MODULE 1

INTRODUCTION

The simplest organic compounds are hydrocarbons as they contain only hydrogen and carbon. Hydrocarbons in which all the carbon–carbon bonds are single ones are alkanes while those which contain a carbon–carbon double bond are alkenes. The reactivity of alkenes and alkanes is quite different. Alkanes react slowly by the process of substitution while alkenes react rapidly, usually by the process of addition.

EXPERIMENT: COMPARING REACTIVITIES OF ALKANES AND ALKENES

Syllabus reference 9.2.1

WORKSHEET2MODULE 1

Copyright © 2007 McGraw-Hill Australia CONQUERINGCHEMISTRY HSC MODULE 1 WS 2

TaskYou are to plan and perform an experiment to compare the reactivities of an alkane and an alkene with bromine water. You may wish to confirm your results by testing the hydrocarbons with acidified potassium permanganate which does not react with alkanes but is decolourised by alkenes.

You will be provided with two sample bottles labelled A and B. One bottle will contain an alkane such as hexane and the other will contain an alkene such as hexene. You will also be provided with bromine water and 0.01 mol/L potassium permanganate (KMnO4) solution which should be acidified with 2 mol/L H2SO4.

HINTS1 Aim: Write an aim for this experiment.2 Equipment: Think about how you might test the samples and what equipment you will need. Write a

list of the equipment.3 Tests should be performed in subdued light as the presence of sunlight can cause an alkane to react.4 Only a small amount (1 mL) of bromine water or acidified potassium permanganate solution is

needed.5 Only a small amount (1 mL) of alkane and alkene is needed for the tests.6 To acidify the potassium permanganate solution add 2 mL of 2 mol/L H2SO4 to 4 mL of 0.01 mol/L

KMnO4.7 Procedure: Write the steps you will need to follow in conducting the tests. Remember you will need

to control your variables.8 The bromine water and potassium permanganate solution will form a separate layer and the test

tube will need to be shaken gently to mix the layers. Allow 5 minutes for reaction.

9 Safety: You should consult the relevant safety sheets or manuals regarding precautions to be taken when handling and disposing of alkanes and alkenes.

SAFETY: Bromine water is poisonous and corrosive and should be handled with care, as should the acidified potassium permanganate solution. If contact with skin occurs wash immediately with plenty of water.

10 Have your equipment list, procedure and safety precautions checked by your teacher before proceeding.

11 Results: a Draw up a results table similar to the one below to record your results.

You may wish to consult your chemistry textbook to help you decide which of the samples is the alkane and which is the alkene.

b Write a balanced equation for the positive reaction with bromine water giving the name and structural formula of each organic reactant and product.

12 Conclusion: Write a generalised statement to compare the chemical reactivities of alkanes and alkenes.

Copyright © 2007 McGraw-Hill Australia CONQUERINGCHEMISTRY HSC MODULE 1 WS 2

SAMPLE

A

B

REACTION WITH Br2 WATER

REACTION WITH ACIDIFIED KMnO4

HYDROCARBON (ALKANE/ALKENE)

NOTE TO TEACHER• Bromine water may be prepared by dissolving 1.95g Br2 in 50 mL water.• Alternatively if using an ampoule of liquid bromine, refrigerate the ampoule then, wearing

rubber gloves and working in a fume cupboard, place the ampoule in 500 mL of distilled water and break it. This bromine water may be stored in a stoppered bottle.

• Cyclohexane and cyclohexene work just as well as hexane and hexane.• Encourage students to use only small amounts of the hydrocarbons to minimise waste.• Be sure to have special waste containers and reinforce that the solution must not be poured

down the sink.• Make sure the area where the students are working is well ventilated.