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Hydrocarbons

Alkanes, alkenes, alkynes, aromatic hydrocarbons.

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Alkanes

IUPAC, preparation, halogenation.

Hydrocarbons — alkanes, alkenes, alkynes, aromatic
Notes

HYDROCARBONS = compounds of C and H only.

Four families:

  • Alkanes (CₙH₂ₙ₊₂) — saturated, single bonds.
  • Alkenes (CₙH₂ₙ) — at least one C=C.
  • Alkynes (CₙH₂ₙ₋₂) — at least one C≡C.
  • Arenes (aromatic) — benzene-ring-based.

ALKANES (paraffins — Latin: "low affinity")

  • General formula CₙH₂ₙ₊₂. sp³, tetrahedral, 109.5°.
  • C-H bond energy ~414 kJ/mol; C-C ~347 kJ/mol.
  • Mostly unreactive (no polar groups, strong bonds).

Preparation:

  1. Wurtz reaction: 2 R-X + 2 Na (in dry ether) → R-R + 2 NaX. (Gives symmetrical alkanes.)
  2. Kolbe electrolysis of carboxylate.
  3. Hydrogenation of alkenes (H₂ + Ni/Pt/Pd).
  4. Reduction of alkyl halides (Zn/HCl, LiAlH₄).

Reactions:

  1. Combustion: complete → CO₂ + H₂O; incomplete → CO/C.
  2. Halogenation (substitution): R-H + Cl₂ → R-Cl + HCl (UV light, free radical mechanism).
  3. Pyrolysis (cracking) at high T → smaller alkanes + alkenes.
  4. Aromatization (n-hexane → benzene with Cr₂O₃/Al₂O₃ at 500°C).

ALKENES (olefins)

  • C=C: 1 σ + 1 π bond. sp², 120°, planar.
  • Geometrical (cis-trans) isomerism around C=C.

Preparation:

  1. Dehydration of alcohols (R-OH → R=R + H₂O, conc. H₂SO₄, 170°C).
  2. Dehydrohalogenation (R-X + alc. KOH).
  3. Dehalogenation of vicinal dihalides (Zn/methanol).
  4. Wittig reaction.

Reactions:

  1. Addition of H₂ (Ni catalyst).
  2. Addition of halogens (Br₂/CCl₄ — decolorization test).
  3. Addition of HX (Markovnikov: H adds to C with more H — gives more stable carbocation).
  4. Anti-Markovnikov with peroxide (Kharasch effect, only HBr).
  5. Acid-catalyzed hydration (H₂SO₄/H₂O → alcohol).
  6. Hydroboration-oxidation (BH₃ then H₂O₂/OH⁻ — anti-Markovnikov, no rearrangement).
  7. Ozonolysis (O₃ then Zn/H₂O → 2 carbonyls; useful to locate position of C=C).
  8. Polymerization (ethene → polyethene).
  9. Oxidation: dilute KMnO₄ (Baeyer) → diol; hot conc. KMnO₄ → cleave to carboxylic acids/ketones.

Markovnikov rule (1869): in addition of HX to unsymmetrical alkene, H goes to C with more H. (Mechanism: more stable carbocation.)

ALKYNES

  • C≡C: 1 σ + 2 π. sp, linear, 180°.

Preparation:

  1. Dehydrohalogenation of vicinal/geminal dihalides (alc. KOH then NaNH₂).
  2. From calcium carbide: CaC₂ + H₂O → HC≡CH + Ca(OH)₂ (industrial acetylene).

Reactions:

  1. Addition of H₂: Lindlar's catalyst (Pd/CaCO₃, poisoned) → cis-alkene; Na/NH₃ (liq) → trans-alkene.
  2. Addition of HX, X₂ (similar to alkenes; 2 equivalents possible).
  3. Acid hydration: H₂SO₄/HgSO₄ → ketone (via enol; Markovnikov). Acetylene gives acetaldehyde (only exception).
  4. Acidic H in terminal alkynes (pKa ~25): NaNH₂ gives sodium acetylide → alkylation.
  5. Polymerization: 3 HC≡CH → benzene (red-hot Cu, 873 K).

