Last Updated: May 2026
NEET Chemistry Hydrocarbons 2027 sits in NCERT Class XI Unit 13 (Organic Chemistry — Some Basic Principles & Techniques follows) and is the single highest-yield chapter for NEET organic. Across NEET 2021-2025 it returned an average of 2.8 questions/year (~11 marks), with combined hydrocarbons + general organic principles giving 5–6 questions. The reason is simple — hydrocarbons are the scaffold for every later organic chapter (alcohols, aldehydes, amines, biomolecules) and reaction-mechanism MCQs lean heavily on alkene/alkyne addition logic.
This guide covers the entire chapter — alkanes, alkenes, alkynes, aromatic hydrocarbons (benzene), reactions, mechanisms, IUPAC nomenclature pitfalls — followed by 40 NEET-style practice MCQs with full answer key, an interactive 10-MCQ quiz, and a NEET 2027 FAQ section.
1. Classification of Hydrocarbons
| Type | General formula | Bonding | Hybridisation | Examples |
|---|---|---|---|---|
| Alkane (saturated) | CnH2n+2 | All C–C single | sp³ | CH4, C2H6, C3H8 |
| Alkene (unsaturated) | CnH2n | One C=C double | sp² | C2H4, C3H6 |
| Alkyne (unsaturated) | CnH2n−2 | One C≡C triple | sp | C2H2, C3H4 |
| Arene (aromatic) | — | Delocalised π | sp² | Benzene, toluene |
2. Alkanes — Methane and Beyond
Preparation
- Wurtz reaction: 2 R-X + 2Na → R-R + 2 NaX (gives symmetric alkanes only; methane cannot be prepared by Wurtz).
- Decarboxylation (sodalime): RCOONa + NaOH/CaO → R-H + Na2CO3.
- Reduction of alkyl halide: R-X + Zn / HCl → R-H.
- Kolbe’s electrolysis: anode reaction of carboxylate salt → alkane (even-numbered).
Properties
- Boiling point ↑ with chain length; branching ↓ b.p. (lower surface area).
- Inert to acids/bases (saturated, only weak C–C, C–H bonds).
- Halogenation (free-radical chain mechanism): CH4 + Cl2 → CH3Cl → CH2Cl2 → CHCl3 → CCl4. Reactivity: F2 > Cl2 > Br2 > I2.
- Combustion: complete (CO2 + H2O) or incomplete (CO + soot).
- Pyrolysis (cracking): high MW alkane → low MW alkane + alkene (industrial petroleum process).
Conformations of Ethane
Ethane shows two extreme conformations: eclipsed (highest energy, torsional strain ~12.55 kJ/mol) and staggered (lowest energy, all H atoms anti). Newman projection diagrams are routinely asked. Conformations are interconverted by rotation about the C–C single bond — they cannot be isolated.
3. Alkenes — Olefins
Preparation
- Dehydrohalogenation of alkyl halide (alc. KOH) → Saytzeff’s product (more substituted alkene).
- Dehydration of alcohol with conc. H2SO4 (170 °C) → alkene.
- Dehalogenation of vicinal dihalide with Zn dust → alkene.
- Partial reduction of alkyne with Lindlar’s catalyst (Pd/CaCO3 / Pb poison) → cis-alkene.
Reactions
- Addition of H2 (Pt/Pd/Ni) → alkane.
- Addition of HX: follows Markovnikov’s rule — H goes to the C with more Hs. With peroxide → anti-Markovnikov (Kharasch effect) for HBr only.
- Addition of H2O (acid catalysed): Markovnikov → alcohol.
- Addition of H2SO4 → alkyl hydrogen sulphate; hydrolysis → alcohol (industrial).
- Addition of halogen (Br2/CCl4): instant decolourisation — classic test for unsaturation.
- Hydroboration-oxidation: B2H6 then H2O2/OH- → anti-Markovnikov alcohol.
- Ozonolysis: O3 then Zn/H2O → cleaves C=C into two carbonyls; lets you locate position of double bond.
- Polymerisation: ethene under high T/P → polythene.
- Oxidation: cold dil. KMnO4 (Baeyer’s reagent) → vicinal diol; hot conc. KMnO4 → cleaves to acids/ketones.
Mechanism — Markovnikov Addition of HBr to Propene
Step 1: H+ adds to give the more stable secondary carbocation (CH3-CH+-CH3) — not the primary (CH3-CH2-CH2+) — due to hyperconjugation + inductive stabilisation.
Step 2: Br− attacks → 2-bromopropane. Stability order of carbocation: 3° > 2° > 1° > methyl. This single principle drives ~30% of NEET organic MCQs.
4. Alkynes — Acetylene and Friends
Preparation
- From CaC2 + H2O → C2H2 + Ca(OH)2 (industrial).
- Dehalogenation of vicinal tetrahalide (Zn) → alkyne.
- Dehydrohalogenation of vicinal/geminal dihalide (alc. KOH then NaNH2).
Reactions
- Addition of H2 (Pd-BaSO4 / Lindlar) → cis-alkene; Na/liq. NH3 → trans-alkene.
