Electromeric Effect

Given

A proton (H⁺) attacks the C=C bond in propene (CH₃–CH=CH₂).

Required

Identify: (a) whether the effect is +E or −E, (b) which carbon receives the π-electron pair.

Concept

The electromeric effect is the temporary, complete transfer of a π-electron pair when an attacking reagent approaches. In +E, electrons shift toward the reagent; in −E, away from it.

Applicable principle

For an electrophilic attack on C=C: the π-electrons shift toward the carbon that forms a new bond with the electrophile. The direction follows Markovnikov's rule — H⁺ attaches to the carbon with more hydrogens, so the π-pair shifts to the other carbon.

Reasoning

H⁺ is an electrophile. It approaches the π-bond and the electron pair shifts to facilitate bond formation. Since electrons shift toward the attacking species (H⁺), this is a +E effect. The terminal CH₂ (bearing more H atoms) bonds to H⁺, while the π-electrons effectively shift to the internal carbon (CH), forming a secondary carbocation intermediate.

Application

- Electrophile: H⁺ - π-bond shifts: electrons move toward the site of electrophilic attack - Type: +E effect (electrons move toward the reagent) - Result: CH₃–C⁺H–CH₃ (secondary carbocation) — more stable intermediate

Final answer

The protonation of propene exhibits the **+E effect**. The π-electron pair shifts toward the terminal carbon (which bonds to H⁺), generating a 2° carbocation at the internal carbon.

Common trap

Students confuse +E and −E by thinking about where the resulting charge appears rather than where the electrons move relative to the reagent. Remember: +E means electrons shift *toward* the attacking reagent.

Similar NEET-style question

"When HBr adds to but-2-ene, identify the type of electromeric effect at the C=C bond and the direction of π-electron shift." ---