Reactions of hydrocarbons

Alkanes and other hydrocarbons burn in air to produce carbon dioxide and water to release heat in a combustion reaction. Nothing happens when alkanes are mixed with oxygen at room temperature but with the introduction of a spark or flame, a vigorous combustion reaction takes place. Since hydrocarbon combustion is highly exothermic they are used widely as fuels.

Figure 1. Combustion equation

Alkanes are relatively unreactive as a class of compounds but do undergo a few reactions. In addition to combustion alkanes can react with themselves in the presence of a catalyst and undergo an isomerization reaction to convert unbranched alkanes to their branched-chain isomers. Alkanes can also react with halogens such as bromine and chlorine but require energy from increased temperature or UV light to initiate the reactions.

Figure 2. Chlorination of methane

Alkenes react with a much richer variety of compounds than alkanes. The classic reaction of alkenes and alkynes is the addition reaction. Hydrogenation is the addition of hydrogen to multiple bonds and can convert alkenes to alkanes. Halogenation - the addition of bromine or chlorine to alkenes - can yield haloalkanes. The addition of H-X - where X is a halogen or other electronegative group - is also known as hydrohalogenation. Alkenes can also be added together in a polymerization reaction to create polymers from single alkene monomers.

Figure 3. Polymerization of alkenes

Alkynes undergo many of the same reactions as alkenes. Hydrogenation of alkynes can be controlled to produce either alkenes or alkanes by adjusting the stoichiometry of the reaction. Similarly, the addition of other reagents to the alkyne triple bond can be controlled by the careful control of reagent equivalents to produce the desired products. For example an alkyne reaction with bromine can either yield a dihaloalkane or tetahaloalkane by changing the number of equivalents of bromine added.

Figure 4. Bromination of but-1-yne