Organic chemistry
Formulae and homologous series
Organic compounds are based on carbon chains. A homologous series is a family with the same functional group and general formula, similar chemical properties and a trend in physical properties; each member differs by . Names come from the chain length (meth 1, eth 2, prop 3, but 4) and the series ending: -ane (alkanes, ), -ene (alkenes, ), -ol (alcohols), -oic acid (carboxylic acids). Structural isomers have the same molecular formula but different structures.
≈ 40 minFuels and alkanes
Crude oil (petroleum) is a mixture of hydrocarbons, separated by fractional distillation: it's heated and vapours rise up a column that is hotter at the bottom; each fraction condenses at its boiling point. Short-chain fractions (refinery gas, petrol) are volatile, runny and flammable; long-chain fractions (fuel oil, bitumen) are viscous with high boiling points. Alkanes are saturated and unreactive except for combustion and substitution with halogens in UV light.
≈ 40 minAlkenes
Cracking breaks long-chain alkanes into shorter, more useful alkanes and alkenes (and sometimes hydrogen), using heat and a catalyst. It matches supply to demand: there's more long-chain oil than needed, and not enough petrol. Alkenes () contain a C=C double bond, so they undergo addition reactions: with bromine, hydrogen (→ alkanes) and steam (→ alcohols). Bromine water turns from orange to colourless with an alkene but not with an alkane.
≈ 40 minAlcohols and carboxylic acids
Ethanol is made by fermentation (yeast + sugar, 25–35 °C, no air; renewable but slow and impure) or hydration of ethene (steam, 300 °C, 60 atm, phosphoric acid; fast and pure but from non-renewable oil). Ethanol burns cleanly and is a solvent and fuel. Oxidising ethanol gives ethanoic acid, a weak acid that reacts with metals, bases and carbonates. An acid + an alcohol (with conc. catalyst) gives an ester, e.g. ethyl ethanoate, used in perfumes and flavourings.
≈ 45 minPolymers
Polymers are very long molecules made by joining many small monomers. In addition polymerisation, alkene monomers open their C=C bonds and join, with no other product: ethene → poly(ethene). In condensation polymerisation, monomers with two functional groups join and a small molecule (usually water) is lost: dicarboxylic acids + diols → polyesters; + diamines → polyamides (nylon). Proteins are natural polyamides made from amino acids. Most plastics are non-biodegradable, causing litter, harm to wildlife and problems in landfill and incineration.
≈ 40 min