Chemistry 131 Laboratory

Experiment 4: Study and Identification of Hydrocarbons

Look over Introduction the for this lab.
Complete the pre-lab questions for this lab.
Look at the INSTRUCTIONS

Laboratory

Introduction

Hydrocarbons are compounds which contain only carbon and hydrogen. Saturated hydrocarbons are those in which the carbon atoms are "saturated" with hydrogen, that is, they are bonded to as many hydrogens as possible. Unsaturated hydrocarbons on the other hand have at least one double bond, and so the molecule is no longer "saturated" with hydrogen (there are now less hydrogens per carbon atom because of the additional carbon carbon bond). The melting point of hydrocarbons increases with an increasing number of carbons in the molecule. This is generally true because for bigger heavier molecules more energy is required to move them from the liquid to the gas phase. Because of the better packing arrangement of saturated hydrocarbons in the liquid state relative to that of unsaturated hydrocarbons, for molecules with the same number of carbons, the unsaturated molecule will have a higher boiling point.

The prefix on the name of the hydrocarbon will tell you the number of carbon atoms in the molecule. Prefixes for hydrocarbons containing up to 6 carbon molecules are given in the table below. Saturated hydrocarbons (i.e., those with no double bonds) are referred to as alkanes, where the -me denotes saturation. Examples of alkanes are methane, ethane and propane. Alkenes are unsaturated hydrocarbons having one or more double bonds. Ethene (commonly known as ethylene) and propene (commonly known as propylene) are examples of alkenes. Alkynes refer to hydrocarbons having at least one triple bond. Ethyne (commonly called acetylene) is an example of an alkyne.

# of Carbons	Prefix	Alkane*	Formula
1	meth-	methane	CH₄
2	eth-	ethane	CH₃CH₃ (C₂H₆)
3	prop-	propane	CH₃CH₂CH₃ (C₃H₈)
4	but-	butane	CH₃CH₂CH₂CH₃ (C₄H₁₀)
5	pent-	pentane	CH₃CH₂CH₂CH₂CH₃ (C₅H₁₂)
6	hex-	hexane	CH₃CH₂CH₂CH₂CH₂CH₃ (C₆H₁₄)

* Formulae for straight chain alkanes are shown

Alkanes, alkenes, and alkynes can all be categorized under the name aliphatic molecules. Aromatic hydrocarbons on the other hand are a different type of highly unsaturated molecule having benzene as their parent compound. The reactivities of these types of molecules are different, and the organic chemist can take advantage of these differences to help in the identification of unknown hydrocarbons.

In this experiment you will be studying differences between saturated and unsaturated and aromatic hydrocarbons. In the first part of this experiment, you will be testing the reaction of know compounds with bromine. Bromine reacts readily with alkenes at room temperature in the absence of light to form the corresponding dibromide compound. This reaction can be followed by the disappearance of the brown bromine color from the reaction medium, as long as the alkene is in excess.

Alkanes on the other hand show no appreciable reaction under these conditions. If
however the reaction mixture is exposed to sunlight, the reaction will proceed with
production of hydrogen bromide (HBr) as a side product.

The presence of hydrogen bromide can be tested for by placing a piece of moist blue litmus paper above the reaction vessel. Evolution of hydrogen bromide will turn the litmus paper red. This can be used as an additional test for alkanes. Aromatic hydrocarbons will not react with bromine either in the dark, or in sunlight.

A second test that you will be doing involves the reaction of permanganate ion (MnO4-) with alkenes to form glycols or other oxidation products. This is referred to as Baeyer's test for alkenes. Neither alkanes nor aromatic hydrocarbons show reaction with permanganate ion. The reaction can be followed by the disappearance of the purple color of the permanganate ion. The resulting solution will appear clear or brown in color. This test will also show positive results for other types of molecules such as alcohols.

In this lab you will be performing these tests on an alkane, hexane, and alkene, hexene, and on an aromatic hydrocarbon, toluene. Carefully record in a table the results in a table in your lab book. You will then use these tests to determine whether each of dime unknowns is an alkane, alkene or an aromatic hydrocarbon.

Instructions

Reaction of hydrocarbons with bromine

1. Place four small clean dry test tubes into your test tube rack. Measure 1 ml of water into one of the test tubes. Set this tube in your test tube rack.

2. Referring to this as a guide use a disposable pipet and place approximately 1 ml of the hexane into the second tube, one ml of hexene into the third, and I ml of benzene into the fourth.

3. Spill the water out of the first tube and set it to dry.

4. The first part of this procedure requires that the samples be tested for reaction of bromine in the dark. To effect this, wrap each of the tubes completely in aluminum foil, except for the opening. Reserve small pieces of aluminum foil for covering the tops of the tubes when addition of the bromine is complete.

5. In the Hood, add 3 drops of the bromine solution into each of the three samples, taking care to not breath the bromine vapor or spill the bromine on your skin. Cover the tops of each tube with the foil.

6. Check each tube after approximately 1 minute. Any sample that still shows no change after 1 minute should be exposed to bright light for several additional minutes.

7. Note all changes and when they occur in a table in your notebook. Write in your notebook the reaction of Br₂ with each of these substances.

Reaction of hydrocarbons with permanganate ion

8. Using three clean dry test tubes add to the first 1 ml of hexane, to the second, 1 ml of hexene, and to the third, 1 ml of toluene.

9. To each of these test tubes add 2 drops of the potassium permanganate solution. If the reaction occurs you will notice a rapid disappearance of the purple color of the permanganate ion.

10. Note the results of this test in your table, and write the reaction of permanganate ion with the three substances.

Testing of the unknowns

11. Using clean dry test tubes, repeat the above procedures with the three unknowns, without attempting to write reactions.

12. Make a table similar to the one you used for the initial testing. On the basis of the results of your tests, identify the three unknowns as either a saturated hydrocarbon (alkane), an unsaturated hydrocarbon (alkene), or an aromatic hydrocarbon.

Pre-lab Questions

Use Your Text As A Resource

1)If you had found your unknowns to be two alkanes, what physical property could you use to determine which had the greater number of carbons (greater molecular mass)?

2.)Explain why the melting points for branched alkanes would be lower than those for straight chain alkanes having the same number of hydrocarbons (hint: why is the melting point for unsaturated hydrocarbons lower than that of saturated hydrocarbons having the same number of carbons?). Take as an example the melting points for hexane (-134 C), hexene (-95 C), and 2,2-dimethylbutane (-128.5 C). Draw these structures and explain the difference in melting points.

3.)If you wanted to differentiate between an alcohol and an alkene, could you use Baeyer's test? Explain your answer. What if you had an aromatic hydrocarbon and an alcohol?

4.)Draw the structures of benzene, heptane, heptene, pentene, butane, and ethyne.
5.)Which of the above molecules is aliphatic? Aromatic?

Department of Chemistry

Capital University

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