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Explain hyperconjugation or no bond resonance withexample |
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Answer» Solution :Like resonance hyperconjugation ALSO increases stability. In which without writing the bond of `C-H` of alkyle group, different structures are represented. Definition: ..It involves delocalisation of `sigma` electrons of C-H bond of an alkyl group directly attached to an atom of unsaturated system or to an atom with an unshared p-orbital. (a) Hyperconjugation effect in `CH_(3)overset(+)(C )H_(2)` (ETHYL cation) and its structure: In this `CH_(3) overset(+)(C )H_(2)`, the positively charge carbon atom has an empty p-orbital one of the C-H bonds of the METHYL group can align in the plane of this empty p-orbital and the electrons constituting the C-H bond in plane with this p-obital can then be delocalised into the empty p-orbital  In such arrangement, hydrogen of alkyle group is other than `H^(+)` bond. The hyperconjugation structure of `CH_(3) overset(+)(C )H_(2)` are as under  All the three hydrogen of `CH_(3), ""^(1)(H, ""^(2)H and ""^(3)H` becomes `H^(+1)` in structure (I) (II),(III) in these, there is no bond with `H^(+)`. e no `sigma` bond. The positive charge (+) displace and delocalised by these structure and the stability of cation is increase. (b) Relative comparisons of different alkyle cation: Because of the positive (+) charge of carbon, if no of alkyle group attach with the carbon increases then interaction in hyperconjugation is increases so, displacement of positive (+) charge increases hence, stability of carbocation increases `CH_(3)- underset(underset(CH_(3))(|))overset(overset(CH_(3))(+|))(C ) gt (CH_(3))_(2) overset(+)(C )H gt CH_(3) overset(+)(C )H_(2) gt overset(+)(CH_(3)) larr` Stability of carbocation `uarr` as `larr` (c ) Hyperconjugation in alkene: Alkene e.g. in propene displacement of `PI` electron in propene.  Orbital diagram showing hyperconjugation in propene. There are various ways of looking at the hyperconjugative effect. One of the way is to reagrd C-H bond as possessing partial ionic character due to resonance. The hyperconjugation (no bond) resonance structure of propene are as under `H - underset(underset(H)(|))overset(overset(H)(|))(C )- overset(overset(H)(|))(C )= overset(overset(H)(|))(C )- H harr H- underset(H^(+))overset(overset(H)(|))(C )= overset(overset(H)(|))(C )- underset(ul(..))overset(overset(H)(|))(C )- H harr` `H^(+) underset(underset(H)(|))overset(overset(H)(|))(C )= overset(overset(H)(|))(C )- underset(ul(..))overset(overset(H)(|))(C )- H harr` `H- underset(underset(H)(|))overset(H^(+))(C )= overset(overset(H)(|))(C )- underset(ul(..))overset(overset(H)(|))(C )-H` |
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