Aryl halides are much less reactive than haloalkanes towards nucleophilic substitution reactions. Which of the following is NOT a reason for this low reactivity?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The C–X bond is polarised, with a partial positive charge on the carbon atom.
A nucleophile is electron rich, so it attacks the electron-deficient carbon of the C–X bond. Anything that makes the C–X bond harder to break, or the carbocation harder to form, lowers the reactivity. Polarity of the bond works the other way: it is what makes the carbon a target in the first place.
Option (1) The C–X bond acquires a partial double bond character due to resonance. — A reason for the low reactivity. The lone pairs of the halogen are in conjugation with the ring, so the C–X bond is harder to break than a single bond.
Option (2) The carbon atom of the C–X bond is sp2 hybridised and holds the electron pair of the bond more tightly. — A reason for the low reactivity. The sp2 carbon has more s-character and is more electronegative, so the bond is shorter (169 pm against 177 pm for C–Cl in a haloalkane) and stronger.
Option (3) The C–X bond is polarised, with a partial positive charge on the carbon atom. — Not a reason for the low reactivity. The halogen is more electronegative than carbon, so the carbon bears a partial positive charge. This polarity is what allows a nucleophile to attack, which is why haloalkanes undergo nucleophilic substitution at all; it does not explain why aryl halides resist it.
Option (4) The phenyl cation formed by self-ionisation is not stabilised by resonance. — A reason for the low reactivity. An unstable phenyl cation means the SN1 route is ruled out.
NCERT (2026-27 edition):
6.7.2 Reactions of Haloarenes – “Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons:” … “C—Cl bond acquires a partial double bond character due to resonance.” … “The sp2 hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C—X bond more tightly than sp3-hybridised carbon in haloalkane with less s-chararcter.” … “(iii) Instability of phenyl cation: In case of haloarenes, the phenyl cation formed as a result of self-ionisation will not be stabilised by resonance and therefore, SN1 mechanism is ruled out.”
6.3 Nature of C-X Bond – “Halogen atoms are more electronegative than carbon, therefore, carbon-halogen bond of alkyl halide is polarised; the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative charge.”