General Organic Chemistry
19.0 Electromeric effect
19.0 Electromeric effect
The electromeric effect is a temporary effect that takes place between two atoms joined by multiple bonds. It takes place in the presence of an attacking reagent.
The instantaneous shift of electron pair of carbonyl group towards oxygen.
Electromeric effect is of two types,
1. +E effect
2. -E effect
1. +E effect: Transition of electron towards the attacking reagent.
2. -E effect: Transition of electron away from attacking reagent.
Electromeric effect differs from the inductive effect as given below,
| Inductive effect (I effect) | Electromeric effect (E effect) |
| 1. It is a permanent polarization. | 1. It is a temporary polarization. |
| 2. Operate through $\sigma$ bonds. | 2. Operates through $\pi$ bonds. |
| 3. Weak effect since the $\sigma$-bond electrons are strongly held. | 3. Strong effect since the loose $\pi$ electrons shift completely. |
| 4. Charged developed on the carbon joined to the substituent (electron repelling or attracting) is small and is shown as ${\delta ^ - }$ or ${\delta ^ + }$. | 4. Complete transfer of electrons causes the full charge on the carbons, which is shown by $+ $ and $ - $. |
