Question

Why are haloarenes less reactive towards nucleophilic substitution reaction? Give two reasons.

Hint:

Haloarenes are compounds where a halogen atom is attached directly to a benzene ring, for example chlorobenzene (\(C_6H_5Cl\)).

Answer 1

Step 1: Identify what a haloarene is.
A haloarene has a halogen attached directly to the benzene ring, for example chlorobenzene \(C_6H_5Cl\). For nucleophilic substitution a nucleophile must attack the carbon holding the halogen and push the halogen out.

Step 2: First reason - resonance and partial double bond character.
The lone pairs of the halogen overlap with the ring's pi system. This delocalisation gives the C-X bond a partial double bond character, making it shorter and stronger, so the halogen is harder to remove.

Step 3: Second reason - nature of carbon and electron repulsion.
The carbon bearing the halogen is \(sp^2\) hybridised. An \(sp^2\) carbon holds its electrons more tightly (more s-character) than an \(sp^3\) carbon, so the bond is stronger. Also the electron-rich ring repels the approaching nucleophile.

Step 4: Combine the reasons.
Both the stronger, shorter C-X bond and the repulsion of the incoming nucleophile make the attack difficult.

Step 5: Conclude.
Therefore haloarenes are much less reactive toward nucleophilic substitution than haloalkanes.

Answer: Haloarenes are less reactive because (i) resonance gives the C-X bond partial double bond character, making it shorter and stronger and the halogen hard to displace, and (ii) the \(sp^2\) carbon holds the bond more tightly and the electron-rich ring repels the incoming nucleophile.