Natural sources of hydrocarbons, fractional distillation of crude oil and the properties of petroleum fractions, thermal and catalytic cracking, and the unique bonding properties of carbon.
Petroleum is the raw material from which most fuels and organic chemicals are made. Understanding how it is separated into useful fractions and how those fractions are further processed by cracking connects directly to the properties of alkanes and alkenes studied in the pages that follow.
Hydrocarbons are compounds that contain only carbon and hydrogen. The two main natural sources are:
Petroleum (crude oil): a complex mixture of hydrocarbons formed over millions of years from the remains of marine organisms under high pressure and temperature. It is the primary source of fuels and petrochemicals worldwide.
Natural gas: consists mainly of methane (CH₄), with smaller amounts of ethane, propane, and butane. Used directly as a fuel and as a source of hydrogen for the Haber process.
Both are non-renewable — once used, they cannot be replaced on a human timescale.
Crude oil is a mixture of hundreds of hydrocarbons with different chain lengths and boiling points, so it must be separated before it can be used. Fractional distillation does this continuously.
Crude oil is heated to about 400 °C so that most of it vaporises. The hot vapour enters the bottom of a fractionating column, which is hot at the bottom and progressively cooler toward the top. As vapours rise through the column, they cool. Different hydrocarbons condense at different heights, each collecting on a tray at the level where the temperature matches its boiling point. Bubble caps on the trays help vapours contact the liquid already condensed, improving separation.
Larger hydrocarbons have higher boiling points and stronger intermolecular forces — they condense lower in the column, are less volatile, and are harder to ignite. Smaller hydrocarbons have lower boiling points — they rise higher before condensing, are more volatile, and ignite more easily. Gases that do not condense at all exit from the top of the column.
| Fraction | Approx. boiling point | Carbons | Main uses |
|---|---|---|---|
| Refinery gases | Below 25 °C | C₁–C₄ | LPG, bottled gas, heating |
| Petrol / gasoline | 25–75 °C | C₅–C₁₀ | Fuel for cars |
| Naphtha | 75–120 °C | C₅–C₁₀ | Chemical feedstock |
| Kerosene | 120–250 °C | C₁₁–C₁₅ | Jet fuel, paraffin heaters |
| Diesel | 250–350 °C | C₁₅–C₁₉ | Fuel for diesel engines |
| Lubricating oil | 350–400 °C | C₂₀–C₃₀ | Oils, waxes, polishes |
| Fuel oil | 400 °C+ | C₃₀–C₄₀ | Ships, power stations, factories |
| Bitumen | Residue | C₄₀+ | Road surfacing, roofing materials |
Moving from top to bottom of the column: boiling point increases, chain length increases, viscosity increases, colour darkens, volatility decreases, and flammability decreases. Refinery gases that do not condense exit at the top; bitumen that never fully vaporised remains as a residue at the bottom.
Distillation gives fractions in proportions fixed by the oil's composition, but that composition does not match what is actually needed. There is too much of the heavy fractions and not enough petrol. Cracking corrects this imbalance.
Petroleum contains more long-chain, heavy fractions than the market demands, and fewer short-chain fuels. The heavier fractions are broken apart in a process called cracking.
Cracking is the thermal decomposition of large hydrocarbon molecules into smaller, more useful ones. One product is always an alkene (which has a C=C double bond and is used in polymer manufacture); the other product is a shorter alkane.
Example:
(decane → pentane + pentene)
| Method | Conditions | Notes |
|---|---|---|
| Thermal cracking | Very high temperature (~800 °C) and pressure | Produces mainly alkenes |
| Catalytic cracking | Lower temperature (~500 °C), zeolite catalyst | More selective, more efficient; used industrially |
The alkenes produced by cracking are the starting materials for making plastics (polymers) and many other organic chemicals.
The reason cracking produces so many different products, and the reason organic chemistry is so vast in general, comes down to something fundamental about carbon itself.
Carbon (Group IV) forms four covalent bonds. This versatility allows it to build:
The ability of carbon to bond to itself (called catenation) and to hydrogen, nitrogen, oxygen, and halogens means millions of distinct organic compounds are possible.