Question

Explain the following reactions with examples: (i) Kolbe reaction
(ii) Reimer–Tiemann reaction
(iii) Williamson Ether synthesis

Hint:

Kolbe reaction forms alkanes by decarboxylation, Reimer–Tiemann introduces \(-CHO\) into phenols (ortho position), and Williamson ether synthesis is the most common lab method to prepare ethers using \(S_N2\).

Answer 1

(i) Kolbe Reaction:
The Kolbe reaction involves the electrolysis of sodium or potassium salts of carboxylic acids, resulting in the decarboxylation and coupling of alkyl radicals to form alkanes.
\[ 2\,RCOONa \xrightarrow[\text{electrolysis}]{} R{-}R \;+\; 2\,CO_2 \;+\; H_2 \;+\; 2\,NaOH \] Example:
Electrolysis of sodium acetate gives ethane: \[ 2\,CH_3COONa \xrightarrow[\text{electrolysis}]{} C_2H_6 \;+\; 2\,CO_2 \;+\; H_2 \;+\; 2\,NaOH \] (ii) Reimer–Tiemann Reaction:
The Reimer–Tiemann reaction introduces a formyl group (\(-CHO\)) into the aromatic ring of phenols, mainly at the ortho position, using chloroform (\(CHCl_3\)) and alkali (NaOH/KOH).
\[ C_6H_5OH \;+\; CHCl_3 \;+\; 3NaOH \;\longrightarrow\; o{-}HO{-}C_6H_4{-}CHO \;+\; 3NaCl \;+\; 2H_2O \] Example:
Phenol reacts with chloroform in alkaline medium to give salicylaldehyde (2-hydroxybenzaldehyde). (iii) Williamson Ether Synthesis:
The Williamson ether synthesis involves the reaction of a sodium alkoxide with a primary alkyl halide to form ethers. It is a nucleophilic substitution (\(S_N2\)) reaction.
\[ R{-}ONa \;+\; R'X \;\longrightarrow\; R{-}O{-}R' \;+\; NaX \] Example:
Sodium ethoxide reacts with methyl iodide to give ethyl methyl ether: \[ C_2H_5ONa \;+\; CH_3I \;\longrightarrow\; C_2H_5{-}O{-}CH_3 \;+\; NaI \]