Why is phenol more acidic than alcohol?

Phenoxide ion is resonance-stabilised after loss of proton, while alkoxide ions from alcohols do not receive the same resonance stabilisation.

Class 12

Tamil Nadu Class 12 Chemistry Hydroxy Compounds and Ethers — Samacheer Kalvi

Plus Two Chemistry notes on alcohols, phenols, ethers, Lucas test, dehydration, Kolbe reaction, Reimer-Tiemann reaction, and Williamson synthesis.

Updated 6 June 20263 min read

Quick Summary

This chapter links structure with reactivity. Alcohols are tested through classification and oxidation, phenols through acidity and electrophilic substitution, and ethers through preparation and cleavage reactions.

Alcohols

Primary alcohol: carbon bearing -OH attached to one carbon
Secondary alcohol: attached to two carbons
Tertiary alcohol: attached to three carbons

Important Reactions

Dehydration gives alkenes
Oxidation: primary alcohol -> aldehyde -> acid; secondary alcohol -> ketone
Esterification with carboxylic acids gives esters
Lucas test helps classify alcohols

Phenols

Phenol is more acidic than alcohol because phenoxide ion is resonance-stabilised.

High-Value Reactions

Bromination gives 2,4,6-tribromophenol
Nitration gives nitrophenols
Kolbe reaction gives salicylic acid
Reimer-Tiemann reaction gives salicylaldehyde

Ethers

Williamson Ether Synthesis

RONa + R'X -> ROR' + NaX

Best with primary alkyl halides because secondary and tertiary halides favour elimination.

Cleavage

Ethers react with HI or HBr to form alkyl halides and alcohols; anisole cleaves at the alkyl-oxygen bond.

Key Reaction Equations

Dehydration of ethanol (acidic):
   C₂H₅OH →(conc. H₂SO₄, 443 K) CH₂=CH₂ + H₂O

Oxidation:
   1° : CH₃CH₂OH →[O] CH₃CHO →[O] CH₃COOH
   2° : (CH₃)₂CHOH →[O] (CH₃)₂C=O (acetone)
   3° : resists oxidation (no α-H on the C–OH carbon)

Kolbe's reaction (phenol → salicylic acid):
   C₆H₅ONa + CO₂ →(400 K, pressure) o-HO-C₆H₄-COONa →(H⁺) salicylic acid

Reimer–Tiemann (phenol → salicylaldehyde):
   C₆H₅OH + CHCl₃ + NaOH → o-HO-C₆H₄-CHO

Williamson synthesis:
   CH₃CH₂ONa + CH₃CH₂Br → CH₃CH₂-O-CH₂CH₃ + NaBr

Ether cleavage by HI:
   CH₃-O-C₂H₅ + HI → CH₃I + C₂H₅OH   (smaller alkyl → iodide)

Lucas Test (distinguishing alcohols)

Reagent = conc. HCl + anhydrous ZnCl₂
3° alcohol : turbidity immediately
2° alcohol : turbidity in ~5 min
1° alcohol : no turbidity at room temperature

Important Questions

One Mark

1. Name the reagent used in Lucas test. 2. Why is phenol acidic? 3. Write the product of bromination of phenol. 4. Name the reaction used to prepare salicylaldehyde from phenol. 5. Write one use of diethyl ether.

Short Answer

1. Distinguish primary, secondary, and tertiary alcohols. 2. Explain Kolbe reaction. 3. Write Williamson ether synthesis and one limitation. 4. Compare alcohols and phenols in acidity.

Long Answer

1. Explain preparation and important reactions of phenol. 2. Discuss chemical properties of alcohols with suitable equations. 3. Explain cleavage of ethers by HI with examples.

Common Mistakes

Treating phenol exactly like an aliphatic alcohol
Forgetting resonance while explaining phenol acidity
Applying Williamson synthesis to tertiary halides without noting elimination
Writing oxidation of tertiary alcohols as easy under ordinary conditions

Revision Checklist

[ ] Alcohol classification and Lucas test
[ ] Alcohol oxidation and dehydration
[ ] Phenol acidity and named reactions
[ ] Williamson synthesis
[ ] Ether cleavage equations

Frequently Asked Questions

Tertiary alcohols give immediate turbidity, secondary alcohols give turbidity after a few minutes, and primary alcohols react only slowly on heating.