Electromeric Effect Explained with Examples

The electromeric effect is the complete, temporary shift of a pi electron pair to one atom of a multiple bond when an attacking reagent approaches, creating a temporary positive charge on one end and a negative charge on the other.

This shift explains how polar reagents add to alkenes and carbonyl compounds: it is the trigger that starts electrophilic addition and nucleophilic addition reactions.

Key Concepts

• Positive electromeric effect (+E): when an electrophile (electron lover) attacks, the pi electrons shift toward the attacking electrophile. Example: H+ approaching ethene pulls the pi pair toward itself, forming a carbocation on the other carbon.
• Negative electromeric effect (-E): when a nucleophile (electron donor) attacks, the pi electrons shift away from it onto the more electronegative atom. Example: CN- attacking a carbonyl carbon pushes the pi pair onto oxygen, forming an alkoxide ion.
• Temporary nature: the shift lasts only while the reagent is present. Remove the attacking species and the electron pair returns. This is what separates the electromeric effect from the permanent mesomeric (resonance) effect.

Formulas

• Formal charge formula: formal charge = valence electrons − nonbonding electrons − (bonding electrons ÷ 2). All charges are in units of the elementary charge e.
• −E in nucleophilic addition: (CH3)2C=O + CN- → (CH3)2C(CN)−O-. The C=O pi pair shifts completely onto oxygen. The carbonyl carbon changes from sp2 to sp3.

Worked Example

Calculate the formal charge on the oxygen atom of acetone before and after cyanide ion attacks the carbonyl carbon, to see the −E effect numerically.

1. Before attack, in (CH3)2C=O, oxygen has 4 nonbonding electrons and 4 bonding electrons (one double bond). Formal charge = 6 − 4 − (4 ÷ 2) = 6 − 4 − 2 = 0. The oxygen is neutral.
2. Cyanide attacks the carbon, and the pi pair shifts onto oxygen. Now oxygen has 6 nonbonding electrons and 2 bonding electrons (one single bond to carbon).
3. Formal charge = 6 − 6 − (2 ÷ 2) = 6 − 6 − 1 = −1.
4. So the electromeric shift changes oxygen from 0 to −1, a full unit of negative charge, while the carbon gains the CN- group. This is the cyanohydrin intermediate.

Common Misconceptions

• The electromeric effect is permanent like the inductive effect. Wrong. The inductive effect is a permanent polarization through sigma bonds, but the electromeric shift vanishes the moment the attacking reagent is removed.
• Electrons shift in any direction. No. In a +E shift the pi pair moves toward the electrophile; in a −E shift it moves away from the nucleophile onto the more electronegative atom (usually oxygen or nitrogen). The direction is fixed by the nature of the attacker.

Real-World Applications

• Industrial addition reactions: the manufacture of ethanol by acid-catalyzed hydration of ethene begins with a +E shift as H+ attacks the pi bond.
• Cyanohydrin synthesis: the −E shift in carbonyl compounds is used to make cyanohydrins, which are converted into alpha-hydroxy acids and amino acids.

The electromeric effect is the spark that starts polar reactions of multiple bonds: an electrophile pulls the pi pair toward itself (+E), a nucleophile pushes it away onto oxygen or nitrogen (−E), and the shift disappears once the reagent is gone. Recognizing which direction the electrons move lets you predict exactly where the new bond will form.