Chemistry 11–12 · Year 12

Substitution against addition: bromine water with cyclohexane in sunlight and in the dark

Module 7: Organic Chemistry

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

An alkene takes up bromine at once and in the dark, because the double bond adds bromine directly, while an alkane takes up bromine only in light, because that reaction is a substitution which cannot begin until a photon splits a bromine molecule into two atoms. Light is therefore the variable that separates the two mechanisms.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

bromine water irritates eyes and skin and its vapour is irritant
cyclohexane and cyclohexene are highly flammable and harmful
bromine vapour escapes from any tube that still holds bromine when it is unstoppered
a 365 nm ultraviolet lamp harms eyes and skin on direct exposure

Controls

only the teacher dispenses bromine water, and only in a fume cupboard
tubes kept stoppered throughout and unstoppered only by the teacher in the fume cupboard, for disposal
no flames anywhere in the room; a well-ventilated laboratory
gloves and eye protection throughout
the ultraviolet lamp shielded and signed, never looked into, and switched off between readings
all organic and bromine waste to a labelled halogenated waste container, never to the sink

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions. Liquid bromine is never handled in the classroom; CSIS Appendix F advises buying bromine water ready-made when that is all that is needed. The NSW Department of Education Module 8 guide names bromine in an organic solvent or bromine water for this test and directs staff to check CSIS for cyclohexane, cyclohexene and bromine before running it with a class.

What you need

Bromine water, about 0.02 mol/L, 15 mL, bought ready-made as CSIS Appendix F advises or prepared by the technician in a fume cupboard, dispensed by the teacher
Cyclohexane, 1 mL per tube for three tubes
Cyclohexene, 1 mL per tube for three tubes
Six test tubes with well-fitting stoppers, and a rack
Aluminium foil to wrap two tubes, and a lidded box or a dark cupboard
A light source: a sunny windowsill in direct sun, or a 365 nm ultraviolet lamp with a shield and a warning sign
White tile or white card, a stopwatch, and a thermometer for each position

How to do it

Label six stoppered tubes and put 1 mL of cyclohexane in three of them and 1 mL of cyclohexene in the other three.
In the fume cupboard the teacher adds about 2 mL of bromine water to each tube and stoppers it. Record the colour of both layers at once: bromine passes into the upper organic layer, which turns orange-brown.
Wrap one cyclohexane tube and one cyclohexene tube in aluminium foil and stand them in a dark cupboard. Stand one of each in direct sunlight or under the ultraviolet lamp. Leave the third pair in the rack on the bench in ordinary room light as the middle case.
Put a thermometer beside the sunlit pair and beside the dark pair and record both temperatures at every reading, so that light and not warmth is shown to be the variable. Where the sunlit tubes warm by more than 2 degrees Celsius, stand every tube in a water bath at room temperature.
Record the colour of the organic layer in all six tubes against the white tile at 0, 1, 2, 5, 10, 20 and 30 minutes, and note the time at which any layer loses its colour.
After the 30-minute reading, take the stoppered tubes to the fume cupboard, where the teacher unstoppers them and empties them into the labelled halogenated waste container.
Repeat the whole set three times, so every one of the six conditions has three trials, and report the range of decolourising times rather than a single value.
Write the equation for each reaction that took place, state the condition each one needs, and name the step in the alkane mechanism that the dark tube rules out.

What you should see

Cyclohexene decolourises the bromine rapidly in every tube, in the dark as readily as in sunlight, because bromine adds across the carbon-carbon double bond. The textbook equation is C6H10 plus Br2 gives C6H10Br2; in bromine water, as the NSW Department of Education Module 8 guide notes, water also takes part, and 1,2-dibromocyclohexane still forms but is not the main product. The foil-wrapped cyclohexane tube keeps its orange-brown colour for the full 30 minutes. The identical cyclohexane tube in strong direct sunlight or under the ultraviolet lamp loses its colour more slowly than any cyclohexene tube, because substitution is a chain reaction that light has to start: C6H12 plus Br2 gives C6H11Br plus HBr. How long it takes depends on the strength of the light, which is why the class records the time rather than predicting it; the ultraviolet lamp gives the more dependable comparison, and the bench tube in ordinary room light is the middle case. The hydrogen bromide the substitution makes dissolves in the water layer, so it is not tested for here. The pair of cyclohexane tubes is the result the whole investigation rests on: the same chemicals at the same temperature, differing only in light, behave differently, which shows that light starts the alkane reaction. This entry takes the light and dark comparison that the bromine and permanganate unsaturation test carries as one observation and makes it the measured variable, with the chain mechanism modelled.

What changes

What you change: exposure to light: dark, ordinary room light, or direct sunlight or a 365 nm ultraviolet lamp
What you measure: time for the organic layer to lose the bromine colour
What you keep the same: hydrocarbon volume, 1 mL per tube
volume and concentration of bromine water, about 2 mL of 0.02 mol/L from one batch
temperature, recorded at every position and equalised in a water bath where the sunlit tubes warm
tube size and stopper, and the depth of liquid
the same white tile and the same observer for every colour judgement
the times at which readings are taken

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Bromine decolourises any hydrocarbon, so a fading colour proves a double bond (an alkane fades it too in light; the test is only conclusive when light is controlled).
The alkane reaction is slower because alkanes are less reactive (it is slower because it needs an initiation step that light supplies; in the dark it does not go at all on this timescale).
Heat and light do the same thing here (the sunlit tube also warms, which is why the temperature is recorded and equalised; light provides the photon that splits the bromine molecule, and warmth alone does not).
Both reactions give the same product (addition puts bromine across the former double bond, while substitution replaces a hydrogen atom with a bromine atom and releases hydrogen bromide).
The colour fading means the bromine was used up completely (the colour fades as bromine is consumed, but some may remain below the eye's threshold, which is why timing to a pale end point is reported as a range).
A negative result proves no double bond is present (a hindered or slow-reacting double bond gives a false negative, which is why cyclohexene and cyclohexane are the recommended pair).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-14 CH11/12-5
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 7, which it allows in any moduleCH11/12-2 CH11/12-3 CH11/12-4
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taught; read at source in the Year 12 focus area Organic chemistry, which requires learners to explain reactivity differences between alkanes and alkenes during addition and substitution reactionsCH-12-03
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12WS-02 CH-12WS-03 CH-12WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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