Acidity: terminal alkyne > alkene > alkane (sp C is more electronegative).

AROMATIC HYDROCARBONS — BENZENE

  • Molecular formula C₆H₆. Discovered by Faraday (1825).
  • Kekulé proposed alternating double bonds; but actually all C-C bonds are equal (139 pm — between single 154 and double 134).
  • Six delocalized π electrons → resonance, aromaticity.

Hückel's rule: planar cyclic ring with (4n+2) π electrons is aromatic (n = 0, 1, 2, ...).

Properties:

  • Planar, hexagonal (120°).
  • Unusually stable (resonance energy ~150 kJ/mol).

Electrophilic aromatic substitution (EAS):

  1. Halogenation (Br₂ + FeBr₃).
  2. Nitration (HNO₃ + H₂SO₄).
  3. Sulphonation (conc. H₂SO₄).
  4. Friedel-Crafts alkylation (R-Cl + AlCl₃).
  5. Friedel-Crafts acylation (RCOCl + AlCl₃).

Activating (o-/p- directing): -OH, -NH₂, -OR, -R (alkyl).
Deactivating (m- directing): -NO₂, -CN, -COOH, -SO₃H.
Deactivating but o-/p- directing: halogens.

EXAM HOOKS:

  • Markovnikov vs anti-Markovnikov (peroxide effect — only for HBr).
  • Ozonolysis identifies position of double bond.
  • Hydroboration: anti-Markovnikov, syn addition.
  • Lindlar = cis-alkene; Na/NH₃ = trans-alkene.
  • Acetylene + H₂O/HgSO₄/H₂SO₄ = acetaldehyde (only).
  • Benzene + Cl₂ in sunlight → BHC (gammexane). In presence of FeCl₃ (dark) → chlorobenzene.

Alkenes — Markovnikov

Addition, anti-Markovnikov with peroxide.

Hydrocarbons — alkanes, alkenes, alkynes, aromatic
Notes

HYDROCARBONS = compounds of C and H only.

Four families:

  • Alkanes (CₙH₂ₙ₊₂) — saturated, single bonds.
  • Alkenes (CₙH₂ₙ) — at least one C=C.
  • Alkynes (CₙH₂ₙ₋₂) — at least one C≡C.
  • Arenes (aromatic) — benzene-ring-based.

ALKANES (paraffins — Latin: "low affinity")

  • General formula CₙH₂ₙ₊₂. sp³, tetrahedral, 109.5°.
  • C-H bond energy ~414 kJ/mol; C-C ~347 kJ/mol.
  • Mostly unreactive (no polar groups, strong bonds).

Preparation:

  1. Wurtz reaction: 2 R-X + 2 Na (in dry ether) → R-R + 2 NaX. (Gives symmetrical alkanes.)
  2. Kolbe electrolysis of carboxylate.
  3. Hydrogenation of alkenes (H₂ + Ni/Pt/Pd).
  4. Reduction of alkyl halides (Zn/HCl, LiAlH₄).

Reactions:

  1. Combustion: complete → CO₂ + H₂O; incomplete → CO/C.
  2. Halogenation (substitution): R-H + Cl₂ → R-Cl + HCl (UV light, free radical mechanism).
  3. Pyrolysis (cracking) at high T → smaller alkanes + alkenes.
  4. Aromatization (n-hexane → benzene with Cr₂O₃/Al₂O₃ at 500°C).

ALKENES (olefins)

  • C=C: 1 σ + 1 π bond. sp², 120°, planar.
  • Geometrical (cis-trans) isomerism around C=C.

