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Hyperconjugation

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Hyperconjugation - Lesson Summary

Effects of hyperconjugation on the stability:
A general stabilizing interaction which involves the delocalization of sigma electrons of a C-H bond of an alkyl group directly attached to an unsaturated system (or) to a species with an unshared p -orbital such as  Carbocations (or) free radicals is known as hyperconjugation. Hyperconjugation is a permanent effect.  
Ex: Hyperconjugation in ethyl carbo cation.  

One of the carbon-hydrogen bonds of the methyl group aligns in the same plane with the empty p -orbital of the positively charged carbon atom.  

The electrons of this C-H sigma bond then gets delocalised into the empty p orbital. In Carbocations, delocalisation of electrons useful in dispersing the positive charge on the carbocation to the adjacent atoms and stabilises it. Thus, the carbocation is stabilised by hyperconjugation.  
 

 


Greater the number of alkyl  groups attached to a positively charged carbon  atom, greater is the hyperconjugation interaction and stabilisation of the cation.

Stability decreasing order among different alkyl Carbocations:


Hence, the tertiary(3 0)-Carbocation in which there are three alkyl groups attached to the positive carbon is more stable than a secondary (2 0) carbocation with two alkyl groups.  

Similarly, a secondary carbocation is more stable than a primary (1 0)carbocation. Therefore, the order of stability decreases from tertiary carbocation to methyl cation. Hyperconjugation also explains the stability of certain olefins over other allieds..  
Ex:  Propene is more stable than ethene because in propene there are three H-C hyperconjugated bonds which lead to greater stability.

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