Alkene bonding and unsaturation

An alkene is a hydrocarbon containing at least one carbon–carbon double covalent bond. A hydrocarbon is a compound made only of carbon and hydrogen atoms. In a double covalent bond, the two carbon atoms share two pairs of electrons; in a single covalent bond, they share one pair.

Alkenes are unsaturated because the carbon atoms joined by the double bond are not bonded to the maximum possible number of hydrogen atoms. If the double bond becomes a single bond, each of those carbon atoms can bond to an additional hydrogen atom.

For example, ethene, $$\ce{CH2=CH2}$$, has a carbon–carbon double bond and four hydrogen atoms. The corresponding saturated hydrocarbon, ethane, $$\ce{CH3-CH3}$$, has a carbon–carbon single bond and six hydrogen atoms.

A saturated hydrocarbon has only single bonds between its carbon atoms. The carbon–carbon double bond therefore distinguishes an alkene from a saturated hydrocarbon.

Cracking of larger alkanes

Cracking breaks larger alkane molecules into smaller molecules. It takes place at a high temperature in the presence of a catalyst, which speeds up the reaction.

The products can include smaller alkanes, alkenes and hydrogen. For example, one possible cracking reaction breaks decane into octane and ethene:

In this reaction, decane is the larger alkane, octane is a smaller alkane and ethene is an alkene. The atoms in the original molecule are rearranged into the smaller molecules.

Why larger alkanes are cracked

Cracking converts larger alkanes into products that are in greater demand. Smaller hydrocarbons are useful as fuels, while alkenes are useful starting materials for making other chemicals.

Aqueous bromine test for unsaturation

Aqueous bromine, also called bromine water, is orange-brown. It can distinguish a saturated hydrocarbon from a hydrocarbon containing a carbon–carbon double bond.

An unsaturated hydrocarbon contains a carbon–carbon multiple bond. When bromine reacts with a carbon–carbon double bond, it adds across the bond: the double bond becomes a single bond, and a bromine atom attaches to each of the two carbon atoms. The orange-brown colour disappears as bromine is consumed.

A saturated hydrocarbon has only single bonds between its carbon atoms. It does not decolourise aqueous bromine under the conditions of this test, so the orange-brown colour remains.