Hydrocarbons for NEET 2026: Alkanes, Alkenes, Alkynes — Complete Guide

Hydrocarbons is the chapter that opens the door to all of Organic Chemistry. Get it right and the next ten chapters — haloalkanes, alcohols, aldehydes, amines, biomolecules — become pattern recognition. Get it wrong and Organic feels like a bottomless pit for the rest of Class 12.

NEET 2026 will pull 2–3 questions directly from this chapter and another 4–5 questions that depend on the reactions you learn here. That is up to 8 questions, or 32 marks, riding on Hydrocarbons alone. This guide is the same walkthrough I run with my one-to-one NEET students — focused, exam-targeted, and built around the reactions NCERT explicitly highlights.

Why Hydrocarbons Matters for NEET 2026

Three reasons this chapter is non-negotiable:

1. It is a direct-question chapter. NEET 2024 pulled 2 questions from Hydrocarbons. NEET 2025 pulled 3. The pattern is stable — expect at least 2 in 2026.
2. It feeds every later Organic chapter. The reactions of alkenes (addition), alkynes (acidic hydrogen) and benzene (EAS) repeat across Haloalkanes, Alcohols, Aldehydes/Ketones and Aromatic Amines. One chapter, ten chapters of leverage.
3. It is mechanism-heavy, not memorisation-heavy. Once you understand carbocation stability, free-radical chain reactions, and EAS, the chapter compresses to 12–15 core reactions.

Alkanes: Saturated and Stable

Alkanes are saturated hydrocarbons with general formula CₙH₂ₙ₊₂. All bonds are single — sp³ hybridized carbons, tetrahedral, 109.5° bond angles. Because they have no π-bonds, alkanes are unreactive under normal conditions. That single sentence kills 90% of "which alkane reacts faster" trap questions.

The 3 Alkane Reactions You Must Know

• Halogenation (free-radical substitution). Methane + Cl₂ in UV light gives CH₃Cl, then CH₂Cl₂, then CHCl₃, then CCl₄. The mechanism has three steps: initiation (Cl₂ → 2Cl·), propagation (Cl· abstracts H, then alkyl radical attacks Cl₂), termination (radicals combine).
• Combustion. CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O. Releases heat — the basis of LPG and petrol.
• Pyrolysis / cracking. High temperature breaks long alkane chains into shorter alkanes + alkenes. Industrial source of ethylene.

Reactivity Order in Halogenation

Tertiary H > Secondary H > Primary H. Why? Tertiary radicals are more stable due to hyperconjugation. This is also why 2-methylpropane gives mainly 2-chloro-2-methylpropane on chlorination — a favourite NEET trap.

Alkenes: Where the Magic of Addition Happens

Alkenes have at least one C=C double bond. General formula CₙH₂ₙ. The double bond is a sigma + pi — the pi bond is weak and electron-rich, which makes alkenes electrophile magnets. Every alkene reaction is just electrophilic addition.

Markovnikov's Rule

When HX adds to an unsymmetrical alkene, H goes to the carbon with more hydrogens, X goes to the carbon with fewer hydrogens. Why? Because the carbocation intermediate is more stable when the positive charge is on the more substituted carbon.

Example: Propene + HBr → 2-bromopropane (not 1-bromopropane). The intermediate secondary carbocation is more stable than the primary one.

Peroxide Effect (Anti-Markovnikov / Kharasch Effect)

Add a peroxide (R-O-O-R) and HBr only: the mechanism flips to free-radical, and Br adds to the less-substituted carbon. Propene + HBr + peroxide → 1-bromopropane.

Ozonolysis

Alkene + O₃, then Zn / H₂O → two carbonyl compounds. The C=C breaks cleanly at the double bond and each carbon becomes a C=O. Reductive ozonolysis (Zn/H₂O) gives aldehydes and/or ketones. Oxidative ozonolysis (H₂O₂) gives ketones and carboxylic acids.

This is the highest-yield reaction in the chapter for NEET because it lets the examiner ask "identify the alkene that gives these two products" — the reverse-direction problem.

Other Must-Know Alkene Reactions

• Hydrogenation (H₂ / Ni, Pt or Pd) → alkane
• Halogenation (Br₂ in CCl₄) → vicinal dibromide. The disappearance of brown Br₂ colour is the classic test for alkenes.
• Hydration (H₂O / H₂SO₄) → alcohol, Markovnikov product
• Hydroboration-oxidation (B₂H₆, then H₂O₂/OH⁻) → alcohol, anti-Markovnikov product
• Oxidation with cold dilute KMnO₄ (Baeyer's reagent) → vicinal diol. The decolourisation of purple KMnO₄ is another classic alkene test.

