Hydrocarbons

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Hydrocarbons — Key Facts for NEET/JEE

Hydrocarbons contain only carbon and hydrogen atoms. They are classified as:

1. Saturated (Alkanes — CₙH₂ₙ₊₂):

• Only single bonds (σ-bonds)
• Tetrahedral geometry at each carbon (sp³ hybridisation)
• General formula: CₙH₂ₙ₊₂
• Examples: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀)
• Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O (exothermic, ΔH = −890 kJ/mol)

2. Unsaturated:

Alkenes (CₙH₂ₙ) — One double bond:

• Trigonal planar geometry (sp² hybridisation)
• Examples: Ethene (C₂H₄), Propene (C₃H₆)
• Addition reactions: H₂ (hydrogenation), Br₂ (bromination), HX (Markovnikov addition)

Alkynes (CₙH₂ₙ₋₂) — One triple bond:

• Linear geometry (sp hybridisation)
• Examples: Ethyne (C₂H₂), Propyne (C₃H₄)
• Terminal alkynes have acidic hydrogen (pKa ~25) — react with NaNH₂, AgNO₃ to form acetylides

3. Aromatic (Arenes):

• Benzene (C₆H₆): Planar, aromatic ring with delocalised π-electrons (6 electrons, Huckel’s rule: 4n+2 = 6, n=1)
• Resonance structures: Two Kekulé structures; delocalisation gives extra stability

IUPAC Nomenclature:

• Longest chain → numbering → suffix (-ane for alkane, -ene for alkene, -yne for alkyne)
• Lowest numbers for functional groups and multiple bonds
• Prefixes: di-, tri-, tetra- for multiple substituents
• For alkenes: -en-; for alkynes: -yn-
• Stereodescriptors: E/Z for geometric isomers, R/S for chiral centres

⚡ Exam tip: Markovnikov’s rule — in addition of HX to an alkene, the hydrogen adds to the carbon with more hydrogens already (less substituted carbon). Anti-Markovnikov addition requires peroxides (peroxide effect — Kharasch addition).

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Hydrocarbons — Chemistry Study Guide

Bonding and Hybridisation:

Hybridisation	Geometry	Bond Angle	Example
sp³	Tetrahedral	109.5°	Methane (CH₄), Ethane
sp²	Trigonal planar	120°	Ethene (C₂H₄), Benzene
sp	Linear	180°	Ethyne (C₂H₂), Hydrogen cyanide

Bond Lengths:

• C−C (single): 1.54 Å
• C=C (double): 1.34 Å
• C≡C (triple): 1.20 Å
• C−H: 1.09 Å

Bond Enthalpies:

• C−C: 347 kJ/mol
• C=C: 614 kJ/mol
• C≡C: 839 kJ/mol
• C−H: 414 kJ/mol

Alkane Reactions:

• Halogenation (Free radical substitution): CH₄ + Cl₂ → CH₃Cl + HCl (UV light required; step-wise: Initiation → Propagation → Termination)
• Combustion: Complete (excess O₂): CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O
• Cracking: Large alkanes → smaller alkanes + alkenes (industrial importance)
• Isomerisation: n-butane → isobutane (catalytic reforming)

Alkene Reactions:

• Addition of H₂ (hydrogenation): Pt/Pd catalyst; heats of hydrogenation: ethylene (−136 kJ/mol), propene (−124 kJ/mol); more substituted alkenes are more stable
• Addition of Br₂: Reddish-brown bromine water decolourises — test for unsaturation
• Addition of HX: Markovnikov addition; carbocation intermediate
• Addition of H₂O (hydration): H⁺ catalyst; Markovnikov addition
• Ozonolysis: O₃ followed by Zn/H₂O; cleaves double bond; gives carbonyl compounds
• Polymerisation: Ethene → polyethylene (free radical polymerisation); Propene → polypropylene

Alkyne Reactions:

• Addition of H₂ (2 steps): 1st step → alkene (cis or trans depending on catalyst); Lindlar’s catalyst (Pd/CaCO₃ poisoned) → cis-alkene; Na/NH₃ → trans-alkene
• Addition of halogen (Br₂): Stepwise; decolourises bromine water
• Acidic nature: Terminal alkynes (HC≡C−) react with Na metal → NaC≡C− + H₂; react with Tollens’ reagent (AgNO₃/NH₃) → silver acetylide precipitate
• Preparation from vicinal dihalides: R−CHX−CHX−R + 2NaNH₂ → RC≡CR + 2NaX + 2NH₃

Benzene Reactions (Electrophilic Aromatic Substitution):

• Nitration: HNO₃/H₂SO₄ → nitrobenzene (C₆H₅NO₂)
• Halogenation: Br₂/FeBr₃ → bromobenzene; Cl₂/FeCl₃ → chlorobenzene
• Friedel-Crafts alkylation: RCl/AlCl₃ → alkylbenzene (C₆H₅R); benzoyl chloride for acyl groups
• Sulphonation: Fuming H₂SO₄ → benzenesulphonic acid (C₆H₅SO₃H); reversible
• Oxidation: Alkyl side chains → benzoic acid (KMnO₄/heat); benzene itself is resistant to oxidation

Directing effects: −OH, −NH₂, −O− (ortho/para directors, activating); −NO₂, −CN, −COOH (meta directors, deactivating).

