JEE Mains Chemistry · Hydrocarbons
Friedel–Crafts, Side-Chain Oxidation and Arene Synthesis
Friedel–Crafts alkylation goes through a carbocation that may rearrange, while acylation does not; hot KMnO₄ cuts any side chain with a benzylic H down to –COOH; and the order in which groups are put on a ring decides where they end up.
Why this matters
Nineteen PYQs, sixteen of them multiple choice, and three from 2026. Five are Friedel–Crafts products or the carbocation behind them, including rearranged and ring-closing alkylations. Seven oxidise a side chain with KMnO₄, then often ask for a count of π bonds or a mass of product. Seven give or ask for the order of reagents that builds a disubstituted benzene. Three of the nineteen ask for a number.
Concept 1 of 3: Friedel–Crafts alkylation and acylation
In alkylation, AlCl₃ turns an alkyl halide into a carbocation. If that cation is primary or secondary and a hydride or methyl shift would make it more stable, it shifts before it attacks the ring, so the alkyl group on the product may not match the halide. The product is also more reactive than benzene, so more alkyl groups can go on. Acylation avoids both problems: the acylium ion does not rearrange, and the C=O it puts on the ring deactivates it, so only one acyl group enters.
Definition
- Alkylation: . Rearrangement and polyalkylation are both likely.
- An alkene with HF or H₂SO₄, or an alcohol with acid, can supply the carbocation instead of RCl.
- A chain bearing both the ring and the cation can close a new ring (intramolecular alkylation), usually six-membered.
- Acylation: . No rearrangement, one substitution.
- To attach an unbranched chain, acylate and then reduce C=O to CH₂ (Clemmensen: Zn–Hg, conc. HCl).
| Reagent with benzene and AlCl₃ | Cation formed | Does it rearrange? | Main product |
|---|---|---|---|
| No | Toluene | ||
| (isobutyl chloride) | Primary, shifts to tertiary | Yes (hydride shift) | tert-Butylbenzene |
| Cyclohexene with HF | Cyclohexyl cation | No | Cyclohexylbenzene |
| Acylium ion | No | Butyrophenone (1-phenylbutan-1-one) | |
| , then Zn–Hg/HCl | Acylium ion | No | n-Butylbenzene |
Alkyl halides can rearrange before they attack; acyl chlorides never do, so acylation then reduction gives a straight chain.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2022 · 29 June 2022 · Q134Moderate
Example 1 · Hydrocarbons · Friedel-Crafts, Side-Chain Oxidation and Arene Synthesis
The stable carbocation formed in the above reaction is :
The alkyl group on the ring may differ from the halide
A primary or secondary cation that can shift to a more stable one does so first. Always ask whether a hydride or methyl shift is possible before writing the product.
Polyalkylation is likely, not certain
An alkyl group activates the ring, so a second alkylation can follow. Using excess benzene keeps the monoalkyl product as the main one.
Concept 2 of 3: Side-chain oxidation to benzoic acid
Hot KMnO₄ (or acidified K₂Cr₂O₇) attacks the side chain at the benzylic carbon, the one joined to the ring. If that carbon has at least one H, the whole chain is cut back to one carbon and becomes –COOH, however long the chain was. If the benzylic carbon has no H, as in a tert-butyl group, the chain survives. The ring itself is not touched.
Definition
- Any side chain with a benzylic H → –COOH: methyl, ethyl, propyl, isopropyl, –CH=CH₂ and –CH₂COOCH₃ all give benzoic acid.
- No benzylic H (–C(CH₃)₃, –C(CH₃)₂OH): no oxidation.
- In alkaline KMnO₄ the product is the carboxylate salt; acidifying gives the acid.
- Two side chains give a dicarboxylic acid: p-xylene gives terephthalic acid.
Side-chain oxidation
Worked example
1-tert-Butyl-4-methylbenzene is heated with alkaline KMnO₄ and then acidified. What forms?
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2024 · 8 April 2024 · Q59Moderate
Example 2 · Hydrocarbons · Friedel-Crafts, Side-Chain Oxidation and Arene Synthesis
Major product of the following reaction has ___ -bond.
The whole chain goes, however long
Ethylbenzene, propylbenzene and isobutylbenzene all give benzoic acid, with one carbon left on the ring. The product is never phenylacetic acid.
Check for a benzylic H before oxidising
tert-Butylbenzene and 2-phenylpropan-2-ol have no H on the benzylic carbon, so hot KMnO₄ leaves them alone.
–COOH directs the next group meta
Once the side chain has become –COOH, it is a meta-directing, deactivating group. A nitration after oxidation goes meta; a nitration before it goes ortho and para to the alkyl group.
Concept 3 of 3: Choosing the order of steps for a disubstituted benzene
The first group on the ring decides where the second goes. So work backwards from the target: if the two groups are meta, the first one put on must be a meta director; if they are ortho or para, it must be an ortho-para director. Two more rules narrow the choice: Friedel–Crafts cannot be done on a deactivated ring, and a group can be changed after it is placed (CH₃ to COOH, NO₂ to NH₂), which lets you use its directing effect before it changes.
Definition
- Meta target: put the meta director on first.
- Ortho or para target: put the ortho-para director on first; separate the para isomer.
- Do any Friedel–Crafts step BEFORE adding –NO₂, –COR or –SO₃H.
- Oxidising CH₃ to COOH switches the group from ortho-para to meta directing; reducing NO₂ to NH₂ switches it from meta to ortho-para.
| Target | Order of steps | Why this order | Wrong order gives |
|---|---|---|---|
| m-Bromonitrobenzene | HNO₃/H₂SO₄, then Br₂/FeBr₃ | –NO₂ sends Br meta | o- and p-bromonitrobenzene |
| p-Bromonitrobenzene | Br₂/FeBr₃, then HNO₃/H₂SO₄; separate para | –Br sends NO₂ ortho and para | m-Bromonitrobenzene |
| m-Nitroacetophenone | CH₃COCl/AlCl₃, then HNO₃/H₂SO₄ | Acylation fails on nitrobenzene; –COCH₃ sends NO₂ meta | No reaction at the acylation step |
| 3-Bromobenzoic acid (from toluene) | KMnO₄, then Br₂/FeBr₃ | –COOH sends Br meta | 2- and 4-bromobenzoic acid |
| 4-Bromobenzoic acid (from toluene) | Br₂/FeBr₃, separate para, then KMnO₄ | –CH₃ sends Br ortho and para | 3-Bromobenzoic acid |
Work back from the target: the relationship of the two groups tells you which one went on first.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2021 · Paper 18 · Q47Moderate
Example 3 · Hydrocarbons · Friedel-Crafts, Side-Chain Oxidation and Arene Synthesis
The correct sequential addition of reagents in the preparation of 3-nitrobenzoic acid from benzene is:
Friedel–Crafts must come before any strong deactivator
Once –NO₂, –COR or –SO₃H is on the ring, AlCl₃ reactions stop working. A sequence that nitrates first and alkylates or acylates later is wrong.
A later change of group can flip its direction
–CH₃ directs ortho and para but becomes –COOH (meta) after oxidation. The step order must use each group while it has the directing effect you need.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (1)
- Side-chain oxidation to benzoic acid
Side-chain oxidation
Reference tables (2)
Friedel–Crafts alkylation and acylation5 rows
| Reagent with benzene and AlCl₃ | Cation formed | Does it rearrange? | Main product |
|---|---|---|---|
| No | Toluene | ||
| (isobutyl chloride) | Primary, shifts to tertiary | Yes (hydride shift) | tert-Butylbenzene |
| Cyclohexene with HF | Cyclohexyl cation | No | Cyclohexylbenzene |
| Acylium ion | No | Butyrophenone (1-phenylbutan-1-one) | |
| , then Zn–Hg/HCl | Acylium ion | No | n-Butylbenzene |
Alkyl halides can rearrange before they attack; acyl chlorides never do, so acylation then reduction gives a straight chain.
Choosing the order of steps for a disubstituted benzene5 rows
| Target | Order of steps | Why this order | Wrong order gives |
|---|---|---|---|
| m-Bromonitrobenzene | HNO₃/H₂SO₄, then Br₂/FeBr₃ | –NO₂ sends Br meta | o- and p-bromonitrobenzene |
| p-Bromonitrobenzene | Br₂/FeBr₃, then HNO₃/H₂SO₄; separate para | –Br sends NO₂ ortho and para | m-Bromonitrobenzene |
| m-Nitroacetophenone | CH₃COCl/AlCl₃, then HNO₃/H₂SO₄ | Acylation fails on nitrobenzene; –COCH₃ sends NO₂ meta | No reaction at the acylation step |
| 3-Bromobenzoic acid (from toluene) | KMnO₄, then Br₂/FeBr₃ | –COOH sends Br meta | 2- and 4-bromobenzoic acid |
| 4-Bromobenzoic acid (from toluene) | Br₂/FeBr₃, separate para, then KMnO₄ | –CH₃ sends Br ortho and para | 3-Bromobenzoic acid |
Work back from the target: the relationship of the two groups tells you which one went on first.
Watch out for (7)
- The alkyl group on the ring may differ from the halide→ Friedel–Crafts alkylation and acylation
- Polyalkylation is likely, not certain→ Friedel–Crafts alkylation and acylation
- The whole chain goes, however long→ Side-chain oxidation to benzoic acid
- Check for a benzylic H before oxidising→ Side-chain oxidation to benzoic acid
- –COOH directs the next group meta→ Side-chain oxidation to benzoic acid
- Friedel–Crafts must come before any strong deactivator→ Choosing the order of steps for a disubstituted benzene
- A later change of group can flip its direction→ Choosing the order of steps for a disubstituted benzene
Test yourself on Hydrocarbons
20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.