Alkynes: Triple-Bond Reactivity

Alkynes contain a C≡C triple bond. General formula CₙH₂ₙ₋₂. Triple bond = 1 sigma + 2 pi bonds. The terminal H on a C≡C is acidic — this is what makes alkynes special.

Acidic Hydrogen

The order of acidity: HC≡CH > H₂C=CH₂ > H₃C-CH₃. The reason is the s-character of the carbon: 50% in sp (alkyne), 33% in sp² (alkene), 25% in sp³ (alkane). Higher s-character → electrons held closer to the nucleus → the C-H bond is more polar → more acidic.

Terminal alkynes react with Na to give sodium acetylide and H₂. With ammoniacal AgNO₃ they give a white precipitate (silver acetylide) — this is the distinguishing test for terminal alkynes vs internal alkynes vs alkenes. NEET examiners love this.

Alkyne Addition Reactions

• Add H₂ once → alkene. Add again → alkane. Lindlar catalyst stops at cis-alkene. Na/liq NH₃ stops at trans-alkene.
• Add HX twice → gem-dihalide (both halogens on the same carbon)
• Add H₂O / H₂SO₄ / HgSO₄ → gives a ketone (not an alcohol). The intermediate enol tautomerises to ketone. This is the Markovnikov hydration of alkynes.

Aromatic Hydrocarbons: Benzene & EAS

Benzene (C₆H₆) is special because of aromaticity: cyclic, planar, conjugated, with (4n+2) π-electrons (Huckel's rule, n=1 → 6 electrons). The delocalised π-system is so stable that benzene does not undergo addition like alkenes — it undergoes electrophilic aromatic substitution (EAS).

The 5 EAS Reactions

1. Halogenation: Benzene + X₂ / FeX₃ → chloro/bromobenzene
2. Nitration: Benzene + conc. HNO₃ / conc. H₂SO₄ → nitrobenzene
3. Sulphonation: Benzene + oleum (fuming H₂SO₄) → benzenesulphonic acid
4. Friedel-Crafts alkylation: Benzene + R-Cl / AlCl₃ → alkylbenzene
5. Friedel-Crafts acylation: Benzene + RCOCl / AlCl₃ → aromatic ketone

Directing Effects (Ortho/Para vs Meta)

If the substituent already on benzene donates electrons (-OH, -NH₂, -CH₃, -OR, -X), the next group goes to ortho/para. If it withdraws electrons (-NO₂, -COOH, -CN, -CHO, -SO₃H), the next group goes to meta.

Halogens are the exception — they are mildly deactivating but ortho/para directing because of lone-pair donation by resonance. NEET tests this exception almost every year.

The 7 Hydrocarbons Traps NEET Uses Every Year

1. Peroxide effect with HCl or HI. Wrong. It works only with HBr. The answer is "no peroxide effect — Markovnikov product."
2. "Identify the alkene from ozonolysis products." Reverse-direction. Practise 10 of these from PYQs.
3. Acidic hydrogen ranking across alkanes, alkenes, alkynes, water, alcohols. Order: H₂O > ROH > HC≡CH > NH₃ > H₂C=CH₂ > H₃C-CH₃.
4. Lindlar catalyst vs Na/liq NH₃. Lindlar = cis. Na/NH₃ = trans. Easy 4 marks.
5. Friedel-Crafts with strong deactivating group. Friedel-Crafts does not work on nitrobenzene or benzene with a strong electron-withdrawing group. Examiner tests this.
6. Halogens as ortho/para directors. Despite being weakly deactivating, halogens direct ortho/para. Counter-intuitive — learn it once.
7. Markovnikov hydration of alkyne gives ketone, not alcohol. Via enol tautomerisation. NEET examiners build whole questions around this single fact.

PK Sir's 3-Week Hydrocarbons Mastery Plan

Final Word

Hydrocarbons is not a hard chapter. It feels hard because students try to memorise reaction names without understanding mechanism. Flip that around — learn the mechanism first, the names label themselves — and the chapter becomes one of the highest-scoring units in NEET Chemistry.

If you want a one-to-one mechanism walkthrough — especially for the traps with peroxide effect, Lindlar vs Na/NH₃, or alkyne hydration — book a free demo class. Bring one Hydrocarbons PYQ you have struggled with. We will solve it together.

And if you want to lock in GOC fundamentals first — inductive, resonance, hyperconjugation, carbocation stability — read that guide alongside this one. They are sister chapters.