Reactions of Alkanes
In comparison to alkenes and alkynes, alkanes are relatively unreactive due to the absence of a weaker pi bond in their carbon skeletons. However, there are a few classes of reactions that are commonly performed with alkanes.
The most important reaction that alkanes undergo is combustion. Smaller, more linear alkanes generally oxidize more readily than larger or more branched molecules. Alkanes can be burned in the presence of oxygen to produce carbon dioxide, water, energy, and—in situations with limited oxygen—carbon monoxide. For this reason, alkanes are frequently used as fuel sources, for example, in internal combustion engines.
With the addition of a halogen gas and energy, alkanes can be halogenated with the reactivity of the halogens proceeding in the following order: Cl2>Br2>I2. In this reaction, UV light or heat initiates a chain reaction, involving the homolytic cleavage of the halogen gas. The halogen radicals can then abstract protons from the alkanes, which can then combine or react to form more radicals. Alkanes can be halogenated at a number of sites, and this reaction typically yields a mixture of halogenated products. For example, see Figure 2.
The complex alkanes with high molecular weights that are found in crude oil are frequently broken into smaller, more useful alkanes (along with alkenes and hydrogen gas) by thermal cracking. This process is typically performed at high temperatures, often in the presence of a catalyst. A mixture of products results, and these alkanes and alkenes can be separated by fractional distillation. A factory for thermal cracking is shown in Figure 1.