9.10C–9.11CThe alkanes
Chemistry only — not on the Combined Science specification
The alkanes are the first homologous series to learn, and four of them have to be recalled by name, formula and structure. They follow the general formula CₙH₂ₙ₊₂, so once you know the number of carbons the number of hydrogens follows: double it and add two.
Drawing the displayed structure means showing every atom and every bond as a line. Count the bonds on each carbon before you finish: carbon always forms exactly four, and hydrogen exactly one. If a carbon in your drawing has three or five lines coming off it, the structure is wrong.
Alkanes are described as saturated. That word has a precise meaning here: every bond between the carbon atoms is a single bond, so the molecule holds as many hydrogen atoms as it possibly can. There is no room for more, which is why an alkane will not take part in an addition reaction and does not decolourise bromine water.
Common mistake
"Saturated" does not mean full of something, or unreactive in general — alkanes burn readily. It means no carbon-to-carbon double bonds, and that is the phrase to use.
9.12C–9.13CStructure and formulae of alkenes
Chemistry only — not on the Combined Science specification
Alkenes are hydrocarbons with a double carbon-carbon bond. The general formula for the homologous series of alkenes is CnH₂ₙ.
Alkene molecules are unsaturated because they contain two fewer hydrogen atoms than the alkane with the same number of carbon atoms. Ethene is C₂H₄ where ethane is C₂H₆ — the two missing hydrogens are the space the second bond occupies.
The first four members of the homologous series of alkenes are ethene, propene, butene and pentene.
Alkene molecules can be represented as a molecular formula such as C₃H₆, or as a displayed structural formula showing the double bond as two parallel lines between two carbon atoms.
You should be able to recognise substances that are alkenes from their names or from given formulae in these forms. From a name, the ending -ene says alkene. From a molecular formula, check whether it fits CnH₂ₙ. From a displayed formula, look for the C=C. You do not need to know the names of individual alkenes other than ethene, propene, butene and pentene.
- Alkene
- a hydrocarbon with a double carbon-carbon bond, general formula CnH₂ₙ
- Unsaturated
- describes a molecule containing a carbon-carbon double bond, so it has two fewer hydrogen atoms than the alkane with the same number of carbons
- Homologous series
- a family of compounds with the same general formula and similar chemical properties
9.14C–9.16CReactions of alkenes
Chemistry only — not on the Combined Science specification
Alkenes are hydrocarbons with the functional group C=C. It is the generality of reactions of functional groups that determines the reactions of organic compounds — which is why learning one reaction of the C=C bond tells you how every alkene behaves.
Alkenes react with oxygen in combustion reactions in the same way as other hydrocarbons, but they tend to burn in air with smoky flames because of incomplete combustion.
Alkenes react with hydrogen, water and the halogens by the addition of atoms across the carbon-carbon double bond, so that the double bond becomes a single carbon-carbon bond.
You should be able to describe the reactions and conditions for the addition of hydrogen, water and halogens to alkenes, and to draw fully displayed structural formulae of the first four members of the alkenes and the products of their addition reactions with hydrogen, water, chlorine, bromine and iodine.
Drawing an addition product is mechanical once the idea is clear. Redraw the alkene with the double bond replaced by a single bond, then attach one atom or group from the reagent to each of those two carbon atoms. Every carbon must still have four bonds when you have finished.
Worked example
Draw the displayed formula of the product when propene reacts with bromine, and give its name.
- Propene is CH₂=CH–CH₃. Identify the two carbon atoms joined by the double bond — the first and second.
- Replace the double bond with a single bond between those two carbons.
- Bromine is Br₂, so add one bromine atom to each of those two carbons.
- Check every carbon has four bonds: carbon 1 has two H, one Br and one C; carbon 2 has one H, one Br and two C; carbon 3 has three H and one C.
- Number from the end that gives the bromines the lower positions.
Answer: CH₂Br–CHBr–CH₃, drawn with every bond as a line — 1,2-dibromopropane. The orange bromine is used up, which is why the bromine water is decolourised.
Common mistake
Nothing is lost in an addition reaction — every atom of both reactants ends up in the single product. If your product has fewer atoms than the two reactants together, a hydrogen has gone missing.
- Functional group
- the group of atoms in a molecule that determines how it reacts; it is the generality of reactions of functional groups that determines the reactions of organic compounds
- Addition reaction
- a reaction in which atoms add across a carbon-carbon double bond so that the double bond becomes a single bond
Before you practise
- Alkanes are CₙH₂ₙ₊₂ and saturated — methane, ethane, propane, butane.
- Alkenes are CₙH₂ₙ and unsaturated — ethene, propene, butene — and their functional group is the C=C double bond.
- Unsaturated means there is a carbon-to-carbon double bond, so the molecule can add atoms across it.
- Alkenes decolourise orange bromine water; alkanes leave it orange. That is the test.
- In an addition reaction the double bond opens and one atom joins each carbon — ethene + bromine gives dibromoethane.
- Every carbon must show exactly four bonds in a displayed structure.
- Alkenes burn with a smokier flame than alkanes because they have a higher proportion of carbon.