Organic Hydrocarbons

ORGANIC CHEMISTRY EXPERINIENT--HYDROCARBONS Organic chemistry is the study of carbon compounds. Because this branch of chemistry covers such a large number of compounds, it is broken down into many subdivisions. One of these groups of compounds is the hydrocarbons. These are organic compounds made up of only hydrogen and carbon. In this experiment we will investigate the properties of four families of hydrocarbons. The first family is the alkanes. These are compounds made up of carbons using sp 3 hybridized orbitals only. The compounds are saturated with hydrogens and show very few chemical reactions. They burn to CO 2 and H2O, as do almost all organic compounds, and undergo free radical substitution reactions with Br2 when exposed to ultraviolet or sunlight. (Try this in sunlight.) Examples of alkanes are:

H

H

H

C

C

H

H

H

H

H

H

H

H

C

C

C

C

H

H

H

H

ethane

H

butane

The alkenes contain a carbon-carbon double bond. The carbons use sp2 hybridization on the double bond carbons. The double bond is very reactive and undergoes rapid reactions called addition. The reactions cause loss of the double bond and addition of two atoms to these carbons. Two reagents, which react with the double bond, are bromine and potassium permanganate (Baeyer Test). H

H

+ Br2

C

H

H

ethylene

3

H C

H

3 H

+ 2KMnO4 + 4H2O

C

Br

C

C

H

H

H C

H

Br

H

OH

OH

C

C

H

H

H

H

+ 2 MnO2 + 2 KOH

Alkynes contain a carbon-carbon triple bond. The carbons use sp hybridized orbitals and like the alkenes, undergo rapid bromine addition reactions. The reactions go through two steps: Br H

C

C

H

C

+ 2 Br2 H

acetylene (ethyne)

Br H

C H

Br

Br

C

C

Br

Br

H

The final family of hydrocarbons we will study are the aromatics. These compounds contain the highly stable benzene ring as a part of their structure. Examples.

H H

H

C C

C

C H

CH3 C C

H

H

benzene

toluene

All of the carbons in the benzene ring use sp2 hybridized orbitals. The reactions of aromatic compounds are called substitution and require a catalyst and prolonged heating. An example is: FeBr3 + Br2

Br

+ HBr

Note the second product HBr gas is formed during substitution unlike addition where only one product is formed. Most aromatics burn poorly due to the stable benzene ring and give an unusual sooty flame. This is an easy way to test for the aromatic ring. Experimental Procedures: a. Bromine Addition: Dissolve 5 drops of the test compound in 1 ml of dichloromethane. Add to this solution, dropwise, a 2% Br2/CH2Cl2 solution. Mix and observe after each drop, adding a total of 3 drops. Look for loss of the orange color. Expose those that do not change color to sunlight for a few minutes. What is the equation for this photochemical reaction? Test Compounds: Hexane, 1-hexene, toluene b. Baeyer Test: To 1 ml of 0.5% potassium permanganate solution, add 5 drops of test compound. Shake the test tube well for 1-2 minutes and note the results. Try the same 3 test compounds from part A. A positive test is loss of the purple color and formation of a brown precipitate. c. Combustion Test: Place 1 drop of the reagent on a metal spatula and ignite with a match. Note the appearance of the flame. Try the same 3 compounds. d. Boiling Point Determination: Fill a 50 ml beaker half full of mineral oil. (Be sure the beaker is absolutely dry before using or the oil will splatter). Securely suspend a thermometer with a small test tube attached as shown in the drawing. Place 1/2 ml (10 drops) of unknown in the test tube and inset a 10 cm capillary tube, open end down, into the test tube. Heat the oil slowly while stirring. (The oil should not boil or splatter) When a rapid and continuous stream of bubbles escapes the capillary tube, discontinue heating but continue to stir the liquid. When bubbles cease to escape and just begin to enter the capillary tube, record the temperature, the boiling point of the liquid. Try this method on toluene and compare to the known boiling point.

e.

Unknown Hydrocarbon: Run all of the above tests and take the boiling point of your unknown. Identify your unknown from the following list of hydrocarbons. Test tube thermometer

Capillary Tube Mineral oil

Sample

Boiling Point Determination Alkane Boiling Points n-pentane n-hexane cyclohexane n-heptane n-octane n-nonane

Alkene Boiling Points 36° 69° 81° 98° 126° 151°

1-pentene 1-hexene cyclohexene 1-heptene 1-octene 1-nonene

Aromatic Boiling Points

Alkynes Boiling Points

benzene 80° toluene 111° ethyl benzene 136° o-xylene 144° mesitylene 165° (1,3,5-trimethylbenzene)

1-pentyne 1-hexyne 1-heptyne

30° 63.5° 83° 93° 122.5° 146°

40° 72° 100°

Name:___________________________________ HYDROCARBONS REPORT SHEET 1. Bromine Addition: a. Test compounds giving a positive Br2 test are: b. Give the equation for any reactions observed. (Use structural formulas for organic compounds) 2. Baeyer Test: a. Test compounds giving a positive Baeyer test are: b. Give the equation for any reactions observed. (Use structural formulas for organic compounds) 3. Combustion Test: a. Test compounds giving a positive aromatic test: 4. Draw the structure of the following compounds: cyclohexane

n-hexane

1-pentyne

toluene

5. Unknown Number:__________ Boiling Point:____________ Test Results (Give + or -) Br2 _____ Baeyer _____ Combustion _____

1-heptene

mesitylene

Name of Unknown Hydrocarbon: __________________________ Structural Formula of unknown:

Name:_________________________________ PRELABORATORY ASSIGNMENT--HYDROCARBONS 1.

How does an alkane differ from an alkene?

2.

What 2 tests are used to test for a double or triple bond?

3.

What is the test for an aromatic ring?

4.

A double bond test with Br2/CH2Cl2 gives bubbles of a gas. What does this tell you about the test?

5.

A compound on burning gives a sooty flame. The boiling point test gives a boiling point of 100±2°. The compound is: