Acidity of Phenols
Phenols are weakly acidic organic compounds. They can donate a proton (H⁺) from their –OH group, forming a phenoxide ion. Their acidity is stronger than alcohols but weaker than carboxylic acids.
1. Basic Acidic Behaviour of Phenols
- Phenols react with strong bases like sodium hydroxide (NaOH) to form phenoxide salts.
- They do not react with weaker bases like sodium bicarbonate (NaHCO₃), showing that they are weaker acids than carboxylic acids.
- Phenoxide salts can be converted back into free phenols by adding stronger acids (e.g., HCl or H₂SO₄).
2. Comparison of Acidity
- Phenols have a Ka value of about 10⁻¹⁰.
- Carboxylic acids are stronger acids (Ka ≈ 10⁻⁵).
- Alcohols are much weaker acids (Ka ≈ 10⁻¹⁶ to 10⁻¹⁸).
This means: Carboxylic acids > Phenols > Alcohols in terms of acidity.
3. Why Are Phenols Acidic?
The benzene ring in phenol helps pull electron density away from the oxygen atom of the –OH group. This creates a slight positive charge on oxygen, making it easier to release the proton (H⁺).
4. Stability of Phenoxide Ion
Once phenol loses H⁺, it forms a phenoxide ion. This ion is stabilized by resonance with the benzene ring.
Because the negative charge can spread across the aromatic ring, the phenoxide ion is more stable than an alkoxide ion (from alcohols). This increased stability is the main reason phenols are more acidic than alcohols.
Summary
- Phenols are weak acids but stronger than alcohols.
- The benzene ring withdraws electrons, increasing acidity.
- The phenoxide ion is stabilized by resonance.
- Phenols react with strong bases (NaOH) but not weak bases (NaHCO₃).
Detailed Notes:
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PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/ ACIDITY OF PHENOLS.