Hyperconjugation - Electromeric effect

The electron displacement in an organic molecule may take place in the presence of an appropriate attacking reagent. This kind of electron displacements leads to polarization of the bond. Some of the effects are electromeric effect and hyperconjugation, which are explained below:

Electromeric effect:

The electromeric effect is a temporary effect, mainly experienced in the presence of an attacking reagent in the vicinity of an organic compound having multiple bonds(a double or triple bond). In this effect, the complete transfer of a shared pair of π-electrons to one of the atoms joined by multiple bonds on the demand of an attacking reagent takes place. The effect ceases as soon as the attacking reagent is removed from the domain of the reaction. The electromeric effect is mainly categorized into two categories.

  1. Positive Electromeric Effect (+E effect):

    The positive electromeric effect is defined as the transfer of π−electrons of the multiple bonds to the atom with which the reagent gets attached.

Positive Electromeric Effect

Positive Electromeric Effect

  1. Negative Electromeric Effect (–E effect):

    The positive electromeric effect is defined as the transfer of π−electrons of the multiple bonds to the atom with which the reagent does not get attached.

Negative Electromeric Effect

Negative Electromeric Effect

Hyperconjugation:

Hyperconjugation effect is a permanent effect in which localization of σ electrons of C-H bond of an alkyl group directly attached to an atom of the unsaturated system or to an atom with an unshared p orbital takes place.

Hyperconjugation

Hyperconjugation

From the above figure, we observe that one of the three C-H bonds of the methyl group can align in the plane of the empty p orbital and the electrons constituting the C-H bond in a plane with this p orbital can then be delocalized into the empty p orbital.

We also observe that the hyperconjugation stabilizes the carbocation as it helps in the dispersal of positive charge. Thus, we can say that greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyperconjugation interaction and stabilization of the carbonation.The relative stability on the basis of hyperconjugation is given as,

Hyperconjugation stability

Hyperconjugation stability

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Electromeric effect