Preparation:

  1. Dehydration of alcohols (R-OH → R=R + H₂O, conc. H₂SO₄, 170°C).
  2. Dehydrohalogenation (R-X + alc. KOH).
  3. Dehalogenation of vicinal dihalides (Zn/methanol).
  4. Wittig reaction.

Reactions:

  1. Addition of H₂ (Ni catalyst).
  2. Addition of halogens (Br₂/CCl₄ — decolorization test).
  3. Addition of HX (Markovnikov: H adds to C with more H — gives more stable carbocation).
  4. Anti-Markovnikov with peroxide (Kharasch effect, only HBr).
  5. Acid-catalyzed hydration (H₂SO₄/H₂O → alcohol).
  6. Hydroboration-oxidation (BH₃ then H₂O₂/OH⁻ — anti-Markovnikov, no rearrangement).
  7. Ozonolysis (O₃ then Zn/H₂O → 2 carbonyls; useful to locate position of C=C).
  8. Polymerization (ethene → polyethene).
  9. Oxidation: dilute KMnO₄ (Baeyer) → diol; hot conc. KMnO₄ → cleave to carboxylic acids/ketones.

Markovnikov rule (1869): in addition of HX to unsymmetrical alkene, H goes to C with more H. (Mechanism: more stable carbocation.)

ALKYNES

  • C≡C: 1 σ + 2 π. sp, linear, 180°.

Preparation:

  1. Dehydrohalogenation of vicinal/geminal dihalides (alc. KOH then NaNH₂).
  2. From calcium carbide: CaC₂ + H₂O → HC≡CH + Ca(OH)₂ (industrial acetylene).

Reactions:

  1. Addition of H₂: Lindlar's catalyst (Pd/CaCO₃, poisoned) → cis-alkene; Na/NH₃ (liq) → trans-alkene.
  2. Addition of HX, X₂ (similar to alkenes; 2 equivalents possible).
  3. Acid hydration: H₂SO₄/HgSO₄ → ketone (via enol; Markovnikov). Acetylene gives acetaldehyde (only exception).
  4. Acidic H in terminal alkynes (pKa ~25): NaNH₂ gives sodium acetylide → alkylation.
  5. Polymerization: 3 HC≡CH → benzene (red-hot Cu, 873 K).

Acidity: terminal alkyne > alkene > alkane (sp C is more electronegative).

AROMATIC HYDROCARBONS — BENZENE

  • Molecular formula C₆H₆. Discovered by Faraday (1825).
  • Kekulé proposed alternating double bonds; but actually all C-C bonds are equal (139 pm — between single 154 and double 134).
  • Six delocalized π electrons → resonance, aromaticity.

Hückel's rule: planar cyclic ring with (4n+2) π electrons is aromatic (n = 0, 1, 2, ...).

Properties:

  • Planar, hexagonal (120°).
  • Unusually stable (resonance energy ~150 kJ/mol).

Electrophilic aromatic substitution (EAS):

  1. Halogenation (Br₂ + FeBr₃).
  2. Nitration (HNO₃ + H₂SO₄).
  3. Sulphonation (conc. H₂SO₄).
  4. Friedel-Crafts alkylation (R-Cl + AlCl₃).
  5. Friedel-Crafts acylation (RCOCl + AlCl₃).

Activating (o-/p- directing): -OH, -NH₂, -OR, -R (alkyl).
Deactivating (m- directing): -NO₂, -CN, -COOH, -SO₃H.
Deactivating but o-/p- directing: halogens.

EXAM HOOKS:

  • Markovnikov vs anti-Markovnikov (peroxide effect — only for HBr).
  • Ozonolysis identifies position of double bond.
  • Hydroboration: anti-Markovnikov, syn addition.
  • Lindlar = cis-alkene; Na/NH₃ = trans-alkene.
  • Acetylene + H₂O/HgSO₄/H₂SO₄ = acetaldehyde (only).
  • Benzene + Cl₂ in sunlight → BHC (gammexane). In presence of FeCl₃ (dark) → chlorobenzene.