Organic chemistry: crude oil and polymers
Chemistry · GCSE · The School
Fractional distillation
Crude oil is a mixture of hydrocarbons of different chain lengths. It is heated and fed into a column that is hot at the bottom and cool at the top. Long chains have strong intermolecular forces, so they have high boiling points and condense low down; short chains rise further before condensing. Nothing reacts — this is separation by boiling point, not chemistry.
Alkanes and alkenes
ALKANES are saturated: only single bonds, general formula CnH2n+2, and relatively unreactive. ALKENES are unsaturated: they contain a carbon-carbon double bond, general formula CnH2n, and that double bond makes them reactive. The test is bromine water: orange bromine water stays orange with an alkane and is decolourised by an alkene, because the double bond opens and adds the bromine.
Polymers
In addition polymerisation, many alkene molecules join into one long chain and nothing else is produced — every atom in the monomer ends up in the polymer. Ethene gives poly(ethene); propene gives poly(propene). The strong covalent chains that make polymers useful are the same thing that makes them unreactive and slow to break down, which is the disposal problem stated in one sentence.
A barrel of crude oil enters the column. THE BOTTOM: bitumen and fuel oil condense first — longest chains, highest boiling points, thick and dark, and there is far more of these than anyone wants. THE MIDDLE: kerosene and diesel. THE TOP: petrol and refinery gases, shortest chains, most in demand. So supply and demand are the wrong way round, and CRACKING fixes it: pass a long-chain fraction over a hot catalyst and it breaks into a shorter alkane plus an ALKENE. The alkane goes into petrol; the alkene goes into polymers. Test which is which with bromine water — the alkene decolourises it, the alkane does not. One process, and it answers both the fuel shortage and the polymer supply at once.
Why polymers do not rot away
The same strong, unreactive covalent bonds that make polymers useful for packaging and pipes also stop microorganisms breaking them down, so most plastics are NOT biodegradable and can sit in landfill for centuries. Recycling sorts polymers by type and melts them down to reshape, but only some plastics are practical to sort and re-melt without losing quality. Incineration recovers the energy stored in the bonds but releases carbon dioxide, and can release toxic gases if the temperature is not controlled. None of the three disposal routes is free of a drawback, which is exactly what an exam answer on this topic needs to say.
Addition and condensation polymers differ in what is lost
Addition polymerisation joins alkene monomers with nothing left over — every atom ends up in the polymer. Condensation joins monomers with two functional groups and releases a small molecule, usually water, each time. That difference is why one is hard to break down and the other can often be hydrolysed.