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Hydrocarbons

Ch. 10Std 11

Easy Overview

Hydrocarbons are the simplest organic compounds — just carbon and hydrogen. Yet from methane (natural gas) to benzene (plastics precursor) to giant polycyclic rings, they form the backbone of the petrochemical industry and the starting point for all organic synthesis. Alkanes are saturated (only σ bonds), alkenes have one or more C=C (σ+π), alkynes have C≡C (σ+2π), and arenes contain aromatic rings. Each class has distinct reactions: alkanes undergo free radical substitution, alkenes and alkynes undergo electrophilic addition, and arenes undergo electrophilic aromatic substitution. The source of most hydrocarbons is petroleum — a complex mixture separated by fractional distillation. Cracking converts large alkanes into smaller, more valuable ones. The petrochemical industry transforms these into polymers, solvents, fuels, and countless everyday products.

Alkanes — The Saturated Hydrocarbons

General formula CₙH₂ₙ₊₂. Only σ bonds, sp³ hybridized carbon (109.5°). Tetrahedral around each carbon. Methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀). Physical state: C₁-C₄ (gases), C₅-C₁₇ (liquids), C₁₈⁺ (solids). Boiling points increase with chain length (more London dispersion) and branch decreases BP (reduced surface area). Alkanes are nonpolar, insoluble in water, soluble in nonpolar solvents. They are relatively inert — called paraffins (parum affinis = little affinity).

Preparation of Alkanes — From the Lab to Industry

Hydrogenation of alkenes/alkynes: RCH=CH₂ + H₂ → RCH₂CH₃ (Ni, Pt, or Pd catalyst, room temp). Wurtz reaction: 2RX + 2Na → R-R + 2NaX (dry ether). Only symmetrical alkanes — mixed Wurtz gives all three possible products. Decarboxylation of sodium carboxylate: RCOONa + NaOH (sodalime, CaO) → RH + Na₂CO₃ — produces alkane with one less carbon. Kolbe's electrolysis: 2RCOOK + 2H₂O → R-R + 2CO₂ + H₂ + 2KOH — at anode: carboxylate loses e⁻, forms radical, dimerizes. Grignard reagent: RMgX + H₂O → RH + Mg(OH)X — gives alkane from alkyl halide.

Chemical Reactions of Alkanes — Substitution and Combustion

Free radical halogenation: RH + X₂ → RX + HX (hν or heat). Selectivity: F₂ (too violent, uncontrolled), Cl₂ (3°H > 2°H > 1°H — radical stability), Br₂ (high selectivity — bromination favors most substituted H), I₂ (doesn't react — too slow). Combustion: complete (CO₂ + H₂O, large ΔH), incomplete (CO or C + H₂O). Alkanes are excellent fuels. Pyrolysis (cracking): heat in absence of air → smaller alkanes + alkenes — crucial in petroleum refining. Isomerization: n-alkanes to branched (improved octane rating). Aromatization: alkanes to aromatic compounds over Pt catalyst at high T.

Conformations of Alkanes — Rotational Isomerism

Free rotation around C-C single bond. Ethane: staggered (60° dihedral, more stable, 0 kJ/mol) and eclipsed (0°, less stable, +12 kJ/mol). Barrier to rotation = 12 kJ/mol — fast at room temp. Butane: anti (180°, CH₃ opposite, most stable), gauche (60°, CH₃ at 60°, +3.8 kJ/mol), eclipsed (+16 kJ/mol). Newman projections visualize conformations. Larger groups = higher rotational barriers. At room temp, molecules rapidly interconvert between conformations.

Alkenes — The Unsaturated Hydrocarbons with C=C

General formula CₙH₂ₙ. sp² hybridized carbons at C=C (120°). π bond is weaker (~265 kJ/mol) than σ bond (~348 kJ/mol). Dipole moment: cis-alkenes have net dipole, trans-alkenes generally do not (trans-2-butene μ=0). Physical properties similar to alkanes of same carbon count but slightly higher BP. More polarizable π electrons give slightly stronger London forces. Nomenclature: suffix -ene, lowest number to C=C. Dienes (two C=C): isolated (separated by ≥2 single bonds), conjugated (alternating single-double), cumulative (adjacent C=C).

