Chemical Tests for Alkanes, Alkenes, and Aromatic
Compounds
Introduction
There are four types of hydrocarbons: alkanes, alkenes, alkynes, and aromatic compounds,
each type with different chemical properties. These properties can be used to classify unknown
compounds. In this laboratory you will use some simple chemical tests to determine in which of
these classes of hydrocarbon an unknown sample belongs. For simplicity's sake you will not test for
the presence of alkynes in this laboratory, but appropriate test for these compounds do exist. You
will run the tests for alkanes, alkenes, and aromatics on an unknown compound, along with some
known representative compounds, to observe the effects of different reagents on these compounds.
Alkanes are a rather unreactive family of compounds. They are capable of participating in
only two common types of chemical reaction: combustible and free radical substitution reactions.
Combustion (i. e. burning of fossil fuels) is obviously of great economic importance. Alkanes
combust with a relatively clean flame. Alkenes, alkynes, and aromatic compounds combust with
significant production of carbon monoxide and elemental carbon (soot). Free radical substitution
consists of the replacement of hydrogen atoms with halogen atoms. These reactions occur only when
the reactants are heated or exposed to ultraviolet light. This reaction can be run with fluorine,
chlorine and bromine. The reaction with chlorine is by far the most common since fluorine is much
too reactive and dangerous and the reaction with bromine requires much stronger heat or light. The
radical substitutions with chlorine or bromine occur very, very slowly in a room with artificial
illumination. These reactions can be summarized as follows:
Combustion:
Alkane + O2
Substitution:
Alkane + Cl2
High Temperature
heat or light
CO2 + H2O + heat
Alkyl chloride + HCl
Alkenes are considered the most reactive of the hydrocarbon classes. They can undergo the
same reactions as alkanes in the saturated parts of their structure, but in addition, undergo a wide
variety of addition and oxidation reactions. The bond electrons readily react with reagents which
are electron poor, such as electrophiles and oxidants. The reactions you will study in this laboratory
are as follows:
Addition of bromine:
Alkene + Br2
Alkyl dibromide
Note: No heat or ultraviolet light is required for this reaction. The orange/brown Br2 is
dissolved in dichloromethane. The reaction proceeds with a loss of bromine color, which is a
positive test for the presence of a double bond. This reaction occurs quickly.
Caution: Even in the concentrations used, bromine is highly toxic and can cause burns.
Saturated sodium Thiosulfate solution is provided to neutralize Br2 that has been spilled on skin.
Permanganate Oxidation: (Von Bayer test)
3 Alkene + 2 KMnO4 + 4 H2O
3 Glycol + 2 MnO2 + 2 KOH
Note: In the course of this reaction the purple color of the aqueous potassium
permanganate is replaced by solid brown manganese (IV) oxide (MnO2). The color change is a
Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 1
positive test for the presence of a double bond. Although this is a good test to differentiate
alkanes and aromatics from alkenes, other materials besides alkenes react with aqueous KMnO4.
Addition of Hydrogen Sulfate:
Alkene + H2SO4
Alkyl hydrogen sulfate
Note: Alkenes react with cold concentrated sulfuric acid and appear to go into solution.
Alkanes and most aromatics do not react. Alkenes can also thicken or change color, which also
indicates a reaction.
Aromatic compounds, like alkanes, are relatively unreactive. They burn but, like alkenes,
they have a very sooty flame, even sootier than alkenes. Aromatics also undergo electrophilic
aromatic substitution reactions. Although aromatics are highly unsaturated, they do not participate in
addition reactions like the alkenes, since the loss of double bonds would lead to a loss of resonance
and aromatic character. These aromatic substitution reactions generally occur in the presence of
Lewis acid catalysts. The aromatic reaction you will study in this laboratory is as follows
Friedel-Crafts Alkylation:
+
R
AlCl3
R
+
Cl
H
Cl
Note: The test you will perform in the lab, though more complex than that shown above,
is basically the same reaction. The final product is a large molecule with a characteristic color.
The production of a color, as listed in Table 1, is evidence of an aromatic compound
CAUTION: Aluminum chloride, in its dry or anhydrous state, is extremely reactive and
produces hydrochloric acid or hydrogen chloride gas when exposed to water or water vapor.
This reaction should be performed in the hood.
