Chemistry · Ch 8 — Organic Chemistry – Some Basic Principles and Techniques

Electromeric Effect (E Effect)

8.7.8

Electromeric Effect (E Effect)

The Electromeric Effect: A Temporary Electronic Shift

The electromeric effect is a temporary electronic effect. It is observed only in organic compounds that contain a multiple bond (a double or triple bond) and only in the presence of an attacking reagent. The effect is defined as the complete transfer of a shared pair of π-electrons to one of the atoms joined by the multiple bond, on the demand of the attacking reagent. As soon as the attacking reagent is removed from the reaction's domain, the effect is annulled — the electrons return to their original positions.

This effect is represented by the symbol E, and the shifting of electrons is shown by a curved arrow ( $\rightharpoonup$ or $\curvearrowright$ ) pointing from the bond to the atom receiving the electrons.

Watch out

The electromeric effect is not a permanent feature of the molecule. It is induced only by the approach of an attacking reagent. Do not confuse it with the permanent inductive effect.

Two Types of Electromeric Effect

The electromeric effect is classified into two distinct types, depending on which atom of the multiple bond receives the π-electrons.

1. Positive Electromeric Effect (+E effect)

In the +E effect, the π-electrons of the multiple bond are transferred to that atom to which the attacking reagent gets attached. The reagent itself becomes bonded to that atom.

Example: Addition of a proton ( $H^+$ ) to an alkene.

Consider a general alkene, $>C=C<$ . When a proton ( $H^+$ ) approaches, the π-electrons are completely transferred to the carbon atom that will bond with the $H^+$ . This leaves the other carbon atom with a positive charge.

>C=C< \; + \; H^+ \longrightarrow \; >\overset{+}{C}-\underset{H}{C}<

Here, the attacking reagent ( $H^+$ ) attaches to the carbon that received the π-electrons. The curved arrow shows the movement of the π-electron pair towards that carbon.

2. Negative Electromeric Effect (–E effect)

In the –E effect, the π-electrons of the multiple bond are transferred to that atom to which the attacking reagent does not get attached. The reagent attaches to the other atom.

Example: Addition of a cyanide ion ( $^-CN$ ) to an alkene.

When a cyanide ion ( $^-CN$ ) approaches an alkene, the π-electrons are completely transferred to the carbon atom that will not bond with the $^-CN$ . The $^-CN$ then attaches to the other carbon atom.

>C=C< \; + \; {}^-CN \longrightarrow \; >\underset{CN}{C}-\overset{-}{C}<

In this case, the attacking reagent ( $^-CN$ ) attaches to the carbon that did not receive the π-electrons. The curved arrow shows the movement of the π-electron pair towards the carbon that will not bond with the reagent.

Tip

A simple way to remember the difference: In the +E effect, the attacking reagent goes to the atom that gains the π-electrons. In the –E effect, the attacking reagent goes to the atom that loses the π-electrons.

Predominance Over the Inductive Effect …

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