Chemistry · Hydrocarbons · NEET
Benzene has 6 pi electrons spread evenly over the ring (delocalised). This makes it very stable, more stable than an open chain of 3 double bonds. If benzene added a reagent (like Br2 adds to an alkene), it would break this stable delocalised ring and lose that stability. So benzene refuses addition under normal conditions. Instead the ring keeps its stability by replacing just one H atom with the new group. That is why arenes mainly do electrophilic SUBSTITUTION, while alkenes/alkynes do electrophilic ADDITION. This exact idea is asked directly in NEET.
NCERT lists exactly five: (1) Nitration - conc. HNO3 + conc. H2SO4 gives nitrobenzene, electrophile is NO2+ (nitronium ion). (2) Halogenation - Cl2 or Br2 with a Lewis acid (anhydrous AlCl3 or FeCl3/Fe) gives chlorobenzene/bromobenzene, electrophile is Cl+ or Br+. (3) Sulphonation - fuming H2SO4 (SO3) gives benzenesulphonic acid, electrophile is SO3. (4) Friedel-Crafts alkylation - alkyl halide R-X + anhydrous AlCl3 gives alkylbenzene, electrophile is R+ (carbocation). (5) Friedel-Crafts acylation - acyl halide RCOCl + anhydrous AlCl3 gives an aryl ketone, electrophile is the acylium ion RC≡O+.
NCERT gives 3 steps for the SE mechanism: (a) Generation of the electrophile E+ (for example, a Lewis acid like AlCl3 takes a halide from Cl2 to make Cl+; H2SO4 protonates HNO3 to make NO2+). (b) Formation of the carbocation intermediate - E+ attacks the ring and forms an unstable, positively charged ring called the arenium ion (or sigma complex); this ion is resonance-stabilised. (c) Loss of a proton (H+) from the carbon that got attacked, which restores the stable aromatic ring and gives the product. Remember: benzene loses H, keeps its aromatic ring.
Alkylation adds an alkyl group (R-, e.g. -CH3, -CH(CH3)2) using an alkyl halide R-X and anhydrous AlCl3; the electrophile is a carbocation R+. Acylation adds an acyl group (R-CO-) using an acyl halide RCOCl (or an anhydride) and anhydrous AlCl3; the electrophile is the acylium ion RC≡O+, and the product is an aryl ketone. Key exam trap: in alkylation the R+ carbocation can rearrange to a more stable one (e.g. n-propyl+ shifts to isopropyl+, so benzene + n-propyl chloride gives CUMENE, not n-propylbenzene). Acylium ions do NOT rearrange, so acylation gives a clean product.
Yes, and this is a favourite NEET trap. With a Lewis acid catalyst (anhydrous AlCl3 or Fe/FeCl3), Cl2 becomes Cl+ and does electrophilic SUBSTITUTION, replacing one H to give chlorobenzene (with excess Cl2 you get hexachlorobenzene, C6Cl6). But with UV/sunlight and NO catalyst, Cl2 becomes free radicals and ADDS to the ring, giving benzene hexachloride C6H6Cl6 (BHC/gammexane). So the catalyst decides: AlCl3 = substitution; UV light = free-radical addition.
AlCl3 is electron-poor, so it is a Lewis acid (electron-pair acceptor). Its job is to GENERATE the electrophile. In halogenation it pulls a Cl- off Cl2 to make Cl+ (AlCl4-). In Friedel-Crafts alkylation it pulls X- off R-X to make the carbocation R+. In acylation it pulls Cl- off RCOCl to make the acylium ion. It must be ANHYDROUS (water-free), because water destroys AlCl3 by hydrolysing it, so no electrophile would form.
Among the following, the reaction that proceeds through an electrophilic substitution is:
Match List-I (Reaction) with List-II (Reagents). A. Cyclohexene -> open-chain dicarbonyl; B. Benzene -> benzophenone (C6H5-CO-C6H5); C. Cyclohexanol -> cyclohexanone; D. Ethylbenzene -> benzoic acid. Reagents: I. Anhyd. AlCl3; II. CrO3; III. KMnO4/KOH, Δ; IV. (i) O3, (ii) Zn-H2O.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An electrophile (E+) is an electron-loving, electron-poor species that attacks a region rich in electrons. Benzene's delocalised pi cloud is electron-rich, so it attracts electrophiles like NO2+, Cl+, SO3, R+ and the acylium ion RC≡O+.
The benzene ring is a cloud of pi electrons, so it is electron-rich. It naturally attracts electron-poor electrophiles (E+), making electrophilic substitution easy. Nucleophiles are also electron-rich, so the ring repels them, making nucleophilic substitution hard. This is NCERT question 9.19.
When the electrophile E+ bonds to a ring carbon, that carbon becomes sp3 and the ring gains a positive charge spread over the other carbons. This positively charged, resonance-stabilised intermediate is the arenium ion or sigma complex. Losing an H+ from it restores the stable aromatic ring.
It depends on the group. Electron-donating groups (like -OH, -NH2, -OCH3, -CH3) activate the ring and make further substitution faster (ortho/para). Electron-withdrawing groups (like -NO2, -COOH, -CHO) deactivate the ring and slow it down (meta). This directive influence is the next topic.
Anhydrous AlCl3 is a Lewis acid that generates the electrophile (R+ or the acylium ion). Water reacts with and destroys AlCl3, so if any moisture is present no electrophile forms and the reaction fails.
