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 Comparison: Phenol vs. Cresol

Phenol (C₆H₅OH) and cresol (methylphenol, C₆H₄CH₃OH) are aromatic compounds with varying acid strengths. Their structural differences dictate which is the stronger acid.

Phenol:

Phenol features a hydroxyl group (-OH) attached to a benzene ring. Its acidity stems from the phenoxide ion's (C₆H₅O⁻) resonance stabilization with the aromatic ring. This delocalization of the negative charge on the oxygen atom enhances conjugate base stability.

Cresol:

Cresol is a phenol derivative with a methyl group (-CH₃) on the benzene ring. The methyl group is electron-donating via inductive and hyperconjugation effects. This increases electron density in the ring, diminishing the phenoxide ion's resonance stabilization capacity. Consequently, cresol is a weaker acid than phenol.

Acidity Comparison:

- Phenol is a stronger acid than cresol. The electron-donating methyl group in cresol destabilizes its conjugate base (phenoxide ion) compared to phenol's. - Phenol's conjugate base is better stabilized by resonance due to the absence of such electron-donating groups.

Conclusion:

Phenol is a stronger acid than cresol. The lack of electron-donating groups, such as the methyl group in cresol, allows for greater conjugate base (phenoxide ion) stabilization in phenol. Conversely, the methyl group in cresol reduces this stabilization.