Chemistry · Haloalkanes And Haloarenes · NEET
It is not a contradiction because two different effects control two different things. The halogen has a strong –I (inductive) effect that pulls electron density OUT of the ring through the sigma bond. This makes the whole ring poorer in electrons, so the ring is less attractive to the electrophile than plain benzene. That is why it is DEACTIVATING (reaction is slower). But the halogen also has lone pairs that go INTO the ring by resonance (+R effect). This resonance sends extra electron density mainly to the ortho and para carbons, not the meta carbons. So when substitution finally happens, it prefers ortho and para. NCERT states it directly: reactivity is controlled by the stronger inductive effect, orientation is controlled by the resonance effect.
During electrophilic substitution, the electrophile adds first and makes a positively charged intermediate (arenium ion / carbocation). If the electrophile attacks ortho or para to the halogen, one resonance structure places the positive charge on the carbon holding the halogen. There, the halogen lone pair can form a bond and share its electrons, giving an extra stable resonance structure. This stabilisation only works for ortho and para attack, so those transition states are lower in energy and those products form. For meta attack, no resonance structure puts the + charge next to the halogen, so no extra stabilisation, so meta is minor.
Slower. Chlorobenzene reacts more slowly than benzene and needs more drastic (harsher) conditions. This is because the halogen's –I effect wins overall and removes electron density from the ring (net deactivation). So do not confuse 'ortho/para directing' with 'fast'. Halogens are the special case: o,p-directing AND deactivating at the same time. Most other o,p-directors (like –OH, –NH2, –OCH3) are activating; halogens are the exception you must remember for NEET.
Because the +R (resonance) donation from the halogen raises electron density mainly at the ortho and para carbons. The meta carbons get little extra electron density. So the electrophile, which wants electron-rich carbons, prefers ortho and para. Also, the stabilised carbocation intermediate only forms for ortho/para attack (halogen lone pair helps), not meta. Both reasons point the incoming group to ortho and para, leaving meta as the minor product.
Inductive (–I) controls SPEED: it withdraws electrons, so the ring is deactivated and the reaction is SLOW. Resonance (+R) controls POSITION: it donates lone-pair electrons to ortho and para, so the product goes ORTHO and PARA. One line to remember: 'Induction slows it, resonance places it.' This exact idea is a favourite NEET one-liner.
Only on the ring C–H positions, not on the carbon holding the halogen. Haloarenes undergo the usual benzene electrophilic reactions (halogenation, nitration, sulphonation, Friedel-Crafts) by replacing a ring HYDROGEN at ortho or para. The C–X carbon already holds the halogen, so a new group goes to a neighbouring (ortho) or opposite (para) position. Note: aryl halides themselves cannot act as the alkyl halide in Friedel-Crafts because their C–X bond has partial double-bond character and does not ionise (a real NEET trap).
Among the following, the reaction that proceeds through an electrophilic substitution is:
Which sequence of reagents is suitable to synthesise chlorobenzene?
Which of the following can be used as the halide component for a Friedel-Crafts (alkylation) reaction?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Deactivating. The strong –I (inductive) effect withdraws electron density from the ring, so haloarenes react more slowly than benzene and need harsher conditions. They are the special deactivating group that is still ortho/para directing.
Because the halogen lone pair donates electron density by resonance (+R) mainly to the ortho and para carbons, and the carbocation intermediate for ortho/para attack is stabilised by the halogen lone pair. Meta attack gets no such help, so meta is the minor product.
The inductive (–I) effect controls reactivity (it deactivates, making the reaction slow), and the resonance (+R) effect controls orientation (it directs the incoming group to ortho and para). NCERT states this exactly.
Slower. Overall the ring is deactivated because the halogen's inductive electron withdrawal is stronger than its resonance donation, so more drastic conditions are needed than for benzene.
No. Its C–X bond has partial double-bond character from resonance, so it does not ionise to a carbocation. Friedel-Crafts needs a halide such as an alkyl halide that can give a carbocation electrophile.
