CHEM 2423 Properties of Hydrocarbons
Dr. Pahlavan
EXPERIMENT 8 – Reactions of Hydrocarbons
Properties and Identification of Hydrocarbons
Purpose:
a) To identify saturated and unsaturated hydrocarbons using properties and reactions.
b) Study substitution and addition reactions.
Equipments
test tube rack (1)
test tube holder (2)
100 mL beaker
medium test tube (6)
Materials
cyclohexene
toluene
0.50 % KMnO4
10% Na2CO3
n-hexane
conc. H2SO4
Br2/CCl4 or Br2/CH2Cl2
unknowns hydrocarbons
Discussion
Hydrocarbons, compounds which contain only carbon and hydrogen, can be classified into several types,
depending on their structure. Aliphatic hydrocarbons are divided into three classes: alkanes have only single
bonds, and are said to be saturated; alkenes and alkynes have carbon-carbon double or triple bonds, and are said to
be unsaturated. Aromatic hydrocarbons are cyclic compounds whose structure is related to that of benzene, with
six -electrons in a six-member ring.
Aliphatic Hydrocarbons such as Alkanes are relatively inert to chemical oxidizing agents such as neutral or
alkaline permanganate, where alkenes are readily oxidized at room temperature. The change in color can be used
as a test for a double bond, provided the molecule contains no easily oxidizable group.
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CHEM 2423 Properties of Hydrocarbons
Dr. Pahlavan
Alkanes react slowly or not at all with bromine at room temperature in the dark, but in the presence of sunlight,
substitution is fairly rapid:
R-H + Br2 /light Æ R-Br + HBr (a substitution reaction)
X
X2 /light
+ ( HBr )
The reaction is easily detected by loss of the bromine color and by evolution of hydrogen bromide.
On the other hand, bromine adds rapidly at room temperature to alkenes in a reaction, which does not require light.
Since the product is colorless, the bromine is rapidly decolorized when added to an alkene.
X
X2 / CH2Cl2
C=C
C- C
X
.
Aromatic Hydrocarbons such as toluene will be used in each of the following experiments. Although formally
unsaturated, C6H6CH3 in the sense that it has multiple carbon-carbon double bonds, toluene does not give the
usual reactions expected of an alkene. It is not easily oxidized, and preferably undergoes substitution rather than
addition reactions.
Permanganate converts cyclohexene into a diol. Since a syn-hydroxylation takes place, the reaction is thought to
involve the formation of an intermediate cyclic manganate ester, which is readily hydrolyzed under the reaction
conditions to yield the glycol. In the course of the reaction purple permanganate is reduced to brown manganese
dioxide (MnO2). Also this reaction is used as a qualitative test for the presence of an alkene (Baeyer's Test ).
This decolorization of bromine water is often used as a test for a carbon-carbon double bond. The addition of
bromine to cyclohexene gives a racemic trans product. Since the product is colorless, the bromine is rapidly
decolorized when added to an alkene.
Br
HO
Br2
KMnO4
H2 O
CH2Cl2
HO
Br
cis- 1,2 -diol
trans-dibromide
The following experiments illustrate some of the fundamental reactions of saturated, unsaturated, and aromatic
hydrocarbons. The three classes sometimes react differently toward the same reagent, in which case it may be used
to distinguish between them.
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CHEM 2423 Properties of Hydrocarbons
Dr. Pahlavan
Experimental Procedure
All tests should be carried out in dry test tubes, and observations should be recorded on the report sheet as each
experiment is performed.
Perform the tests (A) , (B), (C) , and (D) on the following materials
1. The saturated hydrocarbon (alkanes and cycloalkanes) – cycloalkane, pentane, and heptane
2. The unsaturated hydrocarbon (alkenes and alkynes) – cycloalkene and 2-pentene
3. The aromatic hydrocarbon - toluene and benzene
(A) Bromine Test - In a small test tube, add 1 ml. of hydrocarbon to 3-4 ml. of 2% bromine in carbon
Tetrachloride(Br2/CCl4). Shake well and observe after two or three minutes. If the bromine is not
decolorized
prepare a second similar tube and place on tube in your laboratory locker and the other in bright
sunlight. Allow both tubes to stand for ten to fifteen minutes and compare them. Observe the color of
each tube, and whether or not hydrogen bromide was evolved and record the results.
