What is a homologous series, and how do the bonding and reactions of alkanes and alkenes differ?
Describe alkanes and alkenes as homologous series, compare their bonding and reactions, and use the bromine test to distinguish saturated from unsaturated hydrocarbons
A focused answer to the O-Level Combined Science outcome on hydrocarbons. Homologous series and general formulae, saturated alkanes and unsaturated alkenes, combustion and addition, and the bromine test for a double bond.
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What this dot point is asking
SEAB wants you to describe alkanes and alkenes as homologous series, to compare their bonding (saturated single bonds versus an unsaturated double bond) and their reactions (substitution and combustion versus addition), and to use the bromine-water test to tell them apart. The test and the saturated-versus-unsaturated comparison are guaranteed exam content.
The answer
Hydrocarbons and homologous series
A hydrocarbon is a compound of hydrogen and carbon only. A homologous series is a family of organic compounds that:
- share the same general formula,
- have similar chemical properties,
- show a gradual change in physical properties (such as boiling point rising) as the carbon chain lengthens,
- differ from the next member by .
Alkanes and alkenes are two such series.
Alkanes: saturated hydrocarbons
Alkanes have the general formula and contain only single carbon-carbon bonds, so they are saturated. Examples are methane , ethane and propane . Their main reactions are:
- combustion: burning in plenty of oxygen gives carbon dioxide and water, releasing energy (they are used as fuels),
- substitution: with chlorine in ultraviolet light, a hydrogen atom is replaced by a chlorine atom.
Alkenes: unsaturated hydrocarbons
Alkenes have the general formula and contain a carbon-carbon double bond (), so they are unsaturated. Examples are ethene and propene . Because of the reactive double bond, their main reactions are addition reactions:
- with bromine: (this decolourises bromine water),
- with hydrogen (nickel catalyst): addition to the alkane,
- with steam (catalyst): hydration to an alcohol.
The bromine-water test
Bromine water (orange/brown) is the standard test:
- an alkene rapidly decolourises bromine water (the bromine adds across the double bond),
- an alkane does not decolourise it quickly (no double bond to add across).
So decolourising bromine water shows the hydrocarbon is unsaturated.
Examples in context
Example 1. Cracking long alkanes into useful alkenes. Refineries break long-chain alkanes from crude oil into shorter alkanes and alkenes by cracking. The alkenes, such as ethene, are valuable feedstocks for plastics, showing why the difference between saturated and unsaturated hydrocarbons matters industrially.
Example 2. Hardening vegetable oils. Unsaturated vegetable oils contain C=C double bonds. Adding hydrogen across these bonds (using a nickel catalyst) turns them into more saturated, solid fats for margarine, a direct food-industry use of the addition reaction of alkenes.
Try this
Q1. State the general formula of the alkanes and the general formula of the alkenes. [2 marks]
- Cue. Alkanes ; alkenes .
Q2. Describe how you would use bromine water to distinguish ethane from ethene, with the observations. [2 marks]
- Cue. Add bromine water and shake; ethene rapidly decolourises it (orange to colourless), ethane does not.
Q3. Name the type of reaction when ethene reacts with bromine, and write the equation. [2 marks]
- Cue. Addition; .
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksBromine water is added separately to a sample of hexane and a sample of hexene, and both tubes are shaken. Describe what you would see in each tube, name the type of reaction in the tube that reacts, and explain what the test shows about the two hydrocarbons.Show worked answer →
With hexane (an alkane): the bromine water stays orange/brown; there is no quick reaction (no change).
With hexene (an alkene): the bromine water is rapidly decolourised, turning from orange/brown to colourless.
The reaction in the hexene tube is an addition reaction (the bromine adds across the carbon-carbon double bond).
The test shows that hexene is unsaturated (it contains a C=C double bond) while hexane is saturated (only single bonds). The decolourising of bromine water is the standard test for an alkene.
Markers reward no change for the alkane, rapid decolourising for the alkene, addition as the reaction type, and the conclusion that the alkene is unsaturated and the alkane saturated.
Original3 marksDefine the term homologous series, and state two features that members of the same homologous series share, using the alkanes as your example.Show worked answer →
A homologous series is a family of organic compounds with the same general formula, similar chemical properties, and a gradual change in physical properties as the number of carbon atoms increases. Each member differs from the next by .
Two shared features for the alkanes: they all fit the general formula , and they have similar chemical properties (for example they all burn in oxygen and undergo substitution but not addition).
Markers reward a correct definition of a homologous series and two valid shared features such as the same general formula and similar chemical reactions.
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