Hydrocarbons: Structure, Nomenclature, and Industrial Processes

Hydrocarbons are organic compounds composed solely of carbon and hydrogen atoms. They are the simplest organic compounds and form the basis of petroleum and natural gas. They are classified based on the type of carbon-carbon bonds present.

Alkanes are saturated hydrocarbons, meaning they contain only carbon-carbon single bonds. Each carbon atom is sp3 hybridised and forms four single covalent bonds, resulting in a tetrahedral geometry around each carbon. They are relatively unreactive due to the strength of C-C and C-H bonds and the absence of polar bonds. They are non-polar molecules with weak intermolecular forces (van der Waals forces), leading to low boiling points that increase with chain length.

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. The presence of the double bond makes them more reactive than alkanes, readily undergoing addition reactions. The carbon atoms involved in the double bond are sp2 hybridised, resulting in a trigonal planar geometry around each of these carbons. This planar arrangement can lead to cis/trans (geometric) isomerism when each carbon of the double bond is bonded to two different groups.

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond. They are the most reactive of the three classes due to the high electron density of the triple bond, also undergoing addition reactions. The carbon atoms involved in the triple bond are sp hybridised, resulting in a linear geometry around these carbons.

The International Union of Pure and Applied Chemistry (IUPAC) provides a systematic method for naming organic compounds. For hydrocarbons, this involves identifying the longest continuous carbon chain (parent chain), numbering the chain to give substituents the lowest possible numbers, identifying and naming substituents (e.g., methyl, ethyl), and indicating the position of double or triple bonds. Prefixes like 'meth-', 'eth-', 'prop-', 'but-', 'pent-', 'hex-' denote the number of carbon atoms in the parent chain.

Crude oil is a complex mixture of hydrocarbons with varying boiling points. Fractional distillation is a physical separation process used to separate crude oil into different fractions (mixtures of hydrocarbons with similar boiling points). Crude oil is heated and vaporised, then fed into a tall fractionating column. The column is hotter at the bottom and cooler at the top. Vapours rise, cool, and condense at different levels (trays) according to their boiling points. Longer chain hydrocarbons (higher boiling points) condense lower down, while shorter chain hydrocarbons (lower boiling points) rise higher before condensing.

Cracking is a chemical process that breaks down long-chain, less useful hydrocarbons into shorter-chain, more valuable hydrocarbons. This is done to meet the high demand for petrol and lighter fractions, and to produce alkenes for the petrochemical industry. There are two main types: thermal cracking (high temperature and pressure) and catalytic cracking (lower temperature and pressure, using a catalyst like zeolite). Both processes produce a mixture of alkanes and alkenes.