⛽ HYDROCARBONS · extended

📌 5.1 Types & extended classification

Hydrocarbons (C & H) from coal, petroleum, natural gas. Mother of organics — functional groups added to skeleton.

Aliphatic: open chain (straight/branched). Aromatic: benzene-like, Huckel (4n+2)π.

🔸 Saturated (alkanes, cycloalkanes)

🔹 Unsaturated (alkenes, alkynes)

General: alkene CₙH₂ₙ (one C=C), alkyne CₙH₂ₙ₋₂ (C≡C). Ex: 1-propene, 2-butyne, 1,3-butadiene.

🌸 Aromatic (benzene & derivatives)

Benzene C₆H₆, toluene (methylbenzene), phenol (OH), benzoic acid (COOH). Huckel’s rule: 4n+2 π e⁻ (n=1→6π).

🧪 5.2 Alkanes + Cycloalkanes in-depth

📈 Physical properties extended

– Nonpolar, insoluble H₂O, soluble organic solvents.
– BP increases with chain length (more surface area → London forces).
– Straight chain BP > branched (branched = compact, less surface).
– C₁–C₄ gases, C₅–C₁₇ liquids, ≥C₁₈ waxy solids.
– Cycloalkanes: C₃,C₄ gases; ≥C₅ liquids. BP lower than alkanes due to ring strain.

⚛️ Structure (ethane & cyclopropane)

⚡ Reactivity

Alkanes: paraffin (little affinity) – undergo free radical substitution, combustion, cracking. Cyclopropane much more reactive (ring strain).

🔥 5.3 Free radical substitution (mechanism extended)

Homolytic fission: each gets one electron → radicals. Heterolytic: both electrons to one atom.

Chlorination of methane (UV/heat):

Initiation: Cl₂ ☀️ → 2Cl•
Propagation: Cl• + CH₄ → HCl + •CH₃ ; •CH₃ + Cl₂ → CH₃Cl + Cl•
Termination: Cl•+Cl• → Cl₂ ; •CH₃+Cl• → CH₃Cl ; •CH₃+•CH₃ → CH₃–CH₃

Further chlorination yields CH₂Cl₂ (methylene dichloride), CHCl₃ (chloroform), CCl₄ (carbon tetrachloride).

🫧 5.4 Alkenes (olefins) – complete coverage

Structure of ethene

sp² hybridized, trigonal planar, 120°. C=C bond length 1.34Å. One σ (sp²–sp²) + one π (p–p lateral).

Preparation

• Dehydration of ethanol: conc. H₂SO₄, 170°C → ethene + H₂O.
• Dehydrohalogenation: ethyl bromide + alcoholic KOH, 60°C → ethene + KBr + H₂O.

Reactivity & addition reactions (full list)

π bond weaker, exposed to electrophiles.

1. Hydrogenation: H₂, Ni/Pt/Pd, 250–300°C → alkane.
2. Halogenation: Br₂ in CCl₄ → 1,2-dibromoethane (decolourises bromine).
3. Hydrohalogenation: HBr → ethyl bromide. For unsymmetrical: Markovnikov rule (H adds to C with more H). e.g. propene + HBr → 2-bromopropane major.
4. Hydration: H₂O, dil H₂SO₄, 100°C → alcohol.
5. Halohydrin formation: Br₂ + H₂O → bromohydrin (confirmatory test).
6. Epoxidation: peracetic acid → epoxide (three‑membered cyclic ether).
7. Ozonolysis: O₃ then Zn → aldehydes/ketones.
8. Polymerization: high pressure, 200°C → polyethene.

⚙️ 5.5 Alkynes – ethyne & acidity extended

Structure of ethyne

sp hybrid, linear 180°, C≡C 1.20Å. Two π bonds perpendicular.

Physical properties

Colourless, garlic odour (ethyne). Insoluble water, soluble organic solvents. first three members gases, up to C₁₂ liquids.

