Global challenges Study Guide

Crude oil is a mixture of hydrocarbons, which are compounds made of hydrogen and carbon atoms. Many of the hydrocarbons in crude oil are very long-chain alkanes. While some of these long-chain molecules are useful as fuels like bitumen (tarmac) or fuel oil, there is a much higher demand for shorter-chain alkanes, such as those found in petrol and diesel, and for alkenes, which are key building blocks for plastics. Cracking is a process that breaks down large, less useful hydrocarbon molecules into smaller, more valuable ones. This is achieved by heating the long-chain hydrocarbons to a high temperature, often using a catalyst, which speeds up the reaction without being used up itself. The high temperatures and catalysts provide the energy needed to break the strong carbon-carbon bonds within the large molecules. The products of cracking are typically a mixture of shorter-chain alkanes and alkenes. Alkanes are saturated hydrocarbons, meaning they only have single carbon-carbon bonds, while alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. The alkenes produced, such as ethene and propene, are particularly important because their double bonds make them very reactive. This reactivity allows them to be used as monomers, which can be joined together in a process called polymerisation to form long-chain polymers like poly(ethene) and poly(propene), which are used to make plastics. Without cracking, the valuable shorter-chain alkanes and reactive alkenes would be in short supply, limiting the production of many essential materials and fuels we rely on daily.

Organic chemistry deals with compounds containing carbon, usually bonded to hydrogen. To systematically study these vast numbers of compounds, chemists categorise them based on their structure and chemical properties. A functional group is a specific atom or group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule. For example, the hydroxyl group (-OH) is the functional group for alcohols, dictating how all alcohols will typically react. The carboxyl group (-COOH) defines carboxylic acids, giving them acidic properties and characteristic reactions.

A homologous series is a family of organic compounds with the same general formula, similar chemical properties due to the same functional group, and a gradual change in physical properties as the number of carbon atoms increases. Members of a homologous series differ from each other by a -CH₂- unit. For instance, methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) are the first three members of the alkane homologous series. They all have the general formula CnH₂n+₂, exhibit similar chemical reactivity (e.g., combustion), and show a trend in boiling points (increasing with carbon chain length). Recognizing functional groups allows chemists to predict the properties and reactions of new or unfamiliar organic compounds, making the study of organic chemistry more logical and manageable. Identifying members of the same homologous series means understanding that they share these common features despite differing in size.