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Hydrocarbons — Comprehensive Notes

Mechanism — Free Radical Halogenation of Alkanes: Step 1 (Initiation): Cl₂ → 2Cl• (homolytic cleavage, UV light provides energy) Step 2 (Propagation): Cl• + CH₄ → •CH₃ + HCl; then •CH₃ + Cl₂ → CH₃Cl + Cl• (chain reaction continues) Step 3 (Termination): Any two radicals combine:

• Cl• + Cl• → Cl₂
• •CH₃ + Cl• → CH₃Cl
• •CH₃ + •CH₃ → C₂H₆

Mechanism — Electrophilic Addition to Alkenes (Markovnikov): Step 1: H⁺ from HBr approaches the alkene; the π electrons attack H⁺ → carbocation forms on the MORE SUBSTITUTED carbon (stability: tertiary > secondary > primary) Step 2: Br⁻ attacks the carbocation → product is formed

Carbocation stability order: (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺

Anti-Markovnikov Addition (Peroxide Effect): In presence of peroxides (ROOR), the addition of HBr to propene gives 1-bromopropane instead of 2-bromopropane. This is because peroxides generate Br• radicals which attack the less substituted carbon (radical stability is opposite of carbocation).

Electrophilic Aromatic Substitution (EAS) — Detailed Mechanism: Step 1: Generation of electrophile (E⁺) from the attacking reagent Step 2: π-electrons of benzene attack E⁺ → forms sigma complex (arenium ion intermediate) Step 3: Deprotonation by base (Cl⁻, HSO₄⁻, etc.) → restores aromaticity → substituted benzene

Energy profile: The sigma complex is the rate-determining, highest-energy intermediate.

Aromaticity — Huckel’s Rule: Aromatic compounds must have:

1. Cyclic, conjugated system
2. Planar geometry
3. (4n+2) π electrons in the ring (n = 0, 1, 2, …)
4. Examples: Benzene (6π), Pyridine (6π), Pyrrole (6π), Naphthalene (10π), Cyclopentadienyl anion (6π), Tropylium cation (6π)
5. Antiaromatic: Cyclobutadiene (4π), Cyclopropenyl cation (2π) — highly unstable

Geometric Isomerism in Alkenes (E/Z System): E (entgegen = opposed): Two higher priority groups on opposite sides Z (zusammen = together): Two higher priority groups on same side Priority determined by Cahn-Ingold-Prelog (CIP) rules — compare atomic numbers of atoms directly attached to the double bond carbon.

Cyclic Hydrocarbons:

• Cycloalkanes (CₙH₂ₙ): Ring strain due to angle strain (C-C-C angle deviation from 109.5°). Cyclopropane most strained.
• Cyclopropane: 60° angle → high strain → behaves like alkenes (addition reactions)
• Cyclohexane: Chair conformation most stable; all angles ~109.5°; has axial and equatorial positions

Petroleum Refining: Crude oil → fractional distillation → fractions (petrol, kerosene, diesel, lubricating oils) Cracking: Thermal or catalytic splitting of larger alkanes into smaller, more useful ones. Reforming: Alkanes → aromatic or branched alkanes (improves octane rating).

Octane rating: Iso-octane (2,2,4-trimethylpentane) = 100 (anti-knock); n-heptane = 0 (knocking). Higher octane = less knocking.

NEET Pattern Analysis: Hydrocarbons contributes 2–3 questions per year in NEET Chemistry. Key areas: IUPAC naming, Markovnikov addition, E/Z isomerism, aromaticity (Huckel’s rule), benzene reactions and directing effects, and alkyne acidity. Questions involving mechanisms (carbocations, electrophilic addition) are increasingly asked.

⚡ NEET 2023 Qn: Which of the following is the most stable alkene? (a) 2-methylpropene (b) but-1-ene (c) but-2-ene (d) ethene. Answer: 2-methylpropene (most substituted alkene). Stability order: tetrasubstituted > trisubstituted > disubstituted > monosubstituted > ethylene.