Question

Explain the electromeric effect.

Answer 1

Electromeric is a temporary effect which operates in unsaturated compounds (containing > C = C <, > C = 0, etc…) in the presence of an attacking reagent.

Let us consider two different compounds.

• compounds containing carbonyl group ( > C = O)
• unsaturated compounds such as alkenes ( > C = C < )

1. When a nucleophile approaches the carbonyl compound, the n electrons between C and O is instantaneously shifted to the more electronegative oxygen. This makes the carbon electron-deficient and thus facilitating the formation of a new bond between the incoming nucleophile and the carbonyl carbon atom.
   
2. On the other hand when an electrophile such as H⁺ approaches an alkene molecule, the π electrons are instantaneously shifted to the electrophile and a new bond is formed between carbon and hydrogen. This makes the other carbon electron-deficient and hence it acquires a positive charge.
   
3. The electrometric effect, is denoted as E effect. Like the inductive effect, the electrometric effect is also classified as + E and – E based on the direction in which the pair of electrons is transferred to form a new bond with the attacking agent.
4. When the π electron is transferred towards the attacking reagent, it is called + E (positive electrometric) effect.
   
   The addition of H+ to alkene as shown above is an example of +E effect.
   When the π electron is transferred away from the attacking reagent, it is called, -E (negative electrometric) effect.
   
5. The attack of CN^– on a carbonyl carbon, as shown above, is an example of – E effect.