Table 1: Colors Produced by Aromatic Compounds in Friedel-Crafts Test
Compound
Structure
Color produced in
Friedel-Crafts Test
Benzene
Orange or red
Naphthalene
Blue or purple
Anthracene
Green
Phenanthrene
Purple
Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 2
Procedure
For the following tests, use samples of cyclohexene, cyclohexane (or petroleum ether),
toluene, 1-hexene, -pinene (or turpentine) and an unknown sample. As you perform the tests,
record the results on the data sheet.
Ignition (Combustion) Test: For liquids, place 3-4 drops of material on a watch glass, ignite
the liquid and observe whether a sooty flame results. Many samples will be difficult to ignite with a
match, and you will find that a Bunsen burner will reliably ignite all samples that you are testing. For
solids place a small amount of sample on a spatula and place it in the flame of the Bunsen burner.
Again observe and record results.
CAUTION: Keep all other flammable substances well away from the Bunsen burner Perform this
test in the hood.
Bromine Test (to be performed in the HOOD): Place 2 drops of the liquid (or about 15 mg
of solid) in a 10 x 75 mm test tube and add to this 0.5 mL of dichloromethane, CH2Cl2. Next, add 5
to 10 drops, with shaking, of a 2 % solution of Br2 dissolved in dichloromethane. The disappearance
of the orange bromine color as the reagent is added is a positive test for the presence of the double
bond of an alkene.
Potassium Permanganate (Von Bayer) Test: On a spot plate place 0.5 mL of acetone and 2
drops (or about 15 mg of solid). Next, add drop wise with stirring two to three drops of an aqueous
1 % KMnO4 solution. The test is positive if the purple KMnO4 color disappears and is replaced by a
brown precipitate of manganese(IV) oxide. Do not count changes that occur after several minutes.
Run a blank test with acetone to insure that it is not interfering with the test.
Sulfuric Acid Solubility Test: Place one mL of cold concentrated sulfuric acid in a dry
10 x 75 mm test tube and add one drop of the liquid sample from a Pasteur pipet (or a few crystals of
solid) directly into the sulfuric acid. Swirl to ensure mixing and observe. Concentration gradients
(wavy lines in the mixture) and disappearance of liquid or solid indicate solubility. A color change
or change in viscosity of the sulfuric acid is also evidence of reaction.
CAUTION: Concentrated sulfuric acid is extremely corrosive and reacts violently with water.
Friedel-Crafts Test (to be performed in the HOOD.): Place 100 mg of AlCl3 in a test tube.
Heat the tube gently with a Bunsen burner to sublime some of the AlCl3 on to the side of the tube.
Allow the tube to cool to room temperature. Mix one drop (40-50 mg of a solid) of the sample to be
tested with 3-4 drops of dichloromethane. Next, carefully add this mixture to the tube, allowing it to
pass over the sublimed AlCl3. Watch for the formation of color where the reagents come into contact
(see the colors listed in Table 1 for aromatic classification). Production of a color is a positive test
for the presence of aromatic compounds.
Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 3
Report
Name ____________________________________
Chemical Tests for Alkanes, Alkenes and Aromatic Compounds
Part A − Combustion Test
Compound
Type of compound
Observations
cyclohexane
cyclohexene
1-hexene
toluene
-pinene
unknown (# ______)
Part B − Sulfuric Acid Test
Compound
Type of compound
Observations
cyclohexane
cyclohexene
1-hexene
toluene
-pinene
unknown (# ______)
Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 4
Part C − Potassium Permanganate Test
Compound
Type of compound
Observations
cyclohexane
cyclohexene
1-hexene
toluene
-pinene
unknown (# ______)
Part D − Bromine Test
Compound
Type of compound
Observations
cyclohexane
cyclohexene
1-hexene
toluene
-pinene
unknown (# ______)
Part E − Friedel-Crafts Test
Compound
Type of compound
Observations
cyclohexane
cyclohexene
1-hexene
toluene
-pinene
unknown (# ______)
Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 5
What can you determine about your unknown? Your unknown is probably not one of the
samples tested, but based on the five tests performed, classify it as an alkane, alkene, or aromatic
compound and give reasons for your choice.
Problems:
1. Complete and balance the following: (If no reaction occurs, just indicate by N. R.)
a.
CH2=CH2 +
b. C6H6 +
KMnO4 +
H2O
O2
+
Br2
c.
+
KMnO4
+
H2O
d.
+
H2SO4
e.
2. An unknown compound was known to be benzene (C6H6) or cyclohexane or cyclohexene. It
burned with a very sooty flame and when bromine in dichloromethane was added to the sample, the
brown color remained. What was the unknown? Explain why you made the choice you did.
Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 6