Unsaturated hydrocarbons: alkenes and alkynes

Fewer hydrogens, more bonds

Methane, ethane and propane are alkanes. Every carbon in them has four single bonds, and holds the greatest number of hydrogen atoms it can. They are saturated. But carbon has another way to use its four bonds. Two carbon atoms can share more than one pair of electrons.

Unsaturated hydrocarbons

If two neighbouring carbon atoms share two pairs of electrons, there is a double bond. If they share three pairs, there is a triple bond. Because such a carbon has fewer hydrogen atoms than the most that it could hold, the compound is unsaturated.

The double and triple bonds are the reactive parts of the molecule. Alkenes and alkynes react with more substances than alkanes do, and they are the starting point for many plastics.

Alkenes

An alkene has one double bond, C=C. The names end in -ene. The general formula is

• Ethene, $\text{C}_2\text{H}_4$: $\text{CH}_2=\text{CH}_2$.
• Propene, $\text{C}_3\text{H}_6$: $\text{CH}_3-\text{CH}=\text{CH}_2$.
• Hexene, $\text{C}_6\text{H}_{12}$.

There is no methene, because a double bond needs at least two carbon atoms. The name is made as for the alkanes, with the end changed: ethane becomes ethene, and propane becomes propene.

Ethene is a gas that plants make. It makes fruits ripen. A fruit seller who keeps mangoes with a ripe fruit in a closed box is using it. Ethene is also used to make polythene, the plastic of carry bags: many ethene molecules join in a long chain.

Alkynes

An alkyne has one triple bond, C≡C. The names end in -yne. The general formula is

• Ethyne, $\text{C}_2\text{H}_2$: $\text{CH}\equiv\text{CH}$. It is burnt in welding, and it is also called acetylene. Burnt in pure oxygen, it makes a flame hot enough to cut and join steel.
• Propyne, $\text{C}_3\text{H}_4$.

Comparing the three

A simple test

Shake the hydrocarbon with a few drops of bromine water, which is orange-brown. An alkane does not change the colour. An alkene or an alkyne decolourises it, because the bromine adds across the double or triple bond. This is the usual test for unsaturation.

Rings

Carbon atoms can also be in a ring. The ring compounds have the prefix cyclo.

• Cyclohexane, $\text{C}_6\text{H}_{12}$: a ring of six carbon atoms with single bonds. It is saturated, and it has two hydrogens fewer than hexane.
• Benzene, $\text{C}_6\text{H}_6$: a ring of six carbon atoms, with alternate double bonds. It is the parent of many compounds, and it is a very special, stable ring.

How to tell the type from the formula

Compare the number of hydrogens with the alkane of the same number of carbons, $2n + 2$.

• Same number: saturated alkane.
• Two fewer: an alkene or a ring.
• Four fewer: an alkyne (or a compound with two double bonds).

For example, the alkane with eight carbons has $2 \times 8 + 2 = 18$ hydrogens. So $\text{C}_8\text{H}_{16}$ is two fewer. It is an alkene (octene) or a ring (cyclooctane).

You should find that the blue bar can never rise above the grey one. Each step of two down adds one ring or double bond.

Practise with new numbers

Try it in Python

This program tells the kind of hydrocarbon from its formula.

Common mistakes

• Using the formula of the alkane for an alkene. Ethene is $\text{C}_2\text{H}_4$, not $\text{C}_2\text{H}_6$.
• Thinking that every compound with $\text{C}_n\text{H}_{2n}$ is an alkene. It can be a ring, such as cyclohexane.
• Starting the series at one carbon. There is no methene.
• Saying that unsaturated means "has no hydrogen". It means that it has fewer than the greatest number.

Remember these

• Alkene: C=C, $\text{C}_n\text{H}_{2n}$. Alkyne: C≡C, $\text{C}_n\text{H}_{2n-2}$. Both are unsaturated.
• Ethene is $\text{C}_2\text{H}_4$, and ethyne is $\text{C}_2\text{H}_2$.
• Cyclohexane is $\text{C}_6\text{H}_{12}$ (saturated), and benzene is $\text{C}_6\text{H}_6$.
• Bromine water is decolourised by an unsaturated hydrocarbon.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the formula of the alkene with $7$ carbon atoms?

2. Why is there no methene?

3. A gas decolourises bromine water. Can it be ethane?

4. Cyclobutane is $\text{C}_4\text{H}_8$. Is it saturated?