Topic 11: Organic chemistry

Organic chemistry is the study of carbon compounds, almost all of which come originally from crude oil and living matter. Carbon's ability to form four covalent bonds and long chains gives rise to families of related compounds called homologous series. This topic covers how we obtain fuels, how the main families behave and how small molecules are joined into polymers.

Fuels and crude oil

A fuel is a substance that releases useful energy when it burns. The three most important fossil fuels are coal, natural gas and petroleum (crude oil). Natural gas is mostly methane, CH4. Crude oil is a thick, dark mixture of many different hydrocarbons. A hydrocarbon is a compound containing only the elements hydrogen and carbon. Because crude oil is a mixture, it has no fixed boiling point and is not very useful until it is separated into simpler parts called fractions. These fossil fuels are described as finite (non-renewable) because they took millions of years to form and are being used far faster than they are replaced.

Fractional distillation of crude oil

Crude oil is separated by fractional distillation. The oil is heated until it vaporises, and the vapour passes up a tall fractionating column that is hot at the bottom and cooler at the top. Each fraction condenses at the height where the temperature matches its boiling range. Molecules with shorter chains have lower boiling points and rise to the top, while larger molecules with higher boiling points stay near the bottom. Going up the column the fractions become lighter in colour, less viscous (more runny), more flammable and easier to ignite. The main fractions, from the top downward, are: refinery gas (used for bottled gas and heating), gasoline/petrol (fuel for cars), naphtha (a feedstock for making chemicals), kerosene/paraffin (jet fuel), diesel oil (fuel for lorries and trains), fuel oil (fuel for ships and power stations) and finally bitumen (used to surface roads and roofs).

Homologous series and general features

A homologous series is a family of similar compounds that share the same general formula and have similar chemical properties. Members differ from the next by a CH2 unit, so as the chain grows the boiling point gradually increases. All members of one series contain the same functional group, the atom or group of atoms responsible for the characteristic reactions of that family. Compounds with the same molecular formula but different structures are called isomers. The main series studied are alkanes, alkenes, alcohols and carboxylic acids.

Alkanes and substitution

Alkanes are saturated hydrocarbons, meaning they contain only single carbon-carbon bonds and hold the maximum possible number of hydrogen atoms. Their general formula is CnH2n+2, giving methane CH4, ethane C2H6, propane C3H8 and butane C4H10. Alkanes are generally unreactive, but they do two important things. First, they burn: complete combustion in plenty of oxygen produces carbon dioxide and water, for example CH4 + 2O2 gives CO2 + 2H2O. Second, in the presence of ultraviolet light they undergo a substitution reaction with chlorine, in which a hydrogen atom is replaced by a chlorine atom, for example CH4 + Cl2 gives CH3Cl + HCl.

Alkenes and the bromine-water test

Alkenes are unsaturated hydrocarbons because they contain a carbon-carbon double bond, which is their functional group. Their general formula is CnH2n, giving ethene C2H4 and propene C3H6. Alkenes are made by cracking, in which large alkane molecules from heavy fractions are broken into smaller, more useful molecules using heat and a catalyst. The double bond makes alkenes much more reactive than alkanes. They take part in addition reactions, where a small molecule adds across the double bond. The standard test to tell an alkene from an alkane uses bromine water: an alkene decolourises orange/brown bromine water (turning it colourless), while a saturated alkane leaves it orange.

Alcohols and the production of ethanol

Alcohols are a homologous series whose functional group is the -OH group, with the general formula CnH2n+1OH. The first members are methanol CH3OH and ethanol C2H5OH. Alcohols burn in oxygen to give carbon dioxide and water, which is why ethanol is used as a fuel. Ethanol can be made in two ways. Fermentation uses yeast to convert sugars (glucose) into ethanol and carbon dioxide at around 30 to 40 degrees Celsius in the absence of air; it uses a renewable raw material but is slow and gives a dilute, impure product. Catalytic addition (hydration) reacts ethene with steam over a phosphoric acid catalyst at high temperature and pressure; it is fast and continuous and gives pure ethanol, but uses ethene from finite crude oil.

Carboxylic acids

Carboxylic acids form a homologous series whose functional group is -COOH, with the general formula CnH2n+1COOH. The most familiar member is ethanoic acid, CH3COOH, the acid in vinegar. They are weak acids but still show typical acid reactions: they turn damp litmus paper red and react with reactive metals to give a salt and hydrogen, with bases to give a salt and water, and with carbonates to give a salt, water and carbon dioxide. Ethanoic acid can be formed when ethanol is oxidised, for example by air or by oxidising agents. The salts of carboxylic acids are named ending in -oate, so ethanoic acid forms ethanoates.

Polymers and problems with plastics

Polymers are very large molecules built by joining many small molecules called monomers. In addition polymerisation, many alkene monomers add together with no other product, so the empty space at the double bond simply links the units. For example many ethene molecules form poly(ethene). In condensation polymerisation two different monomers join and a small molecule, usually water, is released at each link; nylon and Terylene (a polyester) are made this way. Most addition polymers are non-biodegradable, so they are not broken down by microorganisms and build up as litter in the environment, including in the oceans. Disposing of them by burning can release toxic gases, while landfill takes up space, so reducing, reusing and recycling plastics is important.