- Addition of HX, H2O — both Markovnikov. With H2O / H2SO4 / HgSO4 → keto enol tautomerism gives ketone (acetylene gives acetaldehyde — exception, only terminal alkyne with no extra C).
- Acidic H — terminal alkynes give Na/Cu/Ag acetylides (test: white ppt with AgNO3-NH3, red ppt with Cu2Cl2-NH3).
- Polymerisation: 3 C2H2 / red-hot Fe tube / 873 K → benzene (Berthelot).
5. Aromatic Hydrocarbons — Benzene
Structure & Aromaticity (Hückel rule)
Benzene (C6H6) — planar, cyclic, fully conjugated, 6 π-electrons (4n+2, n=1). All C-C bonds equal length 139 pm (between single 154 and double 134). Resonance energy ~150 kJ/mol → unusually stable. Hückel’s rule: aromatic if planar + cyclic + conjugated + (4n+2) π electrons.
Preparation
- Cyclic polymerisation of acetylene.
- Decarboxylation of sodium benzoate with NaOH/CaO.
- Reduction of phenol with Zn dust.
Electrophilic Aromatic Substitution (EAS) — the workhorse
| Reaction | Reagent | Product |
|---|---|---|
| Halogenation | Cl2 / FeCl3 (or Br2/FeBr3) | Chlorobenzene / Bromobenzene |
| Nitration | conc. HNO3 + conc. H2SO4 (50–60 °C) | Nitrobenzene |
| Sulphonation | fuming H2SO4 | Benzene sulphonic acid |
| Friedel-Crafts alkylation | R-X / anhyd. AlCl3 | Alkyl benzene |
| Friedel-Crafts acylation | R-COCl / anhyd. AlCl3 | Aryl ketone |
EAS mechanism: generation of electrophile → π attack of benzene → arenium ion (Wheland intermediate) → loss of H+ → aromatic product.
Directing Effects
| Group | Effect | Directs to | Examples |
|---|---|---|---|
| Activating | +M / +I | ortho, para | -OH, -NH2, -OR, -NHR, -R, -X (X is ortho/para directing but deactivating) |
| Deactivating | -M / -I | meta | -NO2, -CN, -COOH, -SO3H, -CHO, -COR |
Halogens are the textbook exception: ortho/para directing but deactivating. NEET-favourite trap.
Side-chain Reactions of Toluene
- Oxidation of -CH3 with KMnO4 / K2Cr2O7 → -COOH (benzoic acid).
- Free-radical halogenation of -CH3 (Cl2 / hν) → benzyl chloride.
6. Carcinogenicity & Toxicity
Polynuclear aromatic hydrocarbons (PAHs) like benzo[a]pyrene from incomplete combustion are potent carcinogens — repeatedly tested as a 1-mark NEET item.
Useful internal links: NEET 2027 Drishti Chemistry · NEET 2027 syllabus · free chapter notes · NEET 2027 FAQ · join Drishti Chemistry batch.
40 NEET Practice MCQs — Hydrocarbons
- General formula of alkane: (a) CnH2n (b) CnH2n+2 (c) CnH2n−2 (d) CnH2n+1
- Wurtz reaction is used to prepare: (a) Alkene (b) Alkyne (c) Symmetrical alkane (d) Aldehyde
- Methane cannot be prepared by: (a) Wurtz (b) Decarboxylation (c) Reduction of CH3I (d) Kolbe electrolysis of CH3COONa
- Lower b.p. with branching is due to: (a) Higher MW (b) H-bonding (c) Lower surface area (d) Polarity
- Halogenation of methane proceeds via: (a) Ionic (b) Free radical (c) Concerted (d) Carbene
- Lindlar’s catalyst is: (a) Pd/CaCO3 + lead poison (b) Ni/Al2O3 (c) Pt/C (d) Cu chromite
- Markovnikov rule predicts: (a) H to less-H carbon (b) H to more-H carbon (c) X to terminal (d) X to centre
- Anti-Markovnikov addition (Kharasch) works only with: (a) HCl (b) HBr (c) HI (d) HF
- Test for C=C / C≡C: (a) FeCl3 (b) AgNO3 (c) Br2/CCl4 decolourisation (d) Tollens
- Bayer’s reagent is: (a) hot conc. KMnO4 (b) cold dil. alkaline KMnO4 (c) Br2 in CCl4 (d) Cu2Cl2
- Ozonolysis cleaves: (a) C-C (b) C=C (c) C-H (d) C=O
- Hybridisation in benzene: (a) sp (b) sp² (c) sp³ (d) sp³d
- Number of π electrons in benzene: (a) 4 (b) 6 (c) 8 (d) 10
- C-C bond length in benzene: (a) 134 pm (b) 139 pm (c) 154 pm (d) 120 pm
- Friedel-Crafts catalyst: (a) Anhyd. AlCl3 (b) FeBr3 (c) Conc. H2SO4 (d) NaOH
- Nitration of benzene needs: (a) dil HNO3 (b) conc HNO3 + conc H2SO4 (c) HNO3 + KMnO4 (d) HNO3 alone
- Activating ortho/para directing group: (a) -NO2 (b) -COOH (c) -OH (d) -CN