(B) Aqueous Potassium Permanganate (Baeyer's Test) – In a small test tube, add 1 ml (10 drops). of
hydrocarbon to a mixture of 3 ml. of dilute potassium permanganate solution (0.5 % KMnO4 solution)
and 3 ml. of dilute sodium carbonate solution (10% Na2CO3 solution) and shake the tube for 1-2
minutes, and note the results. (Note -Dilute sulfuric acid may be substituted for sodium carbonate. This
substitution gives a better test reagent for certain purpose since manganese dioxide is not precipitated
in the acid solution)
(C) Sulfuric Acid Test – In a small test tube add 1 ml. of hydrocarbon, cautiously and with gentle shaking,
to about 3 ml. of concentrated sulfuric acid. Shake the tubes well and note the results. Observe whether
heat evolved and whether the hydrocarbon dissolves. Discard the contents by pouring them into a
beaker containing at least 50 mL of water.
Although alkanes are inert to cold, concentrated sulfuric acid, alkenes react by addition. The product,
alkyl hydrogen sulfate, is soluble in concentrated sulfuric acid.
(D) Solubility – Add about 2 ml. of water in a small test tube and add 2 or 3 drops of the hydrocarbon to be
tested. Shake the mixture to determine whether the hydrocarbon is soluble (a colorless second layer may
be hard to see). Record your results and save the mixture for Test (F).
(F) Relative Density – Reexamine the mixtures prepared in part (E) and decide in each case whether the
hydrocarbon is more dense (sinks) or less dense than water (floats).
DISPOSAL: Please dispose all waste from these test tubes experiments in the appropriate waste container
as directed by instructor.
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CHEM 2423 Properties of Hydrocarbons
Dr. Pahlavan
EXPERIMENT 8 – Properties of Hydrocarbons
Data and Results (Prperties of Hydrocarbons)
REPORT FORM
Name _______________________________
Instructor ___________________________
Date ________________________________
Tests (A) and (B) are commonly used for unsaturation. Test (C) is also used to a limited extent for the same
purpose. Record the results in a given tabular form such as that shown below.
OBSERVATIONS ON HYDROCARBONS
Reagent Used
Alkane
Toluene
Alkene
Br2/CH2Cl2 (light)
0.5% KMnO4
Conc. H2SO4
H2O (solubility)
(S,INS,SS)
Relative Densities
(L=low, H=high)
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Unknown #__
Unknown
#__
CHEM 2423 Properties of Hydrocarbons
Dr. Pahlavan
Pre-Laboratory Questions–EXP 8
Name:
Due before lab begins. Answer in space provided.
1. Draw skeletal (line –bond) structures for the following compounds.
cyclohexane
cyclohexene
toluene
2. What are the general formulas for alkanes and cycloalkanes?
Alkanes
cycloalkanes
3. Write equations for the reaction of 1-butene with the following reagents.
a) Br2/CH2Cl2
c) KMnO4/ hot
4. How could you distinguish octane from 1-octene by a simple chemical test?
5. What would you expect the difference between reactivity of the following pairs? Please e explain your
answers.
a) hexane and Cyclohexane
b) hexane and cyclohexene
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CHEM 2423 Properties of Hydrocarbons
Dr. Pahlavan
Post-Laboratory Questions–EXP 8
Name:
Due after completing the lab.
1. Draw condensed structure formulas for the following compounds.
cyclohexane
cyclohexene
toluene
2. What colors are bromine and potassium permanganate solutions?
3. What are the possible distinct alkenes structural formulas for a molecular formula of C4H8? Please name each
structural formulas.
4. Write equations for the reaction of cyclohexene with the following reagents.
a) Br2/CH2Cl2
b) KMnO4/cold
5. How could you distinguish 1-octanol from 1-octene by a simple chemical test?
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