Organic Chemistry

Organic chemistry is the study of carbon compounds. Many organic compounds are obtained from crude oil, while others occur naturally in plants and animals or can be manufactured for use as fuels, polymers, pharmaceuticals, perfumes, flavourings, dyes and detergents.

Crude oil is a finite resource formed from the remains of ancient biomass, mainly plankton buried in mud. It is a mixture containing a very large number of compounds, most of which are hydrocarbons made only from hydrogen and carbon atoms.

Alkanes are saturated hydrocarbons containing only single carbon-carbon bonds. They form a homologous series with the general formula CₙH₂ₙ₊₂. The first four alkanes are methane, ethane, propane and butane.

The hydrocarbons in crude oil are separated into fractions by fractional distillation. Molecules within a fraction have similar numbers of carbon atoms and therefore similar boiling points. The fractions are used as fuels and as feedstock for the petrochemical industry.

As hydrocarbon molecules become larger, their boiling points and viscosity increase while their flammability generally decreases. Complete combustion of hydrocarbons in oxygen produces carbon dioxide and water and releases energy.

Cracking breaks large hydrocarbon molecules into smaller, more useful molecules. Catalytic cracking and steam cracking can produce shorter-chain alkanes and alkenes. Cracking is useful because there is high demand for smaller fuel molecules and for alkenes used to manufacture other chemicals.

Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond, $C=C$. Their general formula is CₙH₂ₙ, and the first four members are ethene, propene, butene and pentene. Alkenes decolourise bromine water because bromine adds across the double bond.

Alkenes undergo addition reactions because of their carbon-carbon double bond. They react with hydrogen, water and halogens such as chlorine, bromine and iodine, with atoms adding across the double bond so that it becomes a single carbon-carbon bond.

Alcohols contain the functional group –OH. Methanol, ethanol, propanol and butanol are the first four members of the homologous series. Alcohols can react with sodium, burn in air, dissolve in water to different extents and react with oxidising agents.

Ethanol can be produced by fermentation of sugar solutions using yeast. Students should know the conditions required for fermentation and should be able to recognise alcohols from their names or formulae.

Carboxylic acids contain the functional group –COOH. Methanoic acid, ethanoic acid, propanoic acid and butanoic acid are the first four members. They react with carbonates, dissolve in water and react with alcohols, and Higher Tier students should understand why they are weak acids in terms of partial ionisation and pH.

Addition polymerisation occurs when many alkene monomers join together to form a long-chain polymer without producing another substance. The repeating unit contains the same atoms as the monomer, and students should be able to identify monomers and polymers and draw repeating units.

Condensation polymerisation occurs when monomers containing two functional groups join together, producing a polymer and a small molecule such as water. Students should understand the relationship between the functional groups in the monomers and the repeating units of the polymer.

Amino acids contain two different functional groups and can undergo condensation polymerisation to form polypeptides. Different amino acids can be joined in different sequences to form proteins.

DNA is a naturally occurring polymer essential for life. DNA molecules consist of two polymer chains made from four different nucleotide monomers arranged in a double helix. Other naturally occurring polymers include proteins, starch and cellulose.