General Organic Chemistry
18.0 Mesomeric effect
18.0 Mesomeric effect
The permanent polarization of a group conjugated with a $\pi$ bond or a set of alternate $\pi$ bonds is transmitted through the $\pi$ electrons of the system resulting in a different distribution of electron in the unsaturated chain. This kind of electron redistribution in unsaturated compounds conjugated with electron-releasing or electron-withdrawing groups or atoms is called mesomeric effect.
We know that carbonyl group is a resonance hybrid.
When the carbonyl group is conjugated with a carbon chain of alternate single and double bonds, the positively charged carbonyl carbon exerts electron transfer towards itself via the $\pi$ electrons. Thus, the polarization of the carbonyl group is transmitted via the $\pi$ electrons of the carbon chain.
Similarly, the lone pair on the nitrogen atom repels the $\pi$ electrons of the conjugated chain of the amino compound.
The $\pi$ electrons of molecules are delocalized due to the mesomeric effect resulting in the number of resonance structures which give stability to the ions. Therefore, this kind of electron transfer is also called resonance effect or conjugative effect besides mesomeric effect.
This is a permanent effect in the ground state of the molecule which is indicated by the dipole moment. The electron-attracting mesomeric effect is indicated by -M effect and the electron-repelling mesomeric effect is indicated by the +M effect.
+M effect possessing groups are,
-M effect possessing groups are,
The low reactivity of halogens bonded to unsaturated carbon is due to the +M effect of the halogen. The C-Br bond in vinyl bromide has a partial double bond character due to the +M effect of bromide with the consequent low reactivity of bromide.
The acidity of phenol is due to the +M effect of OH group. The mesomeric transfer of the lone pair on the oxygen atom of phenol to the $\pi$ electrons of the benzene ring results in several resonance structures with a positive charge on the oxygen atom. This aids the hydrogen atom of OH group to leave as the proton.
The ionization is specially aided due to the formation of the relatively more stable phenoxide ion.
The charge delocalization in phenoxide ion affords greater stability over phenol in which charge separation occurs in the canonical forms.
Hence, phenol prefers to ionize i.e. it is acidic.
The essential differences between inductive effect and mesomeric effect are,
| Inductive effect (I effect) | Mesomeric effect (M effect) |
| 1. I effect can operate in saturated and unsaturated compounds. | 1. M effect can operate in unsaturated and conjugated compounds. |
| 2. I effect involves electrons of $\sigma$ bonds. | 2. M effect involves electrons of $\pi$ bonds and lone pairs. |
| 3. I effect is transmitted over a short distance. The effect dies out rapidly since the $\sigma$-bond electrons are tightly held. | 3. M effect is transmitted with undiminished intensity right up to the end of the unsaturated chain since the effect involves loose $\pi$ electrons. |
