Why are haloarenes less reactive than haloalkanes towards nucleophilic substitution? How does a nitro group at the ortho or para position change this? Give equations.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the fact. Haloarenes are much less reactive than haloalkanes towards nucleophilic substitution.
2 · Give the reasons.
Resonance gives C-Cl partial double bond character
The carbon is sp², so the bond is shorter and stronger
The phenyl cation is not stabilised, so SN1 is ruled out
The electron-rich ring repels the nucleophile
3 · Show the drastic conditions. C₆H₅Cl + 2NaOH → C₆H₅ONa + NaCl + H₂O (623 K, 300 atm); acidify to get C₆H₅OH.
4 · Show the effect of -NO₂.
4-Nitro: 443 K
2,4-Dinitro: 368 K
2,4,6-Trinitro: just warm water
5 · Draw the resonance structures. Draw the intermediate formed when OH⁻ adds to C-1 of 1-chloro-4-nitrobenzene, and show the negative charge moving onto the oxygen of the -NO₂ group. Label: carbanion intermediate, charge on NO₂.
6 · Link to real life. One route to paracetamol passes through 4-nitrophenol made this way.
7 · Close in one line. So electron-withdrawing groups at ortho and para positions make substitution easier.
Scoring tip: Name the position (ortho or para) every time; a meta -NO₂ does not help by resonance.
