Hyperconjugation - Practice Questions & MCQ

Hyperconjugation is a general stabilising interaction. It involves delocalisation of σ 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. The σ electrons of C—H bond of the alkyl group enter into partial conjugation with the attached unsaturated system or with the unshared p orbital. Hyperconjugation is a permanent effect.
To understand hyperconjugation effect, let us take an example of CH₃CH₂⁺(ethyl cation) in which the positively charged 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 orbital can then be delocalised into the empty p orbital as shown in the figure given below:

This type of overlap stabilises the carbocation because electron density from the adjacent σ bond helps in dispersing the positive charge.

In general, greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyperconjugation interaction and stabilisation of the cation. Thus, we have the following relative stability of carbocations:

Hyperconjugation is also possible in free radicals, alkenes and alkylarenes.

In general, greater the number of hyperconjugative structures, greater is the stability.

The IUPAC names and common names of some of the compounds are given below:

                        
IUPAC name - Methylbenzene                            
Common name - Toluene

IUPAC name - Ethenylbenzene
Common name - Styrene

IUPAC name - 2-Methylphenol
Commoon name - o-Cresol

Electrophiles: These are those species that accept electrons. Usually, they are positively charged species.

Nucleophiles: These are those species which has a tendency to donate electrons pairs or react at electron-poor sites.