Chemistry Class 11 NCERT Solutions: Chapter 12 Organic Chemistry Some Basic Principles and Techniques Part 5

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Q: 10. Explain why alkyl groups act as electron donors when attached to a π system.

Answer

When an alkyl group is attached to a system, it acts as an electron3donor group by the process of hyperconjugation. To understand this concept better, let us take the example of propene.

Q 10 Structure of Hyperconjugation

Q 10 Structure of Hyperconjugation

Q 10 Structure of Hyperconjugation

In hyperconjugation, the sigma electrons of the bond of an alkyl group are delocalised. This group is directly attached to an atom of an unsaturated system. The delocalisation occurs because of a partial overlap of a –s sigma bond orbital with an empty orbital of the bond of an adjacent carbon atom. The process of hyperconjugation in propene is shown as follows:

Q 10 1 Structure of Hyperconjugation in Propene

Q 10 1 Structure of Hyperconjugation in Propene

Q 10 1 Structure of Hyperconjugation in Propene

This type of overlap leads to a delocalisation (also known as no3bond resonance) of the electrons, making the molecule more stable.

Q 10 2 Structure of Overlap Leads to a Delocalisation of the Electrons

Q 10 2 Overlap Leads to Delocalisation of Electrons

Q 10 2 Structure of Overlap Leads to a Delocalisation of the Electrons