductive effect. • It is distance independent similar to resonance. • Hyperconjugation also increases the stability of molecule. Greater is the number of hyperconjugation greater is the stability because it is also a stabilizinh phenomenon similar to resonance. www.OrganicChemistry.co.in
Hyperconjugation • As no bond structures are obtained similar to resonance so hyperconjugation is also called as no bond resonance. • One point should always kept in mind that the stability of carcanion is not decided by hyperconjugation in general case. For that we use the concept of M effect or I effect. • To calculate the number of hyperconjugation or no bond resonance always count the hydrogen present over the carbon adjacent to the carbon having + charge, free radical or double C=C bond. • If proton is released from the system completely then such effect is called resonance & if proton is present in the vicinity with no bonding with any atom then same effect is called as hyperconjugation. • One most important point must always kept in mind that concept of hyperconjugation is only applied to carbocation, free radical and alkenes. So never think the concept of hyperconjugation in C=O, C=N, C=S, C≡N etc. www.OrganicChemistry.co.in
