Question

Which is a stronger acid – phenol or cresol? Give reason.

Hint:

Electron-donating groups like the methyl group in cresol reduce the acidity of phenols by stabilizing the negative charge on the oxygen less effectively.

Answer 1

Acidity of Phenol vs Cresol 

Phenol (C₆H₅OH) and cresol (methylphenol, C₆H₄CH₃OH) are both aromatic compounds, but their acid strengths differ. To determine which one is a stronger acid, we need to examine their structures and how the substituents affect the acidity.

1. Phenol:

Phenol contains a hydroxyl group (-OH) attached to a benzene ring. The acidity of phenol is due to the ability of the phenoxide ion (C₆H₅O⁻) to stabilize the negative charge by resonance with the aromatic ring. The negative charge on the oxygen atom is delocalized into the ring, which helps stabilize the conjugate base.

2. Cresol:

Cresol is a methyl-substituted phenol, where the methyl group (-CH₃) is attached to the benzene ring. The methyl group is an electron-donating group through both inductive and hyperconjugation effects. This electron donation increases the electron density on the aromatic ring and reduces the ability of the phenoxide ion to stabilize the negative charge via resonance. As a result, cresol is less acidic than phenol.

3. Comparison of Acidity:

- Phenol is more acidic than cresol because the methyl group in cresol donates electrons to the ring, making the conjugate base (phenoxide ion) less stable. - In phenol, the absence of such electron-donating effects allows for better stabilization of the conjugate base through resonance, making phenol a stronger acid.

Conclusion:

Phenol is a stronger acid than cresol because the absence of electron-donating groups like the methyl group allows the conjugate base of phenol (phenoxide ion) to be more stabilized, whereas the electron-donating methyl group in cresol reduces this stabilization.