Alkene Classification Tests
Two common types of unsaturated compounds are alkenes and alkynes,
characterized by the carbon-carbon double and triple bond, respectively, as the functional
group. There are no simple direct ways to prepare solid derivatives of unsaturated
aliphatic compounds having no other functional groups. Two common qualitative tests
for unsaturation are the reaction of the compounds with bromine in carbon tetrachloride
and with potassium permanganate. In both cases a positive test is denoted by
decolorization of the reagent.
A. Bromine in Carbon Tetrachloride Bromine will add to the carbon-carbon double
bond of alkenes to produce dibromoalkanes and with alkynes to produce
tetrabromoalkanes. When this reaction occurs, molecular bromine is consumed, and its
characteristic dark red-brown color disappears if bromine is not added in excess. The
rapid disappearance of the
bromine color is a positive test
for unsaturation.
The test is not unequivocal, however, because some alkenes do not react with bromine,
and some react very slowly. In the case of a negative test, therefore, the potassium
permanganate test should be performed.
EXPERIMENTAL
To three test tubes, add 20 drops of:
a) cyclohexane
b) cyclohexene
c) your unknown
Add 0.1M Bromine in CCl4 dropwise to the cyclohexane, until a red-brown color
persists for 30 seconds.
Add 0.1M Bromine in CCl4 dropwise to the cyclohexene, until a red-brown color
persists,
OR until you have added ten (10) drops more than you added to the cyclohexane.
Add 0.1M Bromine in CCl4 dropwise to your product, until a red-brown color persists
OR until you have added ten (10) drops more than you added to the cyclohexane.
Dispose of the contents of the test tubes into the “Halogenated Organic Solvent”
Bottle
Failure to do so will result in a 2 point deduction from your score
B. Potassium Permanganate (the Baeyer Test) A second qualitative test for unsaturation, the Baeyer test, depends on the ability of potassium permanganate to oxidize the
carbon-carbon double bond to give alkanediols or the carbon-carbon triple bond to give
carboxylic acids
The permanganate is destroyed in the reaction, and a brown precipitate of MnO2 is
produced. The disappearance of the characteristic color of the permanganate ion is a
positive test for unsaturation. However, care must be taken, since compounds containing
certain other types of functional groups (for example, aldehydes, containing the --CH=O
group) also decolorize permanganate ion.
EXPERIMENTAL:
In a test tube, dissolve 1 or 2 drops of cyclohexane in 20 drops 95% ethanol, then add
0.1M
potassium permanganate solution drop-wise, swirling the test tube from time to time, and
observing the results. Count the number of drops added before the purple permanganate
color persists. (To test for permanganate color; with your stirring rod, place a drop of
solution from your test tube onto a piece of filter paper. The appearance of a purple ring
around the outer edge of your spot indicates the presence of permanganate.) Repeat this
process with cyclohexene, and your unknown. A significant difference in the number of
drops used in your cyclohexane test versus the number used in your cyclohexene and
unknown tests constitutes a positive test for un-saturation. You do not have to reach the
permanganate color end point with cyclohexene or your product, just be sure that ten (10)
more drops were added to these tubes than was added to the cyclohexane tube.
Dispose of the contents of the test tubes into the “Recovered Organic Solvents”
bottle.
Failure to do so will result in a 2 point deduction from your score
Che 301
Su’2003
Lab Report
Title: Alkene Tests
Name:
Date:
Test: Bromine in CCl4
Equation:
What indicates a positive test:
Known Compounds
Results / Observations
Cyclohexane
Cyclohexene
Products
Results / Observations
Cyclohexene
Conclusions:
Your Cyclohexene behaved similarly to which known(s) ?
Test: 0.1M KMnO4
Equation:
What indicates a positive test:
Known Compounds
Results / Observations
Cyclohexane
Cyclohexene
Products
Results / Observations
Cyclohexene
Conclusions:
Your Cyclohexene behaved similarly to which known(s) ?