Preparation of Alkenes

Dehydration of alcohols: RCH₂CH₂OH → RCH=CH₂ + H₂O (conc. H₂SO₄ or Al₂O₃, 170°C). Follows Zaitsev rule: more substituted alkene is major. Dehydrohalogenation of alkyl halides: RCH₂CH₂X + alc. KOH → RCH=CH₂ + KX + H₂O. Carbocation intermediate with rearrangement possible. Dehalogenation of vicinal dihalides: RCHBrCHBrR + Zn → RCH=CHR + ZnBr₂ (stereospecific — anti elimination). Cracking of alkanes: large alkane → smaller alkane + alkene. From alkynes: partial reduction with Lindlar's catalyst (Pd/BaSO₄ poisoned with quinoline, gives cis-alkene) or Na/NH₃ (gives trans-alkene).

Electrophilic Addition Reactions of Alkenes

Markovnikov's rule: H⁺ adds to the carbon with more H already (forms more stable carbocation). HX addition: CH₃CH=CH₂ + HBr → 2-bromopropane (major) — 2° carbocation intermediate. H₂SO₄ addition: forms alkyl hydrogen sulfate, then hydrolyzed to alcohol (industrial hydration). H₂O addition (acid-catalyzed): follows Markovnikov — gives alcohol. Halogen addition: Br₂/CCl₄ → vicinal dibromide (red-brown Br₂ decolorized — test for unsaturation). Anti addition: cyclic bromonium ion intermediate → trans product. Oxymercuration: Hg(OAc)₂/H₂O then NaBH₄ → Markovnikov alcohol, no rearrangement. Hydroboration: BH₃/THF then H₂O₂/OH⁻ → anti-Markovnikov alcohol, syn addition.

Oxidation of Alkenes

KMnO₄: cold, dilute, basic → vicinal diol (cis-1,2-diol, syn dihydroxylation — Baeyer's test: purple to brown MnO₂ indicates unsaturation). Hot, acidic KMnO₄ → cleavage at C=C: terminal C=C gives CO₂ + carboxylic acid; R₂C=CR₂ gives two ketones; RCH=CR₂ gives carboxylic acid + ketone. Ozonolysis: O₃ then Zn/H₂O → cleavage gives aldehydes/ketones — identifies position of C=C. Catalytic hydrogenation: H₂ + Pd/Pt/Ni → alkane (syn addition). Epoxidation: peroxyacid (m-CPBA) → epoxide (three-membered ring). Anti dihydroxylation: epoxide + H₃O⁺ → trans-diol.

Dienes and Conjugated Systems

1,3-butadiene (CH₂=CH-CH=CH₂) has conjugated double bonds — alternating single and double. Delocalization: π electrons spread over four carbons → 12 kJ/mol stabilization. 1,2-addition (kinetic, -80°C) gives 3-bromo-1-butene; 1,4-addition (thermodynamic, 40°C) gives 1-bromo-2-butene. Diels-Alder reaction: [4+2] cycloaddition between conjugated diene and dienophile (C=C with electron-withdrawing group) → six-membered ring. One of the most powerful reactions in organic synthesis. Stereospecific: cis-diene geometry preserved, suprafacial on both components.

Alkynes — The Triple Bond Hydrocarbons

General formula CₙH₂ₙ₋₂. sp hybridization (180° linear). C≡C bond length ~120 pm (shorter than C=C 134 pm, C-C 154 pm). Bond strength 839 kJ/mol — stronger than C=C (614) but π bonds are not twice as strong due to less effective sideways overlap of 2p orbitals. Terminal alkynes (RC≡CH) are weakly acidic (pKa ~25) — can be deprotonated by strong bases (NaNH₂, Grignard reagents) to form acetylide ions (RC≡C⁻). Acidity due to high s-character (50%) of C-H bond — more electronegative sp carbon stabilizes negative charge.

