Question

Describe two methods of preparation of alkyl halides. Also give equations. Explain the following reactions of alkyl halides with examples: (i) Nucleophilic substitution reaction (ii) Elimination reaction.
OR
Write chemical equations of chlorination, nitration, sulphonation, Friedel-Crafts reaction and Wurtz (Fittig) reaction of chlorobenzene.

Hint:

For alkyl halides, recall preparation from alcohols or alkenes; a nucleophile swaps out the halide (SN1/SN2), while alcoholic KOH triggers beta-elimination to an alkene. For chlorobenzene, remember chlorine is an ortho/para director.

Answer 1

Option 1 (alternative approach)

Step 1: Preparation of alkyl halides (two methods).
Method A - Free-radical halogenation of alkanes: Alkanes react with chlorine or bromine under UV light through a free-radical chain to give alkyl halides.
\(CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl\)
Method B - Finkelstein (halide exchange): An alkyl chloride or bromide is converted to the iodide using NaI in dry acetone; NaCl/NaBr precipitates and drives the reaction.
\(R\text{-}Cl + NaI \xrightarrow{acetone} R\text{-}I + NaCl\downarrow\)
(Alcohols also give halides with PCl₅: \(R\text{-}OH + PCl_5 \rightarrow R\text{-}Cl + POCl_3 + HCl\).)

Step 2: Nucleophilic substitution - seen through its mechanism.
Because the C-X bond is polar, carbon is electron-poor and is attacked by a nucleophile Nu\(^-\). In SN2 the nucleophile approaches opposite the leaving group in one step (configuration is inverted); in SN1 the halide leaves first to make a carbocation, which the nucleophile then traps.
Illustration: \(CH_3Br + OH^- \rightarrow CH_3OH + Br^-\)

Step 3: Elimination - seen through its mechanism.
Elimination competes with substitution. A strong base (alcoholic KOH) removes a beta-hydrogen as the halide leaves, creating a pi bond (an E2 process). A beta-hydrogen is essential, and the more stable, more substituted alkene predominates.
Illustration: \(CH_3CH_2CH_2Br \xrightarrow{alc.\ KOH} CH_3CH=CH_2 + HBr\)

Option 2 (alternative wording) - chlorobenzene reactions.
Chlorination: \(C_6H_5Cl + Cl_2 \xrightarrow{FeCl_3} 1,4\text{-}C_6H_4Cl_2 + HCl\)
Nitration: \(C_6H_5Cl + HNO_3 \xrightarrow{conc.\ H_2SO_4} 4\text{-chloronitrobenzene} + H_2O\)
Sulphonation: \(C_6H_5Cl + H_2SO_4(conc.) \rightarrow 4\text{-chlorobenzenesulphonic acid} + H_2O\)
Friedel-Crafts alkylation: \(C_6H_5Cl + CH_3Cl \xrightarrow{anhyd.\ AlCl_3} 4\text{-chlorotoluene} + HCl\)
Fittig (Wurtz-type) reaction: \(2C_6H_5Cl + 2Na \xrightarrow{dry\ ether} C_6H_5\text{-}C_6H_5 + 2NaCl\)
\(\boxed{\text{Diphenyl is the coupling product}}\)