Question

(a) Explain the resonance effect in nucleophilic substitution reactions of haloarenes. By giving the example of chlorobenzene, write short notes on the following:

• (i) Wurtz-Fittig reaction
• (ii) Friedel-Crafts reaction

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OR

(a) What happens when – (write chemical equations only)?

• (i) Ethyl bromide reacts with sodium ethoxide?
• (ii) Ethyl chloride reacts with alcoholic KCN?
• (iii) Chlorobenzene reacts with nitric acid?
• (iv) n-butyl chloride reacts with alcoholic KOH?
• (v) Ethyl bromide reacts with magnesium metal in the presence of dry ether?

Hint:

Haloarenes resist nucleophilic substitution due to resonance stabilization. Wurtz-Fittig and Friedel-Crafts reactions introduce alkyl groups.

Answer 1

Solution:

Step 1: Resonance Effect in Haloarenes:

Haloarenes (e.g., chlorobenzene) show resonance due to delocalization of lone pair electrons on the halogen. This results in partial double bond character, reducing reactivity towards nucleophilic substitution.

\[ \text{C}_6\text{H}_5\text{Cl} \leftrightarrow \text{Various Resonating Structures} \]

Step 2: Wurtz-Fittig Reaction:

• This reaction involves the coupling of aryl halides with alkyl halides using sodium in dry ether.
• Reaction:

\[ C_6H_5Cl + 2Na + C_2H_5Cl \xrightarrow{\text{Dry Ether}} C_6H_5C_2H_5 + 2NaCl \]

Step 3: Friedel-Crafts Reaction:

• Aromatic compounds undergo electrophilic substitution with alkyl halides in the presence of a Lewis acid catalyst (\( AlCl_3 \)).
• Example:

\[ C_6H_6 + CH_3Cl \xrightarrow{AlCl_3} C_6H_5CH_3 + HCl \]

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What happens when – (write chemical equations only)?

(i) Ethyl bromide reacts with sodium ethoxide?

\[ C_2H_5Br + C_2H_5ONa \rightarrow C_2H_5-O-C_2H_5 + NaBr \]

(ii) Ethyl chloride reacts with alcoholic KCN?

\[ C_2H_5Cl + KCN \xrightarrow{\text{alc.}} C_2H_5CN + KCl \]

(iii) Chlorobenzene reacts with nitric acid?

\[ C_6H_5Cl + HNO_3 \xrightarrow{H_2SO_4} o-C_6H_4ClNO_2 + p-C_6H_4ClNO_2 \]

(iv) n-butyl chloride reacts with alcoholic KOH?

\[ C_4H_9Cl + KOH \xrightarrow{\text{alc.}} C_4H_8 + H_2O + KCl \]

(v) Ethyl bromide reacts with magnesium metal in the presence of dry ether?

\[ C_2H_5Br + Mg \xrightarrow{\text{Dry Ether}} C_2H_5MgBr \]