How are families of carbon compounds named, and how do we tell saturated from unsaturated?
Homologous series: alkanes, cycloalkanes and alkenes, their general formulae and naming, saturated and unsaturated molecules, isomers, addition reactions and the bromine test for unsaturation.
An SQA National 5 Chemistry answer on the homologous series of alkanes, cycloalkanes and alkenes, covering general formulae, naming the first eight, saturated and unsaturated molecules, isomers, addition reactions and the bromine water test for unsaturation.
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What this key area is asking
The SQA wants you to recognise the homologous series of alkanes, cycloalkanes and alkenes, use their general formulae, name and draw the first members, tell saturated from unsaturated, recognise isomers, describe addition reactions of alkenes, and use bromine solution to test for unsaturation. This is the naming and structure backbone of Nature's Chemistry.
What a homologous series is
The alkanes
The cycloalkanes
Cycloalkanes are alkanes joined into a ring. They have the general formula and are also saturated. Examples are cyclopropane, cyclobutane and cyclopentane. Because the carbons join in a ring, two hydrogen atoms are "lost" compared with the straight-chain alkane of the same number of carbons, which is why the formula is rather than .
The alkenes
Saturated and unsaturated
Addition reactions and the bromine test
Because the double bond in an alkene can open up, alkenes take part in addition reactions, where a small molecule adds across the double bond and no other product is made. Alkenes add hydrogen (hydrogenation), halogens such as bromine, and water (hydration).
This gives the standard test for unsaturation:
Isomers
Worked example: naming and the general formula
Examples in context
The homologous series explain everyday fuels and materials. The alkanes in natural gas and petrol burn as fuels; the alkenes such as ethene are the building blocks of plastics like poly(ethene), made by joining many alkene molecules in addition polymerisation. The bromine test is used in industry to check whether a sample of oil or fat is saturated or unsaturated, which is exactly the chemistry behind the labels on cooking fats.
Try this
Q1. Give the general formula of the alkanes and of the alkenes. [2 marks]
- Cue. Alkanes ; alkenes .
Q2. State what you would see when bromine solution is added to propene, and what this shows. [2 marks]
- Cue. The bromine is quickly decolourised; this shows propene is unsaturated.
Q3. Write the molecular formula of propane. [1 mark]
- Cue. (alkane, ).
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 2019 style3 marksBromine solution is added to two test tubes, one containing hexane and one containing hexene. Describe what you would see in each, and explain what the test shows.Show worked answer β
Markers reward the observation for each tube and the conclusion about bonding.
With hexene (an alkene), the orange or brown bromine solution is quickly decolourised, turning colourless. With hexane (an alkane), the bromine solution stays orange or brown and does not change quickly.
This is the test for unsaturation. Hexene is unsaturated because it contains a carbon to carbon double bond, which reacts with the bromine in a quick addition reaction. Hexane is saturated, with only single carbon to carbon bonds, so it does not react in the same way and does not decolourise the bromine.
SQA N5 2021 style3 marksPentane and 2-methylbutane are isomers with the molecular formula C5H12. Explain what is meant by isomers, and give the general formula of the alkane family.Show worked answer β
A 3 mark answer needs a correct definition of isomers and the correct general formula.
Isomers are compounds that have the same molecular formula but a different structural formula, meaning the atoms are joined together in a different arrangement. Pentane has a straight chain of five carbons, while 2-methylbutane has a branched chain, yet both contain five carbon and twelve hydrogen atoms.
The general formula of the alkanes is CnH2n+2, where n is the number of carbon atoms. For five carbons this gives C5H12, which matches both isomers.
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Sources & how we know this
- SQA National 5 Chemistry Course Specification β SQA (2019)