- Deactivating but ortho/para director: (a) -OH (b) -NH2 (c) -Cl (d) -CH3
- Toluene + KMnO4 (hot) → (a) Benzaldehyde (b) Benzyl alcohol (c) Benzoic acid (d) Benzene
- Hückel’s rule: aromatic if π electrons = (a) 4n (b) 4n+1 (c) 4n+2 (d) 4n+3
- Acetylene + Cu2Cl2/NH4OH → (a) White ppt (b) Red ppt (c) Black ppt (d) No reaction
- 2 mol acetylene + dimerisation in red-hot tube gives: (a) Vinyl acetylene (b) Benzene (c) Methane (d) Cyclooctatetraene
- Acetylene + H2O / Hg2+ / H2SO4 → (a) Ethanol (b) Acetaldehyde (c) Acetic acid (d) Acetone
- Hybridisation of carbon in acetylene: (a) sp (b) sp² (c) sp³ (d) sp²d
- Conformation with maximum stability in ethane: (a) Eclipsed (b) Staggered (c) Skew (d) Gauche
- Saytzeff’s rule predicts product for: (a) Substitution (b) Addition (c) Elimination (d) Rearrangement
- Cracking of n-hexane gives: (a) Methane only (b) Mixture of low MW alkanes & alkenes (c) Aromatic only (d) Alcohols
- Most stable carbocation: (a) Methyl (b) 1° (c) 2° (d) 3°
- Hydroboration-oxidation of propene gives: (a) 1-propanol (b) 2-propanol (c) Acetone (d) Propanal
- Polythene is polymer of: (a) Propene (b) Ethene (c) Acetylene (d) Benzene
- Number of σ + π bonds in benzene: (a) 12σ + 3π (b) 6σ + 3π (c) 12σ + 6π (d) 18σ + 3π — counting C-H σ too
- Ozonolysis (reductive) of 2-butene gives: (a) 2 CH3CHO (b) 2 HCHO (c) CH3COCH3 (d) CH3COOH
- Sulphonation reagent: (a) Conc. H2SO4 (b) Fuming H2SO4 (c) NaHSO3 (d) SO2
- Strongest acidic H is in: (a) CH4 (b) CH2=CH2 (c) HC≡CH (d) C2H6
- Electrophile in Friedel-Crafts alkylation is: (a) R+ (b) R- (c) R• (d) :CR2
- Benzyl chloride is formed from toluene + Cl2 in presence of: (a) FeCl3 (b) hν (light) (c) H2SO4 (d) AlCl3
- Cyclic polymerisation of acetylene to benzene needs: (a) Pt/H2 (b) Red-hot iron tube (c) AlCl3 (d) HgSO4
- Oxidation product of cold dil KMnO4 on ethene: (a) Ethanal (b) Ethylene glycol (c) Acetic acid (d) Ethanol
- Number of monochlorination products of n-butane: (a) 1 (b) 2 (c) 3 (d) 4
- Carcinogenic PAH from incomplete combustion: (a) Naphthalene (b) Anthracene (c) Benzo[a]pyrene (d) Phenanthrene
Answer Key (NEET Practice — Hydrocarbons)
| 1-b | 2-c | 3-a | 4-c | 5-b | 6-a | 7-a | 8-b |
| 9-c | 10-b | 11-b | 12-b | 13-b | 14-b | 15-a | 16-b |
| 17-c | 18-c | 19-c | 20-c | 21-b | 22-b | 23-b | 24-a |
| 25-b | 26-c | 27-b | 28-d | 29-a | 30-b | 31-a | 32-a |
| 33-b | 34-c | 35-a | 36-b | 37-b | 38-b | 39-b | 40-c |
Interactive 10-MCQ Quiz
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FAQ — NEET Chemistry Hydrocarbons 2027
How many marks does Hydrocarbons carry in NEET?
Direct hydrocarbons questions average 2.8/year (2021-2025) — that is ~11 marks. Combined with general organic principles + tests of unsaturation, total impact rises to ~16 marks/paper.
Which sub-topic gives most NEET questions?
Reactions of alkenes (Markovnikov, anti-Markovnikov, ozonolysis, hydroboration) lead. Followed by EAS directing effects on substituted benzenes, and acidic-H tests for terminal alkynes.
What is the carbocation stability rule for NEET organic?
3° > 2° > 1° > methyl. Allylic and benzylic cations rival 3° due to resonance. Almost every NEET addition mechanism question hinges on this.
Are halogens activating or deactivating in EAS?
Deactivating but ortho/para directing. -I dominates over +M for ring-attacking electrophiles, but lone pair resonance still places the partial negative charge at o/p positions. Classic NEET trap.
Why is Lindlar’s catalyst poisoned with lead?
Pd alone reduces alkyne all the way to alkane. Adding Pb / quinoline poisons Pd just enough to halt at cis-alkene. NCERT-line answer.
Lock NEET 2027 Chemistry early. Drishti NEET Chemistry batches at NEET Gurukul include weekly mechanism sprints, EAS pattern drills, and full-length AIIMS-pattern mocks. Explore all NEET 2027 courses →