JEE Main Chemistry Hydrocarbons 2027: Alkanes, Alkenes, Alkynes & Benzene Reactions
Free-radical halogenation, Markovnikov and peroxide effect, ozonolysis, alkyne chemistry and electrophilic substitution in benzene, with worked examples.
Edurack
September 28, 2026
Hydrocarbons is the first chapter where reactions start to pile up, so a good strategy matters more than raw memory. The key is to learn each reaction as a mechanism, with its reagent, its intermediate and its selectivity, instead of as a stand-alone equation.
Learn the intermediate, and the product follows. Carbocations decide alkenes, and the sigma complex decides benzene.
Chapter at a Glance
| Snapshot | Detail |
|---|---|
| NTA unit | Unit 15 of 20: Hydrocarbons |
| Priority (trend-based) | Moderate |
| Typical question style | Product-prediction MCQs and named-reaction based questions |
| Best first step | Master alkene addition and benzene substitution first |
Priority reflects past-paper trends, not an official NTA weightage.
What the NTA Syllabus Covers
- Classification, isomerism, IUPAC nomenclature, preparation, properties and reactions
- Alkanes: conformations (Sawhorse and Newman projections of ethane), mechanism of halogenation
- Alkenes: geometrical isomerism, electrophilic addition, Markownikoff's rule and peroxide effect, ozonolysis, polymerisation
- Alkynes: acidic character, addition reactions, polymerisation
- Aromatic hydrocarbons: benzene structure and aromaticity, electrophilic substitution (halogenation, nitration, Friedel-Crafts), directive influence of groups
Master These Topics
1. Alkanes: Conformations and Free-Radical Halogenation
In ethane, the staggered conformation is more stable than the eclipsed by about 12 kJ/mol, because of torsional strain in the eclipsed form.
Halogenation proceeds by a free-radical chain mechanism: initiation (light splits X₂), propagation, and termination. Selectivity depends on radical stability, 3° > 2° > 1°.
Worked example: Chlorination of propane has relative reactivity 1° : 2° = 1 : 3.8 per hydrogen. Propane has 6 primary and 2 secondary hydrogens, so 2-chloropropane forms in the ratio 2 × 3.8 = 7.6 versus 6 for 1-chloropropane, about 56% and 44%. Bromine is far more selective than chlorine.
2. Alkenes: Addition Reactions
Electrophilic addition follows Markovnikov's rule: the hydrogen adds to the carbon that already has more hydrogens, giving the more stable carbocation.
- Propene + HBr gives 2-bromopropane.
- Propene + HBr with peroxide gives 1-bromopropane (anti-Markovnikov, radical mechanism). The peroxide effect works only for HBr.
Ozonolysis cleaves the double bond into two carbonyl groups. Worked example: (CH₃)₂C=CHCH₃ gives acetone and acetaldehyde.
Trap: The peroxide effect applies to HBr only, not to HCl or HI.
3. Alkynes
Terminal alkynes are acidic (pKa about 25) because sp carbon is more electronegative, so they react with sodium amide or ammoniacal silver nitrate to form metal acetylides. Hydration with dilute H₂SO₄ and HgSO₄ gives a ketone (acetaldehyde for ethyne). Partial hydrogenation over Lindlar's catalyst gives the cis-alkene, while sodium in liquid ammonia gives the trans-alkene.
4. Benzene and Aromaticity
A compound is aromatic if it is cyclic, planar, fully conjugated and has (4n + 2) pi electrons (Huckel's rule). Benzene, the cyclopentadienyl anion and the tropylium cation are aromatic, while cyclobutadiene is anti-aromatic.
Electrophilic substitution in benzene proceeds through a sigma complex. Key reactions are nitration (HNO₃ + H₂SO₄), halogenation (X₂ + FeX₃), and Friedel-Crafts alkylation and acylation (RCl or RCOCl + AlCl₃).
5. Directing Effects
| Group type | Examples | Effect |
|---|---|---|
| Activating, ortho/para | −OH, −NH₂, −CH₃ | Faster reaction than benzene |
| Deactivating, meta | −NO₂, −CN, −COOH, −CHO | Slower reaction than benzene |
| Deactivating, ortho/para | −Cl, −Br | Slower, but ortho/para directing |
Toluene nitrates at the ortho and para positions, while nitrobenzene gives m-dinitrobenzene. Friedel-Crafts reactions fail on strongly deactivated rings, and alkylation can rearrange the alkyl group.
Common Traps to Avoid
- Applying the peroxide effect to HCl or HI.
- Forgetting that Friedel-Crafts fails on rings with strongly deactivating groups.
- Confusing Lindlar's catalyst (cis-alkene) with Na/liquid NH₃ (trans-alkene).
- Missing that halogens are deactivating yet ortho/para directing.
60-Second Revision Sheet
- Markovnikov: H adds to carbon with more hydrogens; peroxide effect for HBr only
- Ozonolysis gives carbonyl compounds at the double bond
- Aromatic: cyclic, planar, conjugated,
(4n + 2)pi electrons - Meta directors: NO₂, CN, COOH, CHO; ortho/para: OH, NH₂, alkyl, halogens
Your Study Plan
- Day 1: alkane conformations and radical halogenation with selectivity.
- Day 2: alkene addition, Markovnikov, peroxide effect, ozonolysis.
- Day 3: alkyne reactions and aromaticity tests.
- Day 4: electrophilic substitution and directing effects, timed set.
Practice Hydrocarbons Questions Free → (opens in a new tab)
Continue Your Chemistry Journey
- Previous chapter: Basic Principles of Organic Chemistry
- Next chapter: Organic Compounds Containing Halogens
- All JEE Main Chemistry chapters
- Complete JEE Main Syllabus 2027 guide
Frequently Asked Questions
What is Markovnikov's rule?
In the addition of HX to an unsymmetrical alkene, hydrogen attaches to the carbon with more hydrogens, so the halogen goes to the carbon that forms the more stable carbocation.
Why are halogens ortho/para directing but deactivating?
Their lone pairs donate by resonance to the ortho and para positions, but their strong electronegativity withdraws electron density by the inductive effect, making the ring less reactive overall.