Reactions of alkanes and alkenes

Last time

Before we start: last time, in Ethanol and esters, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Same carbon, different behaviour

Ethane and ethene are both hydrocarbons with two carbons. Ethane is happy to sit around in a bottle for years. Ethene, with its C=C double bond, eagerly joins in reactions. That one difference gives two different types of reaction: substitution for alkanes, and addition for alkenes.

Alkanes

Alkanes have only single bonds, so they are saturated and fairly unreactive. They burn completely in plenty of oxygen to make carbon dioxide and water. C 3 H 8 plus 5O 2 gives 3CO 2 plus 4H 2 O. Their other reaction is substitution with chlorine, in ultraviolet light.

Substitution

Methane and chlorine in ultraviolet light: CH 4 plus Cl 2 gives CH 3 Cl plus HCl. A chlorine atom takes the place of a hydrogen atom. That is a substitution reaction. There are two products: the chloroalkane, and hydrogen chloride.

Why UV light?

Without light, methane and chlorine do not react in the cold. The ultraviolet light supplies the activation energy, by breaking the Cl–Cl bond into chlorine atoms. Such a reaction, started by light, is called photochemical.

Going further

With more chlorine, more hydrogens are replaced one at a time: CH 3 Cl, then CH 2 Cl 2, CHCl 3 and CCl 4. Every step makes a molecule of hydrogen chloride as well. CH 3 Cl plus Cl 2 gives CH 2 Cl 2 plus HCl. This gives a mixture of products, so substitution is a poor way to make one pure compound.