Explain why haloarenes are much less reactive than haloalkanes towards nucleophilic substitution reactions.
OR
Which compound in each of the following pairs will react faster in an S2 reaction with OH⁻? Why?
i) CHBr or CHI
ii) (CH)CCl or CHCl
OR
Which compound in each of the following pairs will react faster in an S2 reaction with OH⁻? Why?
i) CHBr or CHI
ii) (CH)CCl or CHCl
Solution
Haloarenes resist nucleophilic substitution because:
(i) \textbf{CHI} — iodide is the better (weaker-base, more polarisable) leaving group than bromide.
(ii) \textbf{CHCl} — methyl offers no steric hindrance to backside attack, while (CH)CCl is tertiary and essentially unreactive by S2.
- Resonance delocalises the halogen lone pair into the ring, giving C–X partial double-bond character (stronger, shorter bond).
- The carbon is sp-hybridised — more electronegative, holding X more tightly than the sp carbon of a haloalkane.
- The electron-rich π-cloud repels the approaching nucleophile (and the planar aryl cation needed for S1 is very unstable).
(i) \textbf{CHI} — iodide is the better (weaker-base, more polarisable) leaving group than bromide.
(ii) \textbf{CHCl} — methyl offers no steric hindrance to backside attack, while (CH)CCl is tertiary and essentially unreactive by S2.
CBSE Class 12 Board · 2008 · Delhi · Subjective