Question

Why is chlorine atom of chlorobenzene less reactive than chlorine atom of chloroethane? Write chemical equations of reactions of chlorobenzene with (A) Cl$_2$ and (B) conc. H$_2$SO$_4$.

Hint:

In aryl halides, resonance stabilisation makes C–Cl bond strong and unreactive toward nucleophiles, but benzene ring favours electrophilic substitution.

Answer 1

Step 1: Reactivity comparison.
- In chloroethane, the C–Cl bond is a simple polar covalent bond; chlorine can easily undergo nucleophilic substitution. - In chlorobenzene, the lone pair of chlorine interacts with the benzene ring (\(\pi\)-resonance). This delocalisation gives partial double bond character to the C–Cl bond. Step 2: Effect of resonance.
Due to resonance: 1. Bond length of C–Cl decreases. 2. Bond strength increases. 3. Nucleophilic substitution becomes difficult. Step 3: Reaction with Cl$_2$.
In presence of FeCl\(_3\), chlorobenzene undergoes electrophilic substitution: \[ C_6H_5Cl + Cl_2 \xrightarrow{FeCl_3} C_6H_4Cl_2 + HCl \] Products: 1,2-dichlorobenzene (ortho) and 1,4-dichlorobenzene (para). Step 4: Reaction with conc. H$_2$SO$_4$.
Sulphonation occurs: \[ C_6H_5Cl + H_2SO_4 \xrightarrow{\Delta} C_6H_4ClSO_3H + H_2O \] Product: p-chlorobenzene sulphonic acid (major). Step 5: Conclusion.
Thus, chlorobenzene is less reactive than chloroethane due to resonance, and it undergoes electrophilic substitution with Cl\(_2\) and H\(_2\)SO\(_4\).