Chemistry · JEE Advanced only

Cyclic Polymerization Reaction of Alkynes (JEE Advanced)

Not in the JEE Main syllabus, but it is in JEE Advanced. Study time: about 1.5 to 2 hours. This is a focused topic with one main reaction to learn. Spend time understanding the mechanism conceptually, memorizing conditions and products for common alkynes (acetylene, propyne, but-2-yne), and practicing 5 to 8 problems involving product prediction..

Last updated: 26 October 2026

Direct answer

Cyclic polymerization of alkynes is a reaction where three molecules of an alkyne combine to form a cyclic aromatic compound, typically benzene or its derivatives.

Cyclic polymerization of alkynes is a reaction where three molecules of an alkyne combine to form a cyclic aromatic compound, typically benzene or its derivatives. When acetylene (ethyne) is passed through a red-hot iron tube at around 873 K, three acetylene molecules trimerize to form benzene. This reaction is also called Berthelot synthesis or cyclotrimerization.

Official syllabus

What the JEE Advanced syllabus says

“cyclic polymerization reaction of alkynes”

Source: JEE Advanced 2026 Information Bulletin, Annexure-I. Closest JEE Main topic: polymerization.

JEE Main covers general polymerization concepts but does not specifically test the cyclic trimerization of alkynes to form aromatic rings. JEE Advanced expects you to know this named reaction and apply it to predict products from substituted alkynes.

Key ideas

What you must understand

Three alkyne molecules join head-to-tail in a cyclic manner to form a six-membered aromatic ring
Acetylene gives benzene; propyne gives 1,3,5-trimethylbenzene (mesitylene)
The reaction requires high temperature (873 K) and a catalyst like red-hot iron tube, organonickel, or chromium-based catalysts
Symmetrical alkynes give a single product; unsymmetrical alkynes can give a mixture of isomeric trimers
This is an example of [2+2+2] cycloaddition and demonstrates how alkynes can be converted to aromatic compounds
Modern catalysts like Wilkinson's catalyst or nickel complexes allow this reaction at lower temperatures

Formulas

Formulas and key facts

3 HC≡CH → C₆H₆ (benzene) at 873 K, red-hot Fe tube
3 CH₃−C≡CH → 1,3,5-trimethylbenzene (mesitylene)
3 RC≡CR → hexasubstituted benzene (symmetrical alkyne case)
Catalyst options: red-hot Fe tube, Ni(0) complexes, Cr-based catalysts
Temperature: typically 873 K (600°C) for thermal process
Product is always aromatic (follows Hückel's rule: 6 π electrons)

Avoid

Common mistakes

Confusing cyclic polymerization with linear polymerization of alkynes (linear gives polyacetylene, cyclic gives benzene derivatives)
Forgetting that three alkyne molecules are needed, not two (it is trimerization, not dimerization)
Not recognizing that unsymmetrical alkynes give isomeric mixtures due to different possible orientations
Writing wrong products for substituted alkynes, for example thinking propyne gives trimethylbenzene with adjacent methyl groups instead of 1,3,5-positions

Worked example

A JEE Advanced-level example

Question. When but-2-yne undergoes cyclic polymerization in the presence of a suitable catalyst, what is the major product formed? Draw its structure and give its IUPAC name.

Show solution

But-2-yne has the structure CH₃−C≡C−CH₃ (a symmetrical internal alkyne)
In cyclic trimerization, three molecules combine to form a benzene ring
Each triple bond contributes two carbons to the ring, so 3 × 2 = 6 carbons form the benzene ring
The methyl groups attached to the triple bond carbons become substituents on the benzene ring
Since but-2-yne is symmetrical, all six positions of the benzene ring get methyl substituents
Product: hexamethylbenzene (C₆(CH₃)₆ or C₁₂H₁₈)

Answer: The major product is hexamethylbenzene (1,2,3,4,5,6-hexamethylbenzene). Structure: a benzene ring with all six hydrogen atoms replaced by methyl groups.

Solved twice independently; both solutions agree.

FAQs

Frequently asked questions

Is Cyclic Polymerization Reaction of Alkynes in the JEE Main syllabus?

No. It is in the JEE Advanced 2026 syllabus only. The closest JEE Main topic is polymerization.

How long does Cyclic Polymerization Reaction of Alkynes take to prepare for JEE Advanced?

About 1.5 to 2 hours. This is a focused topic with one main reaction to learn. Spend time understanding the mechanism conceptually, memorizing conditions and products for common alkynes (acetylene, propyne, but-2-yne), and practicing 5 to 8 problems involving product prediction., if you already know the related JEE Main topic.