Experiment 4. Part B: Properties of Alkanes & Alkenes.

Reference: Ege, Chapters 5, 8

Safety Precautions

Bromine is highly volatile, toxic, and causes severe skin burns. All work with bromine or bromine solutions must be performed in the fume hood.

There are a number of simple tests that may be used to differentiate alkanes from alkenes or to detect the presence of alkenes in alkanes. These tests are based upon the reactivity of alkenes with a variety of reagents to which the alkanes are insensitive. Use cyclohexene as a typical alkene, ligroin (petroleum ether, mostly hexanes) boiling at 66-75 ^oC as a typical mixture of alkanes, and unpurified ligroin as an impure alkane. Write equations for all the reactions that occur and explain how alkanes may be differentiated from alkenes with the use of these tests.

WASTE DISPOSAL: All contents of your test tubes from these test reactions go into "HALOGENATED organic waste". Remember to rinse the residues from your tubes with a little acetone into the same container.

Test Experiments

1. Bromine Water
Measure 3 mL of a 3% aqueous solution of bromine into each of three test tubes and add 1 mL portions of purified ligroin to two of the tubes and 1 mL of cyclohexene to the third. Stopper each tube, shake, and record what you see. Place one of the ligroin-containing tubes in your locker out of the light and expose the other to bright sunlight or hold it close to a light bulb. When a change is noted in the sample exposed to light, compare its appearance with that of the mixture kept in the dark.

2. Bromine in Carbon Tetrachloride
Treat 1 mL samples of purified ligroin, unpurified ligroin, and cyclohexene with 5-6 drops of a 3% solution of bromine in carbon tetrachloride. If decolorization occurs, test for hydrogen bromide with wet litmus. If, however, the bromine color persists illuminate the solution and then, if a reaction occurs, test as before for hydrogen bromide.

3. Acid Permanganate Test
To 1 mL portions of purified ligroin, unpurified ligroin, and cyclohexene in separate test tubes add the permanganate solution dropwise until the purple color persists. Do you observe a difference in behaviour among the substances under test? Account for the differences if any. The test solution is made up of potassium permanganate (1%) in sulfuric acid (10%). (CAUTION: strongly corrosive!)

4. Tests for Unsaturation
Determine which of the following hydrocarbons are saturated and which are unsaturated or contain unsaturated material. Use 0.5 mL to 1.0 mL samples of the trial compounds. Use any of the above tests that seem appropriate.

Pinene, the principal constituent of turpentine oil
Gasoline
Cyclohexane
Cholesterol (make a 10% solution in CCl₄ for the tests).

Food for Thought

1. How would l-hexyne behave in each of the above tests?

2. Would it be possible to differentiate 1-hexyne from cyclohexene by the use of these chemical tests? If your answer is in the negative, think of a test that might be used for this purpose.

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