Alkenes

1. Alkenes are a family of hydrocarbons whose molecules contain a double carbon-carbon covalent bond, written $C=C$.

2. A double covalent bond is made of two shared pairs of electrons between the same two carbon atoms.

3. Alkenes are unsaturated hydrocarbons, because they contain a $C=C$ double bond.

   1. Unsaturated means that more atoms can be added to the molecule when the double bond opens up.

4. The names of alkenes end in -ene, such as ethene and propene.

5. Ethene, $C_2{H}_4$, is the simplest alkene, with the structural formula ${CH}_2={CH}_2$.

   $$\begin{array}{ccc}H& & H\\ |& & |\\ C& =& C\\ |& & |\\ H& & H\end{array}$$
   1. Each carbon atom in ethene still forms four bonds, counting the double bond as two.

6. Propene, $C_3{H}_6$, has the structural formula ${CH}_2={CHCH}_3$.

   $$\begin{array}{ccccccc}H& & H& & H& & \\ |& & |& & |& & \\ C& =& C& -& C& -& H\\ |& & & & |& & \\ H& & & & H& & \end{array}$$

7. Alkenes are more reactive than alkanes, because the double bond is a reactive site where other atoms can join.

Cracking

1. Cracking breaks large alkane molecules into smaller, more useful molecules.

2. The large alkanes come from the heavier fractions of petroleum, such as kerosene and diesel oil.

3. Cracking needs two conditions.

   1. A high temperature, of about $500{}^{\circ }C$, so that the alkane turns into a vapour and its bonds can break.
   2. A catalyst, such as aluminium oxide or silica, which speeds up the breakdown without being used up.

4. The hot alkane vapour is passed over the heated catalyst, and some of its $C-C$ bonds break.

5. The products of cracking are a smaller alkane and an alkene, and sometimes hydrogen.

   1. There are not enough hydrogen atoms in the large molecule for every product to be an alkane, so at least one product must be an alkene.

6. Cracking is a type of thermal decomposition, because heat breaks one substance down into several simpler ones.

7. Decane can be cracked into octane and ethene.

   $$C_{10}{H}_{22}\to C_8{H}_{18}+C_2{H}_4$$

8. Ethane can be cracked to form ethene and hydrogen.

   $$C_2{H}_6\to C_2{H}_4+H_2$$

Reasons for cracking

1. Fractional distillation of petroleum gives more long-chain hydrocarbons than are needed.
2. It gives too little of the short-chain fractions such as petrol, which are in high demand as fuels.
   1. The demand for petrol is greater than the supply from fractional distillation alone.
3. Cracking converts the surplus long-chain alkanes into shorter-chain alkanes that are useful as fuels, such as petrol.
4. Cracking produces alkenes, which are needed as starting materials for making polymers such as poly(ethene) and other chemicals such as ethanol.
5. Cracking can also produce hydrogen, which is used as a fuel and to make other chemicals such as ammonia.
6. Long-chain alkanes are less useful as they are, because they are viscous, hard to ignite and burn with a smoky flame.

Testing for unsaturation

1. Aqueous bromine, also called bromine water, is used to tell a saturated hydrocarbon from an unsaturated one.
2. Add a few drops of orange aqueous bromine to the hydrocarbon and shake the mixture.
3. With an alkene, the aqueous bromine changes from orange to colourless, which is described as decolourised.
   1. The colour disappears because the bromine reacts with the $C=C$ double bond and is used up.
4. With an alkane, the aqueous bromine stays orange, because an alkane has no double bond for the bromine to react with.
5. The test works for any unsaturated hydrocarbon, so a colour change shows that the molecule contains a $C=C$ double bond.