Question

Write short notes on the following:
(i) Williamson's synthesis
(ii) Gattermann reaction
(iii) Kolbe's reaction

Hint:

- Williamson's synthesis is best with primary halides (secondary/tertiary may lead to elimination).
- Gattermann reaction is useful for preparing aromatic aldehydes.
- Kolbe's reaction is industrially important for the synthesis of salicylic acid (precursor of aspirin).

Answer 1

(i) Williamson's Synthesis:
- This is an important laboratory method for the preparation of ethers.
- It involves the reaction of an alkoxide ion with a primary alkyl halide via nucleophilic substitution (SN2).
\[ R{-}O^-Na^+ + R'X \;\longrightarrow\; R{-}O{-}R' + NaX \] Example:
\[ C_2H_5ONa + CH_3I \;\longrightarrow\; C_2H_5{-}O{-}CH_3 + NaI \] (Product: methyl ethyl ether).
(ii) Gattermann Reaction:
- Used for formylation of aromatic compounds (introduction of –CHO group).
- Benzene or substituted aromatic compound reacts with HCN and HCl in the presence of a Lewis acid catalyst (AlCl$_3$ or ZnCl$_2$).
\[ C_6H_6 \xrightarrow[\;ZnCl_2\;]{HCN/HCl} C_6H_5{-}CHO \] (Product: benzaldehyde).
- Sometimes CO and HCl are used with AlCl$_3$/CuCl catalyst for the same transformation.
(iii) Kolbe's Reaction:
- Phenol on treatment with CO$_2$ under high pressure (4–7 atm) and 400 K in the presence of NaOH gives salicylic acid (o-hydroxybenzoic acid).
\[ C_6H_5ONa + CO_2 \xrightarrow[\;400K,\;4-7 atm\;]{} o\text{-HO–C}_6H_4{-}COONa \] On acidification: \[ o\text{-HO–C}_6H_4{-}COONa + HCl \;\longrightarrow\; o\text{-HO–C}_6H_4{-}COOH + NaCl \] (Product: salicylic acid).