Alkenes and Alkynes Reactions for JEE Main 2026

Alkenes and alkynes are the most reaction-rich functional groups in the JEE organic chemistry syllabus. Their π-bonds are the site of electrophilic addition reactions — each reaction adds across the double or triple bond with a characteristic regiochemistry and stereochemistry. Mastering the reagent-product pairs for every standard reaction secures consistent marks in the organic section.

Electrophilic Addition and Markovnikov's Rule

The π bond attacks an electrophile, forming a carbocation at the more stable (more substituted) carbon. The nucleophile then attacks that cation. Markovnikov's rule: H adds to the carbon already bearing more H, and X adds to the more substituted carbon. Examples: CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (not the anti-Markovnikov product). Addition of H₂O (acid-catalysed) follows the same rule: gives the Markovnikov alcohol. For the foundational organic reaction mechanisms context, see our reaction mechanisms guide.

Anti-Markovnikov Addition

Hydroboration-oxidation (BH₃/THF followed by H₂O₂/NaOH): adds H and OH across the double bond with anti-Markovnikov regiochemistry and syn stereochemistry. This sequence is the standard route to primary alcohols from terminal alkenes. Peroxide-promoted HBr addition (radical mechanism): also anti-Markovnikov — Br adds to the terminal carbon. These anti-Markovnikov routes are the main reagent-based contrast questions in JEE: recognise that BH₃ and peroxide/HBr give the opposite regiochemistry to HX alone.

Halogenation, Ozonolysis, and Oxidation

Br₂/CCl₄: adds two Br atoms across the double bond via anti addition (bromonium ion mechanism) → vicinal dibromide. KMnO₄/H₂SO₄ (hot, concentrated): oxidative cleavage of double bond → ketones/acids (aldehydes oxidise further to acids). Ozonolysis (O₃) followed by reductive workup (Zn/H₂O): cleaves double bond → aldehydes/ketones. Ozonolysis with oxidative workup (H₂O₂): aldehydes oxidise to carboxylic acids. Identifying ozonolysis products then reverse-engineering the alkene structure is a classic JEE question type.

Alkynes: Additional Reactions

Terminal alkynes are weakly acidic (pKa ≈ 25) and react with NaNH₂ to form sodium acetylide, which acts as a nucleophile for C–C bond formation. Alkynes add HX twice: first addition follows Markovnikov to give vinyl halide, second gives gem-dihalide. Hydration of alkynes (H₂SO₄/HgSO₄): Markovnikov addition of water to terminal alkynes gives methyl ketone (Markovnikov); to internal alkynes gives a ketone. Lindlar's catalyst (H₂): syn addition → cis-alkene. Na/liquid NH₃ (Birch-type): anti addition → trans-alkene.

Polymerisation

Free-radical addition polymerisation of ethylene gives polythene; of propylene gives polypropylene; of vinyl chloride (CH₂=CHCl) gives PVC; of styrene gives polystyrene. Anionic polymerisation of acrylonitrile or butadiene forms important synthetic rubbers. JEE occasionally asks to identify the monomer from a given polymer repeating unit — break the backbone bonds between every other carbon pair to reveal the monomer's double bond. After mastering all alkene and alkyne reactions, take a free mock test on hydrocarbons.