Daniel Hu

BROMINE WATER PRAC REPORT Introduction:

• Because almost all alkenes add bromine, this reaction is often used as a test for the presence of unsaturation (multiple bonds). • Used to differentiate between an alkane and alkene/alkyne (saturated hydrocarbon/unsaturated hydrocarbon). • If the deep red/brown colour of Br2 disappears, a double or triple bond is present • Bromine water is an equilibrium mixture of bromine and hypobromous acid

Aim

To compare the reactivity of cyclohexene with the corresponding alkane, cyclohexane, in bromine water

Hypothesis

Cyclohexane will react slowly with bromine water, and no decolourisation of bromine water will occur. Cyclohexene will react quickly and spontaneously with bromine water, and bromine water will be decolourised from brown to colourless.

Materials:

• Cyclohexene • Cyclohexane • Bromine water in dropper bottles • Test-tube rack • Droppers • 2 test tubes • Fume cupboard • Darkened, well-ventilated room

Risk Assessment:

Compound Hazard Precautions Reasons for precaution Bromine (aq) Skin/eye irritant Handling bromine Can release toxic Lung sensitiser water should be done fumes, and strong in a fume cupboard or odours so caution performed in a well- must be taken ventilated area, wearing appropriate Prevent toxic safety gear such as substances being goggles, gloves and lab inhaled, ingested, coat. In the event of splashing into eyes eye or skin contact, the or contacting the area should be flushed skin with lots of water. Do not pour down sink, dispose in inorganic waste bottle Daniel Hu

Toxic substances will enter the environment and cause food poisoning. Cyclohexane, Skin/eye irritant The same precautions Can release toxic Cyclohexene Flammable should be followed for fumes, strong Toxic the safe handling of odours, may cyclohexane and combust cyclohexene, as well as keeping them away from sources of ignition since they are both volatile and flammable (keeping a working fire extinguisher close by). Dispose in organic waste bottle

Method:

1. Place two clean and dry small test tubes in a test-tube rack. 2. 2ml of cyclohexane was added to one test tube. 3. 2ml of cyclohexene was added to the other test tube. 4. In the absence of UV light, 1ml of bromine water was added to each liquid. Note the initial colour. 5. Gently swirl/stir each tube to the same extent to mix the chemicals. Place the tubes back into the rack and allow the layers to separate. 6. Stir/mix each thoroughly. The test tubes were left to settle, and then observed.

Justification of Method

- Cyclohexene was used instead of ethylene or propene, as C1 to C4 are gases at room temperature. - Cyclohexene was used instead of hexene as cyclic hydrocarbons are more stable, less toxic than their linear counterparts - Cyclohexene is a clear, colourless liquid and bromine water is an aqueous brown/yellow solution. As a result, possible colour changes can be observed. - Experiment was performed in absence of UV light as exposure to UV light will cause a substitution reaction to occur between an alkane and Br2, where a bromine atom will take the place of a hydrogen atom. This will also cause the bromine water to decolourise, rendering the experiment invalid.

Variables:

Dependent: the reactivity of the substances (alkane or alkene)

Independent: Observation of colour of the mixture after mixing

Controlled: Daniel Hu

- Amount of bromine water/cyclohexane/cyclohexene - Exposure to light - Non-exposure to sunlight - Temperature - Air pressure - Size of test tube

Results:

- It was observed that the cyclohexene immediately decolourised the bromine water from red/brown to colourless, while the cyclohexane remained red/brown. This showed which hydrocarbon decolourised the bromine water, allowing them to be distinguished from each other. (differentiating between alkane and alkene, allowing us to determine whether a double bond is present or not). The cyclohexene reacted with the bromine water, forming 1,2-dibromocyclohexane.

Effect of bromine water on hydrocarbons

Hydrocarbon Colour of bromine water before Colour of bromine water after hydrocarbon hydrocarbon added added

Hexane Red/brown Stays red/brown

Hexene Red/brown Went clear/colourless

Cyclohexane Red/brown Stays red/brown

Cyclohexene Red/brown Went clear/colourless

Balanced equation for the equilibrium reaction where bromine reacts with water is:

Br2 (aq) + H2O (l) → HOBr (aq) + H+ (aq) + Br- (aq) (reversible reaction)

Labelled Diagram:

Daniel Hu

Discussion:

The experiment performed was mostly valid because it used accepted scientific methodology. For instance, all variables excluding the independent and dependent variables were kept constant (controlled) such as using the same volume of cyclohexane/cyclohexene, same volume of Br2, same concentration of Br2, same exposure to light, no exposure to sunlight, same size of test tubes etc. Furthermore, the experiment designed was also appropriate to test the hypothesis, where we could test for the presence of double/triple bonds. On the other hand, the cycloalkane may have reacted slightly because of the slight presence of UV radiation which can cause slow substitution reactions. However, a control wasn’t used in this experiment, which decreases its validity. To address this issue, a container of bromine water with no added chemicals can be used as a control to see if the bromine water would change colour without the addition of cyclohexene. Another way to improve the validity of the investigation is to confirm results using acidified potassium permanganate solution (KMnO4).

The experiment performed was definitely reliable since it was performed more than once, and similar results were achieved each time.

The results achieved were also accurate since the colour changes could be easily observed, especially considering how cyclohexane/cyclohexene are both colourless liquids. Furthermore, the same quantities of chemicals were used in each test tube to allow for accurate comparison. Care was also taken to avoid using excess bromine water which could mask the colour change.

The experiment was also relevant because by performing the investigation, I understood the reactions the different hydrocarbon groups undergo, for instance alkenes undergo addition reactions, while alkanes undergo substitution reactions. I realised that alkenes are far more reactive than alkanes from performing this experiment, and understood that an alkene should be used as the raw material to make a polymer due to its reactivity.

Conclusion:

The results supported the hypothesis. The alkene, cyclohexene reacted quickly with bromine water, changing colour from red/brown to colourless. Decolourisation of the bromine water occurred. The alkane, cyclohexane did not react with bromine water.