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 sp3 hybridized orbitals
only. The compounds are saturated with hydrogens and show very few chemical reactions. They burn to CO2
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).
ethylene
H
3
OH
OH
C
C
H
H
H
C
H
3 H
+ 2KMnO4 + 4H2O
C
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
Br
C
C
Br
Br
Br
H
C
H
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
C
C
H
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: