How can we distinguish between alkenes and alkanes?

Teacher’s Notes

Aims of sheet and practicalThe aims for pupils are to: -

Follow simple instructions. Distinguish between a good test and a poor test. Learn the test to distinguish between alkanes and alkenes.

Apparatus and chemicalsComboplate

Micropipettes.Microspatulas (for stirring).Bromine water.Either cyclohexane or hexane.Either cyclohexene or hex-1-ene.

SAFETY & RISK ASSESSMENTBromine waterThe bromine water is corrosive and contact should be avoided.If any is spilt on the skin it should be washed off immediately with plenty of water.It is desirable to have some sodium thiosulphate ready as it will react with the bromine.

If swallowed:- If any bromine water gets into the mouth wash out the mouth thoroughly with water and give plenty of water to drink.Seek medical attention.

If liquid gets in eyes:- If there is any contact with the eyes and irrigate thoroughly with water for at least 10 minutes.If discomfort persists seek medical attention.

If spilt on skin or clothes:- If the bromine water gets on to skin wash it off thoroughly with water.Remove contaminated clothing and washed before reuse.In severe cases seek medical attention.

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GRIFFIN EDUCATION working with EDU-LAB and The RADMASTE Centre

UNIT C2 TOPIC 5 SYNTHESIS

CyclohexaneThe cyclohexane is a fire hazard and is extremely flammable, therefore, it should not beused near any naked flames. Cyclohexane is irritating to skin, eyes and the respiratory system and appropriate care should be taken when using it.

If swallowed:- If any gets into the mouth, wash out thoroughly with water and give plenty of water to drink. Seek medical attention.

If liquid gets in eyes:- If any gets in the eyes irrigate thoroughly with water for at least 10 minutes.

If spilt on skin or clothes:- If any gets onto the skin wash off thoroughly with water.Remove contaminated clothing and wash before reuse.

The same precautions apply to cyclohexene. The products of the reaction should be washed down the sink (fume cupboard sink if possible) with plenty of water.

Comments on the sheet The data on the burning of the hydrocarbons is there to help students appreciate

that addition of bromine water is a good test. The equation used is the relatively easy ethene and bromine.

C2H4 + Br2 C2H4Br2

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Method

Using bromine water1. Using the micropipette put 5 drops of cyclohexane into well A1.2. Put 5 drops of cyclohexene into well A3.3. Add 5 drops of bromine water to both wells A1 and A3.4. Stir both wells using the thin end of a microspatula.5. Write down your observations in the table below:-

Results

Conclusions1. How can alkenes be distinguished from alkanes using bromine water?______________________________________________________________________________________________________________________________________

2. In another experiment a student burned the alkane and alkene separately and obtained the following results:-

Using your results and bromine water, which test would you use to distinguishbetween alkanes and alkenes?__________________________________________________________________

Explain why______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

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3. Complete the equation below for the reaction between ethene (the simplest alkene)and bromine.

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