Hydrocarbons

Three big piles

Saturated: only single bonds — alkanes (open) and cycloalkanes (rings). General alkane formula CₙH₂ₙ₊₂.

Unsaturated: C=C (alkenes, CₙH₂ₙ) or C≡C (alkynes, CₙH₂ₙ₋₂).

Aromatic (arenes): benzene and relatives — unsaturated on paper, substitution in practice.

Alkanes — make and break

Prep paths: H₂ on alkenes/alkynes (Ni/Pt/Pd); Zn/HCl on alkyl halides; Wurtz (2 R–X + 2Na → R–R); soda-lime decarboxylation of RCOONa → RH (one C less); Kolbe electrolysis of carboxylates.

Physical: C1–C4 gases, then liquids, then solids. Non-polar → dissolve in ether/benzene, not water. Straight chains boil higher than branched isomers of the same formula.

Chemistry is sluggish at room conditions. Under light/heat: free-radical halogenation (CH₄ + Cl₂ → CH₃Cl…). Combustion → CO₂ + H₂O (lots of heat).

Controlled oxidation of CH₄ can give CH₃OH under specific industrial conditions. Steam reforming: CH₄ + H₂O → CO + 3H₂ (Ni, high T).

Aromatisation / reforming: C₆+ alkanes → aromatics with oxide catalysts — industrial, not a school-lab trick.

Alkenes — addition rules

Prep: partial hydrogenation of alkynes; alcoholic KOH dehydrohalogenation of R–X; acid dehydration of alcohols.

Add H₂ (Ni/Pt/Pd) → alkane. Add X₂ in CCl₄ → vicinal dihalide (Br₂ decolourises — classic test).

Add HX → alkyl halide. Markovnikov: H goes to the carbon that already has more H; X to the more substituted carbon.

Peroxide / Kharasch: HBr only, with peroxide → anti-Markovnikov. HCl and HI do not play this game the same way.

Polymerisation: many ethene units → polyethene. Cis/trans (geometric) isomerism when each doubly bonded C has two different groups.

Alkynes

Ethyne from CaC₂ + H₂O industrially. Also from vicinal dihalides via dehydrohalogenation + strong base.

Add two equivalents of H₂ or X₂ stepwise. HX can give geminal dihalides. Warm HgSO₄ / dil. H₂SO₄ + H₂O on ethyne → acetaldehyde.

Hot iron tube: 3 ethyne → benzene (cyclic polymerisation).

Terminal alkynes are acidic enough for salt formation with Ag⁺/Cu⁺ ammoniacal reagents — distinguishes them from alkenes.

Benzene without the mythology

C₆H₆, planar hexagon, all C–C bonds equivalent. Kekulé forms resonate; the hybrid is more stable than a fixed cyclohexatriene.

Prep: ethyne trimerisation; sodium benzoate + soda lime; phenol vapour over hot Zn.

Electrophilic substitution is the main show: nitration (conc. HNO₃ + H₂SO₄ → NO₂⁺), halogenation (FeX₃/AlCl₃), sulphonation (oleum), Friedel–Crafts alkylation/acylation (anhydrous AlCl₃).

Activating groups (–OH, –NH₂, –R…) push electron density and favour o/p. Deactivating groups (–NO₂, –COOH…) slow the ring and often meta-direct.

Burns with a sooty flame — high C%. Smell and non-polar solvents are the everyday physical cues.

Quick compare

Alkanes: substitution (radicals). Alkenes/alkynes: addition. Arenes: substitution that keeps aromaticity.

Alkynes can look “less eager” than alkenes toward some additions because of tighter π clouds on sp carbons — still add, just often need a push.

Baeyer’s reagent (cold alkaline KMnO₄) and Br₂/CCl₄: unsaturation tests. Terminal alkyne: ammoniacal AgNO₃ / CuCl.