In the reactions we will perform in this experiment, hexane will be used to represent the saturated hydrocarbons, cyclohexene will be used as an unsaturated hydrocarbon, and toluene, the aromatic hydrocarbon. Methanol will also be examined. You will use combustion, reactions with halogens and potassium permanganate, as well as solubility to characterize these organic compounds.
As a precaution during these experiments, you should be extremely careful since hydrocarbons are extremely flammable. The Bunsen burner, or other sources of flames, will not be used in the laboratory, unless expressly directed by the instructor.
The Bunsen burner, or other sources of flames, will not be used in the laboratory, unless expressly directed by the instructor (for the combustion part of this experiment, you will ignite your hydrocarbons using a match). All waste chemicals, both liquids or solids, will be disposed of in the appropriate waste containers.
Methyl alcohol is not a hydrocarbon, since it contains the element oxygen, in addition to its carbon and hydrogen atoms. However, alcohols are used during many chemical processes, and like the hydrocarbons, can be used as a fuel. In fact Indy-type race cars (for the Indianapolis 500 race) often use methanol (a 1-carbon alcohol) as a fuel because it burns clean and is saver than conventional fuels because it does not burn as hot.
After you do each part of this experiment, write conclusions that you can draw about each of the chemicals used. For the unknown samples, try to determine which of the four known organic chemicals it most resembles. Include these conclusions at the end of each experimental section in you lab notebook.
One of the most important uses of organic compounds, especially hydrocarbons, is combustion. This is the basis of the internal combustion engine and other heat generating sources such as using methane as the fuel for a gas furnace or water heater and propane for your home barbecue. In fact, most alkanes are so unreactive (you will observe this in today's experiment) that the only reaction that they undergo is combustion, combining with molecular oxygen to produce carbon dioxide and water.
Typical combustion reaction: 2 C4H10 + 13 O2
→ 8 CO2 + 10 H20 + heat energy
Caution: All these combution experiments should be performed
in the hood, or by placing the evaporating dish on the stainless steel lip
of the hood.
Place about 1 mL (no more) of each of the chemicals, in the order shown,
in an evaporating dish, one chemical at a time. Start the chemical burning by
carefully bringing a lighted match, or burning splint, next to the liquid surface.
Repeat this process with an equally small volume of hexane, cyclohexene,
and toluene. (Set up your notebook similar to the table below.) For each of the known chemicals used in this experiment, write complete balanced
combustion reactions. Include these reactions in your notebook.
After you characterize each these compounds, repeat this analysis with kerosene
and each of the two unknown liquids, and make observations.
Based on your observations above, what types of hydrocarbons do you think that
Kerosene, Unknown A, and Unknown B compounds belong?
CAUTION: Carefully dispense the bromine solution under the hood
(you will need about 1 mL total for your experiments) into a small test tube.
Be especially careful not to spill any of the bromine on your hands,
as bromine is an extremely strong oxidizing agent.
Add about 1 mL of the hexane, cyclohexene and toluene to three clean test
tubes. Add about 3 drops of a 5% bromine solution to each tube.
Note any initial observations and any changes, including a change in temperature.
Carefully mix the contents of the tubes, and observe what happens.
Did a reaction take place? A reaction occurs when any of the following takes
place:
Note: For this reaction, pay attention to the color of the Br2
that you are adding. If the Br2 that you are adding just appears
to get fainter in color, then no reaction occurred. If the color of the Br2
that you add disappears, then that is a color change for the Br2,
and a reaction took place between the Br2 and the organic chemical.
The Baeyer test for unsaturated hydrocarbons involves reaction with a hydrocarbon
with alkene (or alkyne) like double bonds. Even though you can draw
double bonds for aromatic hydrocarbons (benzene or toluene), these double
bonds do not react like alkene double bonds. Evidence that an alkene-like
double bound is present is the rapid dissappearance of the permanganate purple
color. Potassium permanganate is a very strong oxidizing agent and
can also oxidize other organic compounds, such as alcohols.
Add 2-3 drops of 0.5% potassium permanganate solution to about 1 mL of hexane,
cyclohexene, and toluene added to separate test tubes. Mix the solutions
and note the results, including color changes, whether the liquide mixture
remains clear, is there change in temperature, etc. Based on your
experiments, what types of bonds in hydrocarbons react with potassium
permanganate?
Using the chemical tests described above, determine whether kerosene belongs
to the alkane, alkene or aromatic hydrocarbon groups. Use Tests B and
C, reaction with bromine and with potassium permanganate. What did
you observe? Can you positively exclude, or include, any of the hydrocarbon
families with these tests using kerosene?
In this part of the experiment, you will prepare acetylene (ethyne) and test
its combustibility.
Fill a 400-mL beaker nearly full of tap water. Fill three test tubes
(18x150 mm) with water as follows: Tube #1 completely full; Tube #2 with15
mL water; and Tube #3 with 6 mL water.
Obtain a small lump of calcium carbide from the reagent bottle and drop it
into the beaker of water. Place your thumb over the full test tube,
invert it, and place the opening of the tube below the water level in the
beaker. Remove your thumb, after the tube is in the water, and collect
the acetylene glass, holding the tube over the bubbling sample. When
the tube is full of gas, and before removing the tube from the water, place
a stopper into the tube. Repeat the collection of acetylene gas for
the other two tubes, keeping the air in the tube before you collect more
acetylene gas. Stopper these tubes as well.
Test the contents of each tube as follows:
What are your observations? Why do these different tubes appear to
react differently? What other chemical is required for these reactions
to proceed besides the acetylene in the tubes? Why is the tube with
the least amount of acetylene (Tube #3) the most explosive of the three?
Water is usually always used as a solvent for most experiments in chemistry.
However, water, which is a polar molecule, and many organic compounds,
which are often not polar, at not miscible. Organic compounds which
are soluble in water usually also contain polar bonds due to the presence
of oxygen, such as the alcohols and acids. Do you expect hydrocarbons,
which contain only hydrogen and carbon to be polar?
To test the solubility of hexane, cyclohexene and toluene in water, add 1
mL (no more) of each hydrocarbon to three clean test tubes containing about
5 mL water. Shake each mixture for a few seconds, and note whether
the organic chemical dissolves in water. If the chemical does not dissolve,
is it more or less dense than water?
Test the miscibility of each hydrocarbon in the other two hydrocarbons. To
do this experiment, add about 1 mL of hexane to two clean and dry test tubes.
Then, add about 1 mL of the cyclohexene and toluene to each tube.
Shake as before, and determine if the chemicals are miscible. If
they are not miscible, which chemical is less dense? How could you
determine this?
Hydrocarbons:
Other Reagents or chemicals:
Go To Experiment:
1 2 3
4 5 6
7 8 9
10
Copyright © Donald L.
Robertson (Modified:
01/26/2010)
Methanol
CH4O
Hexane
C6H14
Cyclohexene
C6H10
Toluene
C7H8
Kerosene-like
Unknown A
Unknown B
B. Reaction with Bromine
C. Reaction with Potassium Permanganate
D. Kerosene
E. Acetylene (optional, at the discretion of the instructor, or a class demo)
F. Solubility Tests
Reagents required for this experiment:
Return to Chem102 Experiments Index