Preparation (elimination)

• Vicinal dihalide (1,2-dibromoethane) + alcoholic KOH → bromoethene, then + KOH → ethyne.
• Tetrahalide (1,1,2,2-tetrabromoethane) + Zn → ethyne + ZnBr₂.

Acidity of terminal alkynes

50% s-character (sp) makes C electronegative, H slightly δ⁺. pKa ~25. Reaction with ammonical AgNO₃ → white ppt (AgC≡CAg); with Cu₂Cl₂ → red ppt (Cu₂C₂). Distinguishes from alkenes.

Addition reactions of alkynes

Hydrogenation: H₂/Pt → alkene → alkane.
Hydrohalogenation: 2 moles HBr → geminal dihalide (Markovnikov).
Hydration: HgSO₄/H₂SO₄, H₂O → enol → tautomerize to carbonyl (acetaldehyde from ethyne, acetone from propyne).
Bromination: 2 mol Br₂ → tetrabromoalkane.
Ozonolysis: O₃ then H₂O₂ → glyoxal (from ethyne).

🌀 5.6 Isomerism – full spectrum

Structural isomerism (same formula, different connectivity)

Chain: C₄H₁₀ → n-butane, isobutane. C₅H₁₂ → n-pentane, isopentane, neopentane.
Position: C₃H₇Cl → 1-chloropropane, 2-chloropropane. C₄H₈ → 1-butene, 2-butene.
Functional group: C₂H₆O → ethanol, methoxymethane (ether). C₃H₆O → propanal, acetone.
Metamerism: C₄H₁₀O → methyl propyl ether, diethyl ether (different alkyl around O).

Stereoisomerism

Chirality / optical isomerism: carbon with four different groups (chiral centre). Enantiomers: non‑superimposable mirror images. Dextrorotatory (+) / levorotatory (–). Racemic mixture = 50:50, no rotation.

Geometrical (cis‑trans): restricted rotation about double bond. e.g. 1,2-dichloroethene: cis (same side), trans (opposite).

🧬 5.7 Benzene & derivatives (extended)

Kekule structure (1865): cyclic, alternate double bonds. Resonance hybrid: all C–C bonds equal (1.39Å). Delocalized π molecular orbitals (six p orbitals overlap above & below).

Physical properties

Colourless liquid, b.p. 80°C, m.p. 5.5°C, density 0.88 g/mL, carcinogenic, insoluble in water, soluble in organic solvents.

Addition vs substitution

Despite unsaturation, benzene resists addition (delocalization stability). Addition: H₂ + Ni (200°C, 30 atm) → cyclohexane; Cl₂ (UV light) → BHC (hexachlorocyclohexane).

Electrophilic substitution mechanism (general)

1. generation of electrophile E⁺ (with catalyst).
2. E⁺ attacks π system → arenium ion (σ‑complex) – nonaromatic.
3. loss of proton restores aromaticity.

Main electrophilic substitutions:

Nitration: conc. HNO₃ + conc. H₂SO₄ (50°C) → nitrobenzene. Electrophile: NO₂⁺.
Halogenation: Cl₂/FeCl₃ or Br₂/FeBr₃ → chlorobenzene.
Sulfonation: fuming H₂SO₄ (SO₃) → benzenesulfonic acid.
Friedel‑Crafts alkylation: R–Cl + AlCl₃ → alkylbenzene.
Friedel‑Crafts acylation: R–COCl + AlCl₃ → acylbenzene (e.g. acetophenone).

Directing effects (ortho/para vs meta)

Ortho/para directors: electron‑releasing (‑OH, ‑OR, ‑CH₃, ‑NH₂, even halogens (though deactivating)).
Meta directors: electron‑withdrawing (‑NO₂, ‑CN, ‑SO₃H, ‑COOH, ‑COR).
Example: nitration of benzoic acid → meta‑nitrobenzoic acid.
Preparation of TNT: toluene nitrated with hot conc. HNO₃/H₂SO₄ → 2,4,6‑trinitrotoluene (ortho/para directing by ‑CH₃).

📊 Comparison of reactivity (alkane, alkene, alkyne)

📝 20 MCQ quiz (full syllabus)

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