Preparation and Reactions of Alkynes

Preparation: dehydrohalogenation of vicinal dihalides (alc. KOH, then NaNH₂). From calcium carbide: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂ — industrial route to acetylene. Reactions: electrophilic addition (two steps — first to alkene, then to alkane). HX addition follows Markovnikov. H₂O addition (HgSO₄/H₂SO₄): terminal alkyne → methyl ketone (via enol tautomerization — vinyl alcohol rearranges to carbonyl). NaNH₂ deprotonation to form acetylide: RC≡CH + NaNH₂ → RC≡C⁻Na⁺ + NH₃. Alkylation: RC≡C⁻ + R'X → RC≡C-R' + X⁻ — extends carbon chain. Ozonolysis: C≡C → carboxylic acids. Partial hydrogenation: Lindlar's → cis-alkene; Na/NH₃ → trans-alkene.

Aromatic Hydrocarbons — Benzene and the Hückel Rule

Benzene (C₆H₆): planar, all C-C 139 pm (between single and double). Kekulé structure (1865) proposed alternating double bonds, but benzene doesn't undergo addition reactions like alkenes. Modern: delocalized π system, all six C-C equivalent. Hückel's rule: aromatic if planar, cyclic, fully conjugated, with (4n+2) π electrons. n=1 → 6π (benzene, pyridine). n=2 → 10π (naphthalene). Anti-aromatic: 4n π electrons (cyclobutadiene, extremely unstable). Non-aromatic: doesn't meet all criteria. Resonance energy of benzene = 150 kJ/mol — extra stability. Electrophilic aromatic substitution: retains aromaticity — characteristic reaction of arenes.

Electrophilic Aromatic Substitution — The Heart of Arene Chemistry

Mechanism: (1) electrophile generation (2) π-complex (weak interaction, reversible) (3) σ-complex (arenium ion, Wheland intermediate — carbocation with positive charge delocalized over ring, resonance stabilized, slower step) (4) deprotonation to restore aromaticity (fast). Nitration: HNO₃/H₂SO₄ → NO₂⁺ (nitronium ion). Sulfonation: H₂SO₄/SO₃ → benzene sulfonic acid (reversible — desulfonation with dilute H₂SO₄ at 100°C). Halogenation: Cl₂/AlCl₃ or Br₂/FeBr₃ → aryl halide. Friedel-Crafts alkylation: RCl/AlCl₃ → alkylbenzene. Rearrangement possible (1° carbocation → 2° then 3°). Friedel-Crafts acylation: RCOCl/AlCl₃ → acyl benzene — no rearrangement, gives ketone.

Directing Effects of Substituents on Benzene

Ortho-para directors (activate ring, except halogens): EDG groups (OH, NH₂, OCH₃, alkyl). Ortho-para directors have lone pairs or +R/+I effect, stabilize σ-complex intermediate (especially ortho and para positions). Activating groups donate electrons by resonance or induction, making ring more reactive than benzene. Ortho-para directing but deactivating: halogens (F, Cl, Br, I) — strong -I withdraws electrons (deactivates) but lone pairs donate by resonance (direct ortho/para). Meta directors (deactivate ring): EWG (NO₂, CN, C=O, COOH, SO₃H, CF₃). Meta directors have -R or strong -I effects, destabilize σ-complex at ortho/para positions more than meta. Strong deactivators: NO₂, CF₃. Moderate: CN, COR, COOH. Weak: halogens.

Polycyclic Aromatic Hydrocarbons (PAHs)

Naphthalene (C₁₀H₈): two fused benzene rings, 10π electrons (n=2). More reactive than benzene — electrophilic substitution occurs at α-position (C1, more stable σ-complex). Anthracene (C₁₄H₁₀): three linearly fused rings. Phenanthrene: three angularly fused rings. PAHs are formed during incomplete combustion of organic matter (cigarette smoke, grilled meat, vehicle exhaust). Some are carcinogenic (benzo[a]pyrene). Health concerns: air pollution from diesel exhaust, wood smoke. PAHs are persistent environmental pollutants, accumulate in soil and sediments, and undergo bioaccumulation in food chains.

Sources of Hydrocarbons — Petroleum and Natural Gas

Petroleum (crude oil): complex mixture of hydrocarbons (alkanes, cycloalkanes, aromatics) formed from ancient marine organisms over millions of years. Fractional distillation: separates by boiling point — refinery gas (<30°C, C₁-C₄), gasoline (30-200°C, C₅-C₁₂), kerosene (150-250°C, C₁₂-C₁₆), diesel (200-350°C, C₁₅-C₂₅), fuel oil (350-400°C), and residue (asphalt, >400°C). Cracking: thermal (high T, high P) and catalytic (zeolites, lower T) — converts large alkanes to smaller, more valuable ones. Reforming: converts straight-chain alkanes to branched and aromatic compounds (higher octane). Octane number: measure of anti-knock quality — isooctane = 100, n-heptane = 0. Leaded gasoline (tetraethyl lead, Pb(C₂H₅)₄) was phased out due to toxicity; modern gasoline uses oxygenates (ethanol, MTBE).

Environmental Aspects of Hydrocarbons

Complete combustion: CO₂ + H₂O — CO₂ is the major greenhouse gas from burning fossil fuels. Incomplete combustion: CO (toxic, binds hemoglobin 200× stronger than O₂), soot (particulate matter). Methane: potent greenhouse gas (25× CO₂ over 100 years) — released from natural gas leaks, agriculture (livestock), landfills. Volatile organic compounds (VOCs): unburned hydrocarbons from vehicles and industry → ground-level ozone formation (photochemical smog). CFCs: ozone depletion. Catalytic converters: Pt/Pd/Rh convert CO→CO₂, unburned HC→CO₂+H₂O, NOx→N₂. Greener alternatives: electric vehicles, hydrogen fuel, biofuels (ethanol, biodiesel).

Key Points

  • •Alkanes: CₙH₂ₙ₊₂, sp³, σ bonds only, free radical substitution
  • •Wurtz reaction: 2RX + 2Na → R-R; mixed Wurtz gives 3 products
  • •Free radical halogenation: 3° > 2° > 1° H reactivity for Cl₂ and Br₂
  • •Alkenes: CₙH₂ₙ, sp², π bond, electrophilic addition
  • •Markovnikov's rule: H⁺ adds to carbon with more H (stable carbocation)
  • •Anti-Markovnikov: via hydroboration (BH₃/H₂O₂, OH⁻) — syn addition, no rearrangement
  • •Baeyer's test: cold KMnO₄ purple→brown indicates unsaturation
  • •Ozonolysis: O₃ then Zn/H₂O cleaves C=C → identifies position
  • •Alkynes: CₙH₂ₙ₋₂, sp, terminal H acidic (pKa 25), deprotonated by NaNH₂
  • •Lindlar's catalyst: Pd/BaSO₄/quinoline → cis-alkene from alkyne
  • •Hückel: (4n+2)π electrons, planar, cyclic, conjugated = aromatic
  • •Electrophilic aromatic substitution: σ-complex intermediate (Wheland)
  • •Ortho-para directors: EDG (activators) and halogens (deactivators)
  • •Meta directors: EWG (NO₂, CN, C=O, COOH, CF₃)
  • •Petroleum: separated by fractional distillation; cracking increases value
  • •Catalytic converters reduce CO, HC, NOx emissions

Practice Questions

  • Write mechanism for CH₃CH=CH₂ + HBr. Show all steps. What is the major product?
  • Explain Baeyer's test and ozonolysis. How would you distinguish ethene and ethyne?
  • Describe Friedel-Crafts alkylation and acylation of benzene. Why does alkylation give rearranged products?
  • How does a benzene ring direct electrophilic substitution? Explain with NO₂ (meta) and OH (ortho-para).
  • Write reactions: (i) Wurtz reaction (ii) Kolbe's electrolysis (iii) hydration of ethyne.
  • An alkene C₅H₁₀ on ozonolysis gives CH₃CHO and (CH₃)₂CO. Identify the alkene and write the reaction.
  • Explain Hückel's rule. Check aromaticity: cyclopentadienyl anion, cycloheptatrienyl cation, pyridine.
  • Describe fractional distillation of petroleum. What is cracking and why is it important?