Chapter 7: Alcohols, Phenols and Ethers
Key Topics: Preparation Methods, Acidity of Phenols vs Alcohols, Distinction Tests (Lucas Test), Kolbe's Reaction, Reimer-Tiemann Reaction, and Williamson Ether Synthesis.
1. Classification & Structure
- Alcohols: Compounds containing hydroxyl ($-\text{OH}$) group attached to a saturated $sp^3$ carbon chain ($\text{R-OH}$).
- Phenols: Compounds containing hydroxyl ($-\text{OH}$) group directly attached to an aromatic ring ($\text{Ar-OH}$).
- Ethers: Compounds formed by replacing hydrogen of a hydrocarbon with an alkoxy ($-\text{OR}$) or aryloxy ($-\text{OAr}$) group ($\text{R-O-R'}$).
2. Methods of Preparation of Alcohols & Phenols
Acid Catalyzed Hydration of Alkenes (Markovnikov's Addition):
$$\text{R-CH=CH}_2 + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{R-CH(OH)-CH}_3$$
Hydroboration-Oxidation of Alkenes (Anti-Markovnikov Product):
$$\text{CH}_3\text{-CH=CH}_2 \xrightarrow{\text{B}_2\text{H}_6 / \text{THF}} (\text{CH}_3\text{-CH}_2\text{-CH}_2)_3\text{B} \xrightarrow{\text{H}_2\text{O}_2 / \text{OH}^-} 3\text{CH}_3\text{-CH}_2\text{-CH}_2\text{OH} + \text{B(OH)}_3$$
Preparation of Phenol from Cumene (Industrial Method):
$$\text{C}_6\text{H}_5\text{-CH(CH}_3)_2 + \text{O}_2 \xrightarrow{\text{Air}} \text{C}_6\text{H}_5\text{-C(CH}_3)_2\text{-OOH} \xrightarrow{\text{H}^+} \text{C}_6\text{H}_5\text{OH} + \text{CH}_3\text{COCH}_3$$
3. Distinction Tests & Acidity Comparison
| Test / Property | Primary ($1^\circ$) Alcohol | Secondary ($2^\circ$) Alcohol | Tertiary ($3^\circ$) Alcohol |
|---|---|---|---|
| Lucas Test ($\text{conc. HCl} + \text{ZnCl}_2$) | No turbidity at room temperature; turns turbid on heating. | Turbidity appears within 5 minutes. | Turbidity appears immediately. |
| Dehydration over Hot Cu ($573\text{ K}$) | Forms Aldehyde ($\text{R-CHO}$). | Forms Ketone ($\text{R-CO-R'}$). | Undergoes dehydration to form Alkene. |
Relative Acidity Order:
$$\text{Phenol} > \text{Water} > 1^\circ \text{ Alcohol} > 2^\circ \text{ Alcohol} > 3^\circ \text{ Alcohol}$$
4. Important Named Reactions of Phenols
Kolbe's Reaction (Synthesis of Salicylic Acid):
$$\text{C}_6\text{H}_5\text{OH} + \text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{ONa} \xrightarrow[(ii) \text{ H}^+]{(i) \text{ CO}_2} \text{o-HO-C}_6\text{H}_4\text{-COOH}$$
Reimer-Tiemann Reaction (Synthesis of Salicylaldehyde):
$$\text{C}_6\text{H}_5\text{OH} + \text{CHCl}_3 + 3\text{NaOH} \xrightarrow{343\text{ K}} \text{o-HO-C}_6\text{H}_4\text{-CHO} + 3\text{NaCl} + 2\text{H}_2\text{O}$$
Reaction of Phenol with Zinc Dust:
$$\text{C}_6\text{H}_5\text{OH} + \text{Zn} \xrightarrow{\Delta} \text{C}_6\text{H}_6 + \text{ZnO}$$
5. Preparation & Cleavage of Ethers
Williamson Ether Synthesis ($S_N2$ Mechanism):
$$\text{R-X} + \text{R'-ONa} \rightarrow \text{R-O-R'} + \text{NaX}$$
$$\text{Note: Alkyl halide (R-X) must be } 1^\circ \text{ to prevent elimination.}$$
Cleavage of Unbalanced Ethers by Hydrogen Halides ($\text{HX}$):
$$\text{R-O-R'} + \text{HX} \rightarrow \text{R-X} + \text{R'-OH}$$
$$\text{If one alkyl group is } 3^\circ\text{, halide forms at } 3^\circ \text{ carbon via } S_N1\text{ pathway.}$$