Hydrocarbons & Aromatic Chemistry

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Hydrocarbons & Aromatic Chemistry — Key Facts for Makerere University (Uganda) Core concept: This topic covers saturated (alkanes) and unsaturated (alkenes, alkynes) hydrocarbons, plus benzene and aromatic chemistry — including properties, reactions, and benzene’s unique stability High-yield points: Alkane reactions (halogenation, combustion), alkene addition reactions, alkynes reactions, benzene structure and aromaticity, electrophilic aromatic substitution reactions ⚡ Exam tip: Make sure you can draw benzene’s Kekulé structure and resonance forms; understand why benzene undergoes substitution rather than addition reactions

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Hydrocarbons & Aromatic Chemistry — Makerere University (Uganda) Study Guide

1. Alkanes (Saturated Hydrocarbons)

General Properties

• Formula: CₙH₂ₙ₊₂ (only single bonds)
• Bonding: All carbon atoms are sp³ hybridized, tetrahedral geometry
• Bond angles: ~109.5°
• Physical state: C₁–C₄ are gases; C₅–C₁₇ are liquids; C₁₈+ are solids
• Solubility: Non-polar → insoluble in water; soluble in non-polar solvents
• Density: Less than water (all alkanes float on water)

Nomenclature

• Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀), Pentane (C₅H₁₂)
• Halogen-substituted alkanes: chloromethane (CH₃Cl), dichloroethane (C₂H₄Cl₂)

Reactions of Alkanes

1. Combustion (Oxidation): Complete combustion (excess O₂): CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O + heat

Example — Methane: CH₄ + 2O₂ → CO₂ + 2H₂O (ΔH = −890 kJ/mol)

Incomplete combustion (limited O₂): 2CH₄ + 3O₂ → 2CO + 4H₂O (partial oxidation) CH₄ + O₂ → C + 2H₂O (soot/carbon produced)

⚠️ Safety note: Methane is explosive when mixed with air (5–15% CH₄ in air).

2. Halogenation (Free Radical Substitution): Alkanes react with halogens (Cl₂, Br₂) in the presence of UV light via a free radical mechanism.

Mechanism — Chlorination of methane: Initiation: Cl₂ → 2Cl• (free radicals generated by UV) Propagation:

• Cl• + CH₄ → HCl + CH₃• (hydrogen abstraction)
• CH₃• + Cl₂ → CH₃Cl + Cl• (chlorine abstraction) Termination:
• Cl• + Cl• → Cl₂
• CH₃• + CH₃• → C₂H₆
• CH₃• + Cl• → CH₃Cl

Products: Mixture of mono-, di-, tri-, and tetra-chlorinated products. Selectivity: 3° > 2° > 1° hydrogen replacement (due to stability of resulting radicals).

3. Isomerization: Straight-chain alkanes can be converted to branched isomers in the presence of AlCl₃/HCl (Friedel-Crafts type).

4. Cracking (Thermal Decomposition): Breaking larger alkanes into smaller, more useful fragments. C₁₆H₃₄ → C₈H₁₈ + C₈H₁₆ (octane + octene) Catalytic cracking uses zeolite catalysts at lower temperatures.

5. Reforming: Converting n-alkanes to branched-chain alkanes or aromatic compounds (aromatic reforming for high-octane fuel).

2. Alkenes (Unsaturated Hydrocarbons — Double Bond)

General Properties

• Formula: CₙH₂ₙ (at least one C=C double bond)
• Bonding: Each double-bonded carbon is sp² hybridized
• Bond angle: ~120°
• C=C bond length: ~134 pm (shorter than C–C single bond, 154 pm)
• Bond energy: ~611 kJ/mol (weaker than C–C single bond, 347 kJ/mol)
• Physical state: C₂–C₄ gases; C₅–C₁₈ liquids; C₁₉+ solids

Nomenclature

• Ethene (C₂H₄), Propene (C₃H₆), But-1-ene, But-2-ene
• Prefixes: eth-, prop-, but-, pent-, hex-, etc.

Types of Alkenes

• Terminal alkene: Double bond at end of chain (CH₂=CH–R)
• Internal alkene: Double bond within the chain
• Cyclic alkene: Double bond in a ring (cyclopentene, cyclohexene)

Reactions of Alkenes

1. Addition Reactions (Electrophilic Addition):

Hydrogenation (with H₂/Ni or Pt or Pd): C=C + H₂ → C–C (alkane) Heat of hydrogenation: ethene (−136 kJ/mol) vs ethane (−156 kJ/mol) — benzene (−208 kJ/mol) is especially stable.

Halogenation (with Br₂ or Cl₂): C=C + X₂ → C–X (vicinal dihalide)

• Br₂ in CCl₄ (red-brown) decolorizes → test for unsaturation
• Cl₂ adds across double bond to give dichloro product

Hydrohalogenation (with HX where X = Cl, Br, I):

• Markovnikov addition: H adds to carbon with MORE hydrogens; X adds to carbon with FEWER hydrogens
• Mechanism: H⁺ attacks C=C forming carbocation; X⁻ attacks carbocation

Example: Propene + HBr → 2-bromopropane (major) + 1-bromopropane (minor)

Hydration (with H₂O/H⁺): C=C + H₂O → alcohol

• Acid-catalyzed addition of water follows Markovnikov rule
• Reverse reaction: dehydration of alcohols (E1 mechanism)

Oxyhalogenation (with X₂ + H₂O): C=C + X₂ + H₂O → halohydrin (X and OH add to adjacent carbons)

• OH goes to the carbon with MORE hydrogens (Markovnikov orientation)
• Halogen goes to the carbon with FEWER hydrogens

2. Polymerization: n CH₂=CH₂ → (−CH₂–CH₂−)ₙ (polyethylene) n CH₃–CH=CH₂ → (−CH(CH₃)–CH₂−)ₙ (polypropylene) Reaction requires a catalyst (Ziegler-Natta catalyst, or free radical initiators).

3. Combustion: CₙH₂ₙ + (3n/2)O₂ → nCO₂ + nH₂O + heat

3. Alkynes (Unsaturated Hydrocarbons — Triple Bond)

General Properties

• Formula: CₙH₂ₙ₋₂ (at least one C≡C triple bond)
• Bonding: Each triple-bonded carbon is sp hybridized
• Bond angle: 180° (linear geometry)
• C≡C bond length: ~120 pm
• Bond energy: ~839 kJ/mol (strongest carbon-carbon bond)
• Physical state: First three (ethyne, propyne, butyne) are gases

Nomenclature

• Ethyne (acetylene, C₂H₂), Propyne (C₃H₄), But-1-yne, But-2-yne
• Terminal alkyne: C≡C at end (H–C≡C–R)
• Internal alkyne: C≡C within chain (R–C≡C–R’)

Reactions of Alkynes

1. Acidic Character of Terminal Alkynes: Terminal alkynes (R–C≡C–H) are weakly acidic (pKa ≈ 25).

• React with Na, NaNH₂ to form acetylide salts: R–C≡CNa + ½H₂
• NaNH₂ is a strong base that deprotonates terminal alkynes
• This reaction proves terminal alkynes are more acidic than regular hydrocarbons

2. Addition Reactions:

Hydrogenation (Reduction to Alkane): R–C≡C–R’ + 2H₂ → R–CH₂–CH₂–R’ (alkane) Using H₂/Pt gives full reduction.

Partial Reduction (to Alkenes): R–C≡C–R’ + H₂ → R–CH=CH–R’ (cis-alkene with Lindlar’s catalyst: Pd/CaCO₃ poisoned with quinoline) R–C≡C–R’ + Na/NH₃ → R–CH=CH–R’ (trans-alkene via sodium in liquid ammonia)

Halogenation: R–C≡C–R’ + X₂ → R–CX=CH–R’ (dihaloalkene) → R–CX₂–CX₂–R’ (tetrahaloalkane)

3. Hydration (Kucherov Rule): R–C≡C–R’ + H₂O → (Hg²⁺/H₂SO₄ catalyst) → R–CO–CH₂–R’ (methyl ketone) ⚠️ Note: The OH adds to the carbon with more hydrogens (Markovnikov), then tautomerizes to ketone.

4. Formation of Acetylides: R–C≡C–H + Cu₂Cl₂ → R–C≡C–Cu (copper acetylide, red precipitate) — used to identify terminal alkynes. R–C≡C–H + AgNO₃ → R–C≡C–Ag (silver acetylide, white precipitate)

4. Benzene and Aromatic Chemistry

Structure of Benzene

• Molecular formula: C₆H₆
• Historical: Kekulé proposed alternating single and double bonds (cyclohexa-1,3-diene)
• Modern understanding: Resonance hybrid — all C–C bonds are equivalent (intermediate between single and double, bond length = 140 pm)

Resonance structures of benzene:

      H                   H
      |                   |
   H–C             C–H
      \\           //
       C=========C
      //           \\
   H–C             C–H
      |                   |
      H                   H

Kekulé Structure A    Kekulé Structure B

The REAL structure is a hybrid where π electrons are delocalized over the ring.

Evidence for Aromaticity (Delocalization):

1. Heat of hydrogenation: Cyclohexene (−120 kJ/mol), but benzene (−208 kJ/mol) — benzene is 150 kJ/mol more stable than expected
2. Bond lengths: All C–C bonds in benzene are equal (140 pm) — intermediate between single (154 pm) and double (134 pm)
3. X-ray crystallography: Confirms hexagonal planar structure with equal bond lengths
4. NMR: Aromatic hydrogens show characteristic chemical shift (δ 7.2 ppm)

Criteria for Aromaticity (Hückel’s Rule):

A compound is aromatic if it:

1. Is cyclic
2. Is planar (or nearly planar)
3. Has (4n + 2) π electrons (where n = 0, 1, 2, 3…)

Benzene: 6 π electrons (n=1) → 4(1)+2 = 6 → aromatic ✓ Cyclobutadiene: 4 π electrons (n=0) → antiaromatic (4n rule, n=1 gives 4n=4) → unstable Naphthalene: 10 π electrons (n=2) → 4(2)+2 = 10 → aromatic ✓ Pyridine: 6 π electrons → aromatic ✓ Furan: 6 π electrons → aromatic ✓

Benzene’s Unreactivity Toward Addition

Benzene does NOT undergo typical alkene addition reactions because:

• Addition would destroy the stable aromatic system (6 π electrons)
• Substitution preserves aromaticity → more energetically favorable
• Substitution reactions are kinetically controlled

Benzene DOES undergo electrophilic aromatic substitution (EAS) — the electrophile replaces H, preserving the aromatic ring.

5. Electrophilic Aromatic Substitution (EAS)

General mechanism:

1. Generation of electrophile: E⁺ is generated from the reagent
2. Attack on aromatic ring: π electrons attack E⁺ → forms sigma complex (arenium ion) — rate-determining step
3. Deprotonation: Loss of H⁺ restores aromaticity

Step 2 — Sigma Complex (key intermediate):

        E
        |
     [cyclohexadienyl cation]
     delocalized positive charge
     over 5 carbons

The positive charge is delocalized (stabilized by resonance).

Important EAS Reactions

1. Nitration (–NO₂): C₆H₆ + conc. HNO₃ + conc. H₂SO₄ → C₆H₅NO₂ + H₂O Reagents: nitrating mixture (HNO₃/H₂SO₄) Electrophile: NO₂⁺ (nitronium ion)

2. Halogenation (–Cl, –Br): C₆H₆ + Cl₂/FeCl₃ → C₆H₅Cl + HCl (chlorobenzene) C₆H₆ + Br₂/FeBr₃ → C₆H₅Br + HBr (bromobenzene) Electrophile: Cl⁺ or Br⁺

3. Friedel-Crafts Alkylation (–R): C₆H₆ + R–Cl/AlCl₃ → C₆H₅R + HCl Reagents: alkyl halide + anhydrous AlCl₃ Electrophile: R⁺ (carbocation) ⚠️ Limitation: Carbocation rearrangements possible; polyalkylation occurs; doesn’t work with strong electron-withdrawing groups.

4. Friedel-Crafts Acylation (–COR): C₆H₆ + R–COCl/AlCl₃ → C₆H₅–COR + HCl Reagent: acyl chloride + anhydrous AlCl₃ Electrophile: RCO⁺ (acylium ion) Advantage: No rearrangement possible; only mono-acylation (deactivated product stops further reaction).

5. Sulfonation (–SO₃H): C₆H₆ + fuming H₂SO₄ → C₆H₅SO₃H + H₂O Electrophile: SO₃H⁺ or SO₃ Reversible reaction — can be removed by heating with steam.

6. Halogenation with F₂/I₂: Direct fluorination and iodination of benzene are difficult; require special conditions.

Directing Effects of Substituents

Substituents already on the ring direct where the incoming electrophile will go:

Ortho/Para Directors (Activating):

• –OH, –NH₂ (strongly activating)
• –OCH₃, –NR₂ (activating)
• –CH₃, –C₂H₅ (alkyl groups, weakly activating)
• –Cl, –Br, –I (weakly deactivating but ortho/para directing)

Meta Directors (Deactivating):

• –NO₂ (strongly deactivating)
• –CN, –COOH, –COOR, –SO₃H (deactivating)
• –CCl₃, –CF₃

Ortho/Para vs Meta:

• Ortho/para directors: New substituent goes ortho or para to the existing group
• Meta directors: New substituent goes meta to the existing group

⚠️ Halogen exception: Cl, Br, I are ortho/para directors but deactivating. This is because their lone pairs stabilize the sigma complex at ortho/para positions.

Example — Nitration of toluene: Toluene (methylbenzene) + HNO₃/H₂SO₄ → mostly ortho-nitrotoluene and para-nitrotoluene (methyl is ortho/para director).

Example — Nitration of nitrobenzene: Nitrobenzene + HNO₃/H₂SO₄ → meta-nitrobenzene only (NO₂ is meta director, strongly deactivating — reaction is slower).

6. Phenols (Ar–OH)

Properties of Phenol

• Formula: C₆H₅OH (hydroxy attached directly to benzene ring)
• Physical: Colorless liquid or crystals with characteristic odor; slightly soluble in water
• Acidic: Phenol is a weak acid (pKa ≈ 10) — more acidic than alcohols (pKa ≈ 16) because the phenoxide ion is resonance-stabilized

Reactions of Phenol

1. Acidity: C₆H₅OH + NaOH → C₆H₅ONa + H₂O (forms sodium phenoxide)
   • Does NOT react with NaHCO₃ (distinguishes from carboxylic acids)
2. Bromination: C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr (2,4,6-tribromophenol, white precipitate) — more reactive than benzene
3. Nitration: C₆H₅OH + HNO₃ → 2,4-dinitrophenol + 4,6-dinitrophenol
4. Esterification: C₆H₅OH + (CH₃CO)₂O → C₆H₅OCOCH₃ + CH₃COOH (phenyl acetate)

7. Exam-Style Questions & Tips

Common exam question patterns at Makerere:

1. “Explain why benzene undergoes electrophilic substitution rather than electrophilic addition”
2. “Write the mechanism for the nitration of benzene”
3. “Predict the major product(s) when [substituted benzene] undergoes [EAS reaction]”
4. “State and explain the directing effect of [substituent] in electrophilic aromatic substitution”
5. “Draw the resonance structures of the sigma complex for ortho, meta, and para attack on toluene”
6. “Compare the reactivity of benzene, phenol, and nitrobenzene toward electrophilic substitution”

⚡ Exam tips:

• For EAS products with mixed ortho/para: the para isomer usually predominates (less steric hindrance)
• For directing effects: draw out resonance structures to see which positions are stabilized
• Benzene’s resistance to addition is due to thermodynamic stability from aromaticity — not steric reasons
• Remember: ALL EAS reactions go through the same sigma complex intermediate

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8. Polycyclic Aromatic Hydrocarbons (PAHs)

Naphthalene (C₁₀H₈)

Two benzene rings fused together. 10 π electrons → aromatic (Hückel: 4(2)+2 = 10).

• Positions: 1,2,3,4,5,6,7,8 (alpha positions: 1,4,5,8; beta positions: 2,3,6,7)
• Substituents at alpha positions are more reactive in electrophilic substitution

Anthracene and Phenanthrene (C₁₄H₁₀)

Three fused benzene rings. Linear (anthracene) vs angular (phenanthrene).

Carcinogenic PAHs

Some PAHs (e.g., benzo[a]pyrene) are carcinogenic. They are produced during incomplete combustion of organic matter (cigarette smoke, car exhaust, grilled meat).

9. Heterocyclic Aromatic Compounds

Five-membered Heterocycles

Pyrrole (C₄H₅N): 5-membered ring with N. 6 π electrons (4 from C, 2 from N lone pair) → aromatic. N’s lone pair is part of the aromatic sextet → not basic. Furan (C₄H₄O): 6 π electrons (4 from C, 2 from O lone pair) → aromatic. Thiophene (C₄H₄S): 6 π electrons (4 from C, 2 from S lone pair) → aromatic.

Six-membered Heterocycles

Pyridine (C₅H₅N): 6 π electrons from ring only. N’s lone pair is perpendicular to the π system → not part of aromatic sextet → N is basic. Aromatic but acts like a base. Pyridine is a weaker base than ammonia because the N’s lone pair is sp² hybridized with more s-character (holds electrons closer, less available for bonding).

Fused Heterocycles

Quinoline: Benzene fused to pyridine Indole: Benzene fused to pyrrole Purine/Pyrimidine: Found in DNA and RNA bases

10. Mechanism of EAS — Detailed Sigma Complex Analysis

For nitration of benzene, the sigma complex intermediate:

Ortho attack on nitrobenzene:

        NO₂
        |
     [H shifts to C bearing NO₂]
     resonance structures show positive charge on positions 2, 4, 6 relative to NO₂

The positive charge CANNOT be delocalized to the position bearing the electron-withdrawing NO₂ group → meta product is favored.

Ortho attack on toluene:

        CH₃
        |
     [H shifts to C bearing CH₃]
     resonance structures show positive charge on positions 2, 4, 6 relative to CH₃

The positive charge CAN be delocalized to the position bearing the electron-donating CH₃ group (which stabilizes it by hyperconjugation/inductive effect) → ortho/para products are favored.

11. Arenes — Alkylbenzenes

Cumene (Isopropylbenzene)

Important industrial chemical. Used in production of acetone and phenol via cumene hydroperoxide rearrangement.

Cumene process: (CH₃)₂CH–C₆H₅ + O₂ → (CH₃)₂CO–C₆H₅ (cumene hydroperoxide) → PhOH + (CH₃)₂CO

Styrene (Phenylethene)

C₆H₅–CH=CH₂ — important monomer for polystyrene.

Toluene (Methylbenzene)

C₆H₅–CH₃ — used as solvent, in TNT production, and as a precursor to benzoic acid and benzaldehyde.

Practice Problems

Q1: Write equations for: (a) Complete combustion of propane (b) Chlorination of methane (initiation, propagation, termination steps) (c) Hydration of propene (d) Addition of HBr to 2-methylpropene

Q2: Predict the major product(s): (a) Nitration of phenol (show all three possible products and name them) (b) Bromination of nitrobenzene (c) Friedel-Crafts alkylation of toluene with 2-chloropropane

Q3: Draw the resonance structures showing how the –OH group directs to ortho/para positions in phenol. Explain why the ortho/para intermediates are more stable than the meta intermediate.

Q4: Arrange in order of increasing reactivity toward electrophilic aromatic substitution: (a) Benzene, phenol, nitrobenzene, toluene, chlorobenzene (b) Benzoic acid, benzene, anisole (methoxybenzene)

Q5: Explain why cyclobutadiene is antiaromatic and therefore very unstable.

Common Mistakes to Avoid

1. Confusing benzene’s stability with resistance to reaction: Benzene is thermodynamically stable (aromatic stabilization ~150 kJ/mol), which is why addition would destroy it — but it still undergoes substitution readily.
2. Forgetting that halogen substituents are ortho/para directors but deactivating: The lone pairs on Cl can donate into the ring, stabilizing ortho/para sigma complexes, but the inductive withdrawal makes the overall reaction slower.
3. Thinking all 6 carbons in benzene are equivalent: In substituted benzenes, ortho, meta, and para positions are distinct and have different reactivity.
4. Forgetting that Friedel-Crafts alkylation can have carbocation rearrangements: 1° alkyl halides with AlCl₃ give 2° or 3° alkylbenzene products due to rearrangement.
5. Mixing up addition and substitution for alkenes vs benzene: Alkenes undergo ADDITION; benzene undergoes SUBSTITUTION (to preserve aromaticity).

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