my text book state that since the pi electron cloud is more polarizable
than sigma electron cloud,so mesomeric effect is usually stronger than
I cannot understand the reason why M effect is stronger than I effect
Please give me the detailed explanation
- 冷風Lv 51 decade agoFavorite Answer
As you know, resonance effect is much stronger than inductive effect.
Mesomeric effect is a form of resonance, so it is stronger than inductive effect.
Inductive effect：shifting electrons through a sigma bond.
Mesomeric effect：moving a pair of electron.
At the moment, I want to introduce more information of mesomeric effect to you.
For example, there are two organic acids, benzoic acid and 4-hydroxybenzoic acid.
4-hydroxybenzoic acid is a weaker acid because of the involvement of the electron pairs in the O atom of the phenolic group(positive mesomeric effect, +M), destabilizing the carboxylate anion formed.
That is to say, in 4-hydroxybenzoate ion, the negative charge in the O atom of COOH group is less delocalized.
There are some groups which have mesomeric effect when compounds attach them.
+M：─OR, ─OOR, ─OH, ─NH2