Qualitative Tests for Alkenes
Qualitative Tests for Alkenes
Aim:
The purpose of this experiment was to use pyridinium bromide perbromide (PBP) and potassium permanganate to distinguish alkenes and alkanes from aromatic compounds. We tested a total of six known compounds (cyclohexane, 1-octene, ethyl acetate, cyclohexene, kerosene, and mineral oil) as well as three unknown compounds (Q_1, Q_2, and Q_3).
Introduction:
Hydrocarbons are organic compounds that consist solely of hydrogen and carbon atoms. Alkanes, alkenes, alkynes, and aromatic compounds are all examples of a hydrocarbon. An alkane is saturated because it cannot further react with hydrogen due to its single bond. On the other hand, an alkene, alkyne, and aromatic compound are all unsaturated and are all capable of reacting with hydrogen under the right conditions. Two common qualitative tests for unsaturation in hydrocarbons are the dicoloration of an aqueous bromine (i.e. pyridinium bromide perbromide or PBP test) and the dicoloration of an aqueous potassium permanganate (KMnO4 test).
In this experiment a bromine test was carried out with pyridinium bromide perbromide (PBP). If the test is unsaturated, the yellow color of the aqueous PBP would disappear as the solution would turn into a clear color. The color change indicates an addition product and a positive alkene or alkyne test. This is an addition reaction because bromine adds to each carbon of the bond, producing a product with a bromine at each end of the bond. However, bromine does not react with all unsaturated compounds; while it reacts with alkenes and alkynes, it does not react with the ring structure of aromatic compounds. In the potassium permanganate test, if the compound tested is unsaturated, an oxidation-reduction reaction will take place as KMnO4 oxidizes the unsaturated compound. An OH group is added to each carbon at each end of the double bond. In a positive test, the compound reduces the purple permanganate ion (MnO4-), forming a precipitate of brown manganese dioxide (MnO2).
Below are balanced equations showing the reaction of cyclohexene undergoing a PBP test as well as a KMnO4 test:
KMnO4 test:
3C6H10 + 2KMnO4 + 4H2O à 3C6H12O2 + 2MnO2 + 2KOH
PDP test:
C6H10 + C5H6Br3N à C6H10Br2 + C5H6BrN
Table of Physical Properties and Hazards:
|
Compound |
Physical Properties |
Hazards |
|
Pyridinium bromide perbromide (C5H6Br3N) |
Molar Mass: 319.822 g/mol Melting Point: -8°C Physical State: Liquid |
Pyridinium tribromide is corrosive and a lachrymator. Prevent eye, skin, and clothing contact. Avoid inhaling or ingesting this compound. Use the fume hood when handling this reagent. |
|
Potassium permanganate (KMnO4) |
Molar Mass: 158.034 g/mