The full lecture, told through the things hydrocarbons actually do in your life — the LPG cylinder under your stove, a burning candle, a ripening banana, the petrol in the bowser, a bromine-water test in the lab, and the strange stability of benzene. Scroll; the panel on the right brings each one to life.
Hydrocarbons are made of carbon and hydrogen only. Alkanes (CₙH₂ₙ₊₂) are saturated — every bond is a single bond, so each carbon carries the maximum number of hydrogens. That makes them fairly unreactive ("paraffins" = little affinity).
| Alkane | Everyday use |
|---|---|
| Methane | natural gas / CNG — cooking & vehicle fuel |
| Propane & butane | the gas inside an LPG cylinder |
| Higher alkanes | petrol, diesel, kerosene, candle wax |
Alkanes burn in plenty of oxygen to carbon dioxide and water, releasing large amounts of heat — this is their main use as fuel:
In limited oxygen, incomplete combustion gives toxic carbon monoxide (CO) and soot (C) — a smoky yellow flame instead of a clean blue one. A candle is just solid long-chain alkane (wax) that melts, vaporises and burns at the wick.
Alkenes (CₙH₂ₙ) are unsaturated — they carry a C=C double bond. The π electrons of that double bond make it a region of high electron density, so alkenes are far more reactive than alkanes.
The simplest alkene, ethene (C₂H₄), is a plant hormone: a ripening banana, apple or mango releases ethene, which speeds the ripening of fruit around it. (Industrially, ethene is also the monomer of polythene.) Alkenes are made by elimination:
Crude oil is mostly long-chain alkanes, but cars need shorter, petrol-sized molecules. In cracking, heat (and a catalyst) break a long chain into smaller fragments — some saturated alkanes for fuel, and some reactive alkenes for the chemical industry.
So the petrol bowser at the pump is filled with the lighter alkanes carved out of heavy oil — and the same process supplies the ethene that becomes plastics.
Because the C=C is electron-rich, alkenes undergo electrophilic addition — a small molecule adds across the double bond, turning it into a single bond:
This is the classic test for unsaturation: shake the compound with orange bromine water. An alkene decolourises it (orange → colourless) as Br₂ adds across the double bond; an alkane leaves it orange. Alkenes also add H₂ (Ni), HX and H₂O.
Benzene is C₆H₆ — a flat hexagonal ring. Kekulé drew alternating single and double bonds, but all six C–C bonds are actually identical. Each carbon is sp² hybridised, and the six left-over p-orbitals merge into a continuous ring of delocalised π electrons above and below the plane.
Picture a stadium crowd doing a Mexican wave around a ring: no single person "owns" the wave — it belongs to everyone at once. In the same way no single bond owns the extra electrons; they belong to the whole ring.
Aromatic hydrocarbons are everywhere: toluene in paint thinners, and the naphthalene mothballs in a wardrobe — two fused benzene rings (C₁₀H₈) that slowly sublime into a pest-repelling vapour. They are all built on the stable aromatic ring.
Benzene reacts with electrophiles by substitution — an electrophile replaces a ring hydrogen, leaving the aromatic ring intact:
| Reaction | Electrophile | Product |
|---|---|---|
| Nitration | NO₂⁺ | nitrobenzene |
| Halogenation | Cl⁺ (AlCl₃) | chlorobenzene |
| Sulphonation | SO₃ | benzenesulphonic acid |
| Friedel–Crafts | R⁺ / RCO⁺ | alkyl- / aryl-ketone |