Hydrocarbons: Alkanes, Alkenes, Alkynes & Benzene

1. Alkanes — Saturated Hydrocarbons (CnH₂n+2)

Alkanes undergo substitution reactions via free radical mechanism. The halogenation proceeds through three stages:

Initiation: X₂ → 2X- (homolytic cleavage by UV/heat) Propagation: R-H + X- → R- + HX; R- + X₂ → R-X + X- Termination: R- + R- → R-R; R- + X- → R-X; X- + X- → X₂

Reactivity of halogens: F₂ > Cl₂ > Br₂ > I₂ (I₂ is thermodynamically unfavorable — reversed by using oxidizing agent) Selectivity (opposite to reactivity): I₂ > Br₂ > Cl₂ > F₂. Bromine is highly selective — preferentially abstracts 3-degree H.

Relative reactivity of H-abstraction: 3-degree H : 2-degree H : 1-degree H = 5.0 : 3.8 : 1.0 (for Cl₂ at 25 degrees C) and 1600 : 82 : 1 (for Br₂ — extremely selective).

Conformational Analysis: Ethane has staggered (most stable, 0 kJ/mol) and eclipsed (least stable, 12.5 kJ/mol torsional strain) conformations. Butane has anti (most stable), gauche (+3.8 kJ/mol), eclipsed (+16 kJ/mol), and fully eclipsed (+19 kJ/mol) conformations.

Key hydrocarbon structures:

Cyclohexane (sp3, all single bonds)

2. Alkenes — Unsaturated Hydrocarbons (CnH₂n)

Alkenes primarily undergo electrophilic addition reactions due to the electron-rich $\pi$ bond.

Mechanism of HX Addition (Markovnikov): Step 1: H+ (electrophile) attacks the $\pi$ bond, adding to the less substituted carbon → more stable carbocation intermediate Step 2: X- (nucleophile) attacks the carbocation

Key Reactions:

• HX addition: Markovnikov (ionic mechanism). Anti-Markovnikov with HBr/peroxide only (Kharasch effect — radical mechanism)
• H₂O/H+ (hydration): Markovnikov addition of water
• X₂ addition: Anti addition via cyclic halonium ion intermediate. 1,2-Dihalide product
• HOX (hypohalous acid): Markovnikov for OH, anti for X — X adds to less substituted carbon (OH to more substituted)
• OsO₄ (syn-dihydroxylation): Syn addition of two OH groups via cyclic osmate ester
• KMnO₄ (cold, dilute, alkaline — Baeyer's reagent): Syn dihydroxylation (decolorizes purple KMnO₄ — test for unsaturation)
• Ozonolysis (O₃ then Zn/H₂O): Cleaves C=C to give aldehydes/ketones. Reductive workup (Zn) prevents further oxidation
• Hydroboration-oxidation (BH₃ then H₂O₂/NaOH): Anti-Markovnikov, syn addition of water. Gives primary alcohol from terminal alkene
• Catalytic hydrogenation (H₂/Pt or Pd or Ni): Syn addition of H₂

Zaitsev's Rule: In elimination reactions (E₁/E₂), the more substituted alkene (more stable) is the major product.

Representative alkene structures:

1,3-Butadiene (conjugated diene)

Styrene (vinyl benzene)

3. Alkynes — Triple-Bonded Hydrocarbons (CnH₂n-2)

Alkynes have TWO $\pi$ bonds — they undergo electrophilic addition but require harsher conditions than alkenes.

Acidity of terminal alkynes: sp C-H (pKa ~25) is acidic enough to react with strong bases:

• NaNH₂ → sodium acetylide (R-C≡C-Na+)
• This is the basis for acetylide alkylation: R-C≡C- + R'-X → R-C≡C-R' (SN₂, only 1-degree and methyl halides work)

Key Reactions:

• HX addition (2 equivalents): Follows Markovnikov twice → gem-dihalide
• X₂ addition (1 eq): Trans-dihaloalkene; (2 eq): Tetrahalide
• H₂O/H+/Hg₂+ (Tautomerization): Terminal alkynes → methyl ketones (via enol intermediate, Markovnikov); internal alkynes → mixture of ketones
• Hydroboration (disiamylborane then H₂O₂/NaOH): Terminal alkynes → aldehydes (anti-Markovnikov)
• Lindlar's catalyst (H₂/Pd-BaSO₄-quinoline): Partial reduction → cis-alkene (syn addition on poisoned catalyst surface)
• Na/NH₃(l) (Birch-type): Partial reduction → trans-alkene (anti addition via radical anion mechanism)
• Ozonolysis: Gives carboxylic acids (or CO₂ from terminal C)
• Polymerization: 3 CH≡CH → benzene (cyclotrimerization, red hot Fe tube)

Naphthalene — fused bicyclic aromatic (10 $\pi$ electrons):

4. Benzene — Aromatic Hydrocarbon

Benzene — 6 $\pi$ electrons, aromatic

Benzene undergoes electrophilic aromatic substitution (EAS) rather than addition, to preserve aromaticity.

General EAS Mechanism: Step 1: Generation of electrophile (E+) Step 2: E+ attacks ring → $\sigma$ complex (arenium ion/Wheland intermediate, non-aromatic) Step 3: Loss of H+ from $\sigma$ complex → substituted benzene (aromaticity restored)

Named EAS Reactions:

Directing Effects:

• Activating + o/p directors: -OH, -NH₂, -OR, -NHCOR, -R (alkyl)
• Deactivating + o/p directors: -F, -Cl, -Br, -I (halogens)
• Deactivating + meta directors: -NO₂, -CN, -CHO, -COR, -COOH, -SO₃H

Birch Reduction: Benzene + Na/NH₃(l)/EtOH → 1,4-cyclohexadiene. Electron-donating substituents: reduction avoids the substituted carbon. Electron-withdrawing: reduction occurs at the substituted carbon.

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