Can COOH show +M effect? Due to lone pairs of O? Isn't COOH acidic in nature, and thus should attract e- rather than donate?

Answer 1

The carboxyl group (-COOH) does not exhibit a significant +M (mesomeric or resonance) effect. The presence of the oxygen atom does involve lone pairs, but these lone pairs are more involved in the acidic nature of the carboxyl group rather than contributing to a mesomeric effect.

Acidic Nature of COOH:

1. The carboxyl group is acidic because the hydrogen ion (H+) from the -COOH group can be readily donated, forming a carboxylate ion (-COO⁻) and a hydronium ion (H₃O⁺).

Resonance Effect:

1. While the oxygen in the carbonyl group does have lone pairs, they are typically not involved in resonance stabilization in a way that would result in a significant +M effect.
2. In resonance structures, electrons are usually delocalized across adjacent atoms. In the case of the carboxyl group, resonance stabilization involves the movement of electrons in the π bonds, not the lone pairs on oxygen.

Electron-Withdrawing Nature:

1. The carbonyl oxygen is electronegative and tends to withdraw electron density from the adjacent carbon, making the carbon atom partially positive.
2. This electron-withdrawing effect contributes to the acidity of the hydrogen in the carboxyl group because the more electron-deficient the hydrogen, the more easily it can be donated.

In summary, while the carboxyl group in -COOH does not exhibit a strong +M effect, the oxygen's lone pairs contribute to the acidic nature of the group by facilitating the donation of a proton (H⁺). The electronegativity of oxygen plays a key role in the overall electronic properties of the carboxyl group.