Question

(i) Write notes on Williamson's synthesis. (ii) Write the chemical equation for the preparation of phenol from cumene. (iii) How will you convert phenol to salicylaldehyde? (2+1½+1½=5)
OR
What happens when (write chemical equations only): (i) Phenol reacts with dil. HNO₃ (ii) Ethyl methyl ether is heated with conc. HI (iii) Phenol is heated with conc. HNO₃ in the presence of conc. H₂SO₄ (iv) Ethyl alcohol reacts with thionyl chloride in the presence of pyridine (v) Phenol reacts with bromine water. (1+1+1+1+1=5)

Hint:

Part 1: Williamson is an S_N2 reaction of alkoxide with a primary alkyl halide; cumene gives phenol plus acetone; phenol to salicylaldehyde is the Reimer-Tiemann reaction with CHCl3/NaOH. Part 2: recall nitration, ether cleavage by HI, picric acid, SOCl2 chlorination and tribromophenol formation.

Answer 1

Option 1

(i) Williamson's synthesis: It is the most important general laboratory method for preparing ethers, both symmetrical and unsymmetrical. An alkyl halide is heated with sodium alkoxide (or sodium phenoxide). The alkoxide ion is a strong nucleophile that attacks the carbon of the alkyl halide by an \( S_N2 \) mechanism, displacing the halide ion and forming the ether linkage: \[ R{-}X + R'{-}\overset{-}{O}Na^+ \rightarrow R{-}O{-}R' + NaX \] For example, diethyl ether is made from ethyl bromide and sodium ethoxide: \[ C_2H_5Br + C_2H_5ONa \rightarrow C_2H_5{-}O{-}C_2H_5 + NaBr \] The reaction works best with primary alkyl halides; secondary and especially tertiary halides tend to undergo elimination (alkene formation) instead. Aromatic ethers such as anisole are made from sodium phenoxide and methyl iodide: \[ C_6H_5ONa + CH_3I \rightarrow C_6H_5{-}O{-}CH_3 + NaI \]

(ii) Preparation of phenol from cumene: Cumene (isopropylbenzene) is oxidised by air to cumene hydroperoxide, which is then treated with dilute acid. The acid cleaves it to phenol and acetone: \[ C_6H_5CH(CH_3)_2 \xrightarrow{O_2 \ (\text{air})} C_6H_5C(CH_3)_2OOH \xrightarrow{H_3O^+} C_6H_5OH + (CH_3)_2C{=}O \] Acetone is a valuable by-product of this industrial route.

(iii) Conversion of phenol to salicylaldehyde (Reimer-Tiemann reaction): Phenol is heated with chloroform and aqueous sodium hydroxide. The NaOH and \( CHCl_3 \) generate the electrophile dichlorocarbene \( (:CCl_2) \), which attacks the ortho position of the phenoxide ring. Hydrolysis of the resulting intermediate on acidification gives an ortho \( -CHO \) group, that is salicylaldehyde (2-hydroxybenzaldehyde): \[ C_6H_5OH + CHCl_3 + 3NaOH \xrightarrow{\Delta} o\text{-}HOC_6H_4CHO + 3NaCl + 2H_2O \]