Chemistry of Hydrocarbons: MDCAT Notes and MCQs

Alkanes (C_nH_2n+2)

• IUPAC: longest chain → number from end nearest the branch → name substituents alphabetically with locants (e.g. 2-methylbutane, 2,2-dimethylpropane).
• sp³, tetrahedral, C–C 1.54 Å. Unreactive ("paraffins"); undergo free-radical substitution in UV light.

Free-radical chlorination of methane

1. Initiation: Cl₂ → 2Cl• (homolytic fission by UV/sunlight).
2. Propagation: Cl• + CH₄ → HCl + •CH₃; •CH₃ + Cl₂ → CH₃Cl + Cl• (chain continues).
3. Termination: radicals combine — Cl• + Cl• → Cl₂; •CH₃ + •CH₃ → C₂H₆; •CH₃ + Cl• → CH₃Cl.

Excess Cl₂ gives CH₂Cl₂, CHCl₃, CCl₄. Ethane similarly gives C₂H₅Cl.

Alkenes (C_nH_2n)

• IUPAC: suffix -ene; longest chain containing C=C; lowest locant to double bond (but-2-ene).
• Ethene: each C is sp²; planar, bond angles ≈ 120°; C=C = 1 σ (sp²–sp²) + 1 π (sideways 2p–2p, above and below plane); C=C 1.34 Å. The exposed π electrons make alkenes reactive towards electrophilic addition (H₂, X₂, HX — Markovnikov's rule, H₂O, KMnO₄ → glycol (Baeyer's test, pink colour discharged), ozonolysis, polymerization).
• Bromine water decolourised → test for unsaturation.
• Preparation: dehydration of ethanol with conc. H₂SO₄ at 170 °C (or Al₂O₃ at 350 °C); dehydrohalogenation of ethyl halide with alcoholic KOH.

Benzene (C₆H₆)

• All six C are sp²; planar hexagon, angles 120°; all C–C bonds equal = 1.397 Å (between single and double). The six unhybridized p orbitals overlap to form a delocalized π cloud above and below the ring (MO treatment).
• Resonance: molecule represented by two or more structures (Kekulé), none of which is real; the actual hybrid is more stable. Resonance energy = difference between the calculated energy of the most stable contributing structure and the actual molecule ≈ 150 kJ/mol for benzene → extra stability.
• Reactivity: more reactive than alkanes but less than alkenes towards addition; prefers electrophilic substitution (retains aromaticity). Does not decolourise bromine water or KMnO₄.
• Addition reactions: H₂/Ni at 150–250 °C → cyclohexane; Cl₂ in sunlight → benzene hexachloride (C₆H₆Cl₆); ozonolysis → glyoxal. Methylbenzene (toluene) + H₂ → methylcyclohexane.

Electrophilic substitution — mechanism

(1) Generation of electrophile E⁺; (2) E⁺ attacks π cloud → arenium ion / σ-complex (carbocation, slow step); (3) loss of H⁺ restores aromaticity.

Methylbenzene is more reactive than benzene (CH₃ activates) and gives ortho + para products; side chain: Cl₂ in sunlight → benzyl chloride; KMnO₄ oxidation → benzoic acid.

Orientation (directing effects)

• Ortho/para directors (activating): –OH, –NH₂, –OCH₃, –CH₃/alkyl. Halogens are o/p-directing but deactivating.
• Meta directors (deactivating): –NO₂, –CHO, –COOH, –COR, –SO₃H, –CN, –NR₃⁺.

Alkynes (C_nH_2n−2)

• IUPAC suffix -yne (ethyne, propyne, but-1-yne, but-2-yne). sp hybridized, linear, C≡C 1.20 Å (1σ + 2π).
• Preparation by elimination: double dehydrohalogenation of vicinal or geminal dihalides with alcoholic KOH / NaNH₂ (CH₂Br–CH₂Br → CH≡CH); also CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂.
• Acidity: terminal alkyne H is weakly acidic because sp carbon has 50% s-character (more electronegative). Reacts with Na/NaNH₂ → sodium acetylide; ammoniacal AgNO₃ → white silver acetylide; ammoniacal Cu₂Cl₂ → red copper acetylide (distinguishes terminal alkynes from alkenes and internal alkynes). Acidity: ethyne > ethene > ethane.
• Hydrogenation: H₂/Ni or Pt → alkene → alkane; Lindlar's catalyst stops at cis-alkene.
• Hydrohalogenation: HX adds twice (Markovnikov) → geminal dihalide (CH≡CH + 2HBr → CH₃CHBr₂).
• Hydration: dil. H₂SO₄ + HgSO₄ at 80 °C → enol → ethanal (acetaldehyde) from ethyne; other alkynes give ketones (propyne → acetone).

Substitution vs addition

Substitution: one atom/group replaces another; saturated and aromatic compounds (alkanes — free radical; benzene — electrophilic). Addition: atoms add across a multiple bond with no loss; unsaturated compounds (alkenes, alkynes — electrophilic; carbonyls — nucleophilic).