Organic chemistry
Fuels from petroleum and the families of organic compounds: alkanes, alkenes, alcohols, carboxylic acids, esters and polymers.
📌 Key points
- A homologous series has the same general formula and functional group, similar chemical properties, and a trend in physical properties; each member differs by CH2.
- Alkanes (CnH2n+2) are saturated hydrocarbons. They are generally unreactive but burn, and react with chlorine in ultraviolet light by substitution: CH4 + Cl2 → CH3Cl + HCl.
- Petroleum is a mixture of hydrocarbons separated by fractional distillation into fractions: refinery gas, gasoline/petrol, naphtha, kerosene/paraffin, diesel oil/gas oil, fuel oil, lubricating oil and bitumen. Lower fractions have longer chains, higher boiling points and higher viscosity.
- Cracking breaks long-chain alkanes into shorter alkanes and alkenes (and sometimes hydrogen) using a high temperature and a catalyst. It meets the demand for petrol and makes alkenes.
- Alkenes (CnH2n) are unsaturated: they have a C=C double bond. Bromine water turns from orange to colourless with an alkene but stays orange with an alkane. Alkenes undergo addition reactions with bromine, hydrogen and steam.
- Ethanol is made by fermentation of glucose with yeast at 25–35 °C without oxygen (renewable but slow, batch process), or by hydration of ethene with steam at 300 °C, 6000 kPa with a phosphoric acid catalyst (fast, continuous, pure product, non-renewable).
- Ethanol is used as a solvent and as a fuel; it burns completely to carbon dioxide and water.
- Carboxylic acids such as ethanoic acid are weak acids. They react with metals, bases and carbonates, and with alcohols (acid catalyst) to form esters and water, e.g. ethanoic acid + ethanol → ethyl ethanoate + water.
- Structural isomers have the same molecular formula but different structural formulae, e.g. butane and methylpropane.
- Addition polymers form from alkenes, e.g. ethene → poly(ethene). Condensation polymers such as nylon (a polyamide) and PET (a polyester) form with loss of a small molecule such as water. Most plastics are non-biodegradable.
📖 Key terms
- Hydrocarbon
- A compound containing hydrogen and carbon only.
- Saturated
- Containing only single carbon–carbon bonds.
- Unsaturated
- Containing at least one carbon–carbon double (or triple) bond.
- Homologous series
- A family of compounds with the same general formula and functional group, and similar chemical properties.
- Functional group
- The atom or group of atoms that decides the chemical properties of a compound.
- Structural isomers
- Compounds with the same molecular formula but different structural formulae.
- Cracking
- Breaking long-chain alkanes into shorter alkanes and alkenes.
- Polymer
- A large molecule made from many small units called monomers.
✏️ Worked example
Name the ester made from methanol and propanoic acid, and write the word equation.
- The first part of the ester name comes from the alcohol: methanol → "methyl".
- The second part comes from the acid: propanoic acid → "propanoate".
- Water is the other product, and an acid catalyst is used.
- Answer: methyl propanoate. Word equation: propanoic acid + methanol → methyl propanoate + water.
For the bromine water test, write "orange to colourless". "Clear" means see-through, not colourless, and loses the mark.
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