Benzene can be made in three main ways: (1) pass ethyne (HC≡CH) through a red-hot iron tube at 873 K so three molecules join into one ring, (2) heat sodium benzoate with soda lime to knock off CO2 (decarboxylation), and (3) heat n-hexane over a catalyst so it loses hydrogen and forms a ring (aromatisation). Memory hook: "3 ethynes make 1 ring, benzoate loses its CO2, hexane loses its H2."
Three routes to benzene: ethyne trimerisation over red-hot iron (873 K), decarboxylation of sodium benzoate with soda lime, and aromatisation of n-hexane over Mo2O3. All three arrows point to the same benzene ring.
Your doubts, answered
How do 3 ethyne molecules make benzene?
When ethyne (HC≡CH) gas is passed through a red-hot iron tube at 873 K, three ethyne molecules join together (cyclic polymerisation / trimerisation). The three triple bonds rearrange into a six-carbon ring with alternating double bonds, which is benzene (C6H6). No atoms are lost here: 3 × C2H2 = C6H6. Remember the exact condition: red-hot iron tube, 873 K.
What is decarboxylation and how does it give benzene?
Decarboxylation means removing CO2 from a carboxylic acid salt. Sodium benzoate (C6H5COONa) is heated with soda lime (a mix of NaOH and CaO). The COONa group leaves as sodium carbonate and CO2, and an H atom takes its place on the ring. The product is benzene. General idea: the salt loses one carbon (the -COO- carbon), so the ring stays but the acid group is gone.
Why does soda lime, not just NaOH, get used?
Soda lime is NaOH mixed with CaO (calcium oxide). CaO is added because pure NaOH is sticky and absorbs water and eats through glass when hot. CaO keeps the mixture dry and solid so it heats safely. For NEET, always write the reagent as 'soda lime' or 'NaOH + CaO', not NaOH alone.
Does benzenediazonium chloride give benzene or phenol?
This is a favourite NEET trap. When benzenediazonium chloride (C6H5N2+Cl-) is warmed with WATER, it gives PHENOL (C6H5OH), not benzene. To get benzene from it, you must use hypophosphorous acid (H3PO2) instead of water. So 'warmed with H2O' = phenol; this is the reaction that does NOT give benzene.
What is aromatisation of n-hexane?
Aromatisation (also called reforming or cyclisation) takes a straight alkane and turns it into an aromatic ring. n-Hexane (C6H14) is heated over a catalyst like Mo2O3 (or Cr2O3/V2O5) at about 773 K and 10-20 atm. It loses hydrogen (dehydrogenation), cyclises, and becomes benzene. This is how benzene is made industrially from petroleum.
⚠️ The NEET trap ✗ Benzenediazonium chloride warmed with water gives benzene. ✓ Benzenediazonium chloride warmed with water gives PHENOL. Benzene is obtained only when it reacts with hypophosphorous acid (H3PO2), not water. 🧠 Diazonium + H2O = phenOL (has O). Diazonium + H3PO2 = benzene (no O).
Real NEET questions
NEET 2025
Which one of the following reactions does NOT give benzene as the product?
A · HC≡CH passed through a red-hot iron tube at 873 K
B · Benzenediazonium chloride (C6H5N2+Cl-) warmed with H2O ✓
C · Sodium benzoate (C6H5COONa) heated with soda lime
D · n-Hexane heated over Mo2O3 at 773 K, 10-20 atm
Solution: Options A, C and D are all standard benzene preparations: (A) cyclic trimerisation of ethyne, (C) decarboxylation of sodium benzoate with soda lime, (D) aromatisation of n-hexane over Mo2O3. But (B) is the trap: benzenediazonium chloride warmed with WATER gives PHENOL (C6H5OH), not benzene. To get benzene you would need H3PO2. So the reaction that does NOT give benzene is (B).
NEET 2019
Among the following, the reaction that proceeds through an electrophilic substitution is:
A · C6H5N2+Cl- --Cu2Cl2--> C6H5Cl + N2
B · C6H6 + Cl2 --AlCl3--> C6H5Cl + HCl ✓
C · C6H5CH3 + Cl2 --UV light--> C6H5CH2Cl + HCl
D · CH3OH + HCl --Δ--> CH3Cl + H2O
Solution: Benzene is an aromatic ring that reacts by electrophilic aromatic substitution. In option B, Cl2 with AlCl3 makes the electrophile Cl+, which substitutes an H on the benzene ring. Option C is free-radical side-chain chlorination (UV light), option A is a diazonium replacement, and option D is a simple substitution on an alcohol. So the electrophilic substitution on benzene is (B).
Solved Hydrocarbons NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What are the three main methods to prepare benzene for NEET?
(1) Cyclic polymerisation of ethyne (3 HC≡CH through red-hot iron tube at 873 K), (2) decarboxylation of sodium benzoate with soda lime, and (3) aromatisation of n-hexane over a catalyst (Mo2O3) at ~773 K. Benzene is also isolated commercially from coal tar.
What is the exact condition for ethyne to benzene?
Pass ethyne gas through a RED-HOT IRON TUBE at 873 K. Three ethyne molecules join into one benzene ring. NEET often tests this exact temperature and condition.
What does sodium benzoate give with soda lime?
It gives benzene. The -COONa group leaves as CO2 (as sodium carbonate) and is replaced by H. This loss of CO2 is called decarboxylation. Soda lime is NaOH + CaO.
How is benzene made in industry?
By aromatisation (reforming) of petroleum alkanes. n-Hexane heated over Mo2O3 at 773 K and 10-20 atm loses hydrogen, forms a ring, and becomes benzene. Benzene is also obtained from coal tar.
Why does the diazonium salt give phenol and not benzene with water?
With hot water the C-N2+ bond is replaced by -OH, giving phenol. Only hypophosphorous acid (H3PO2) replaces N2+ with -H to give benzene. This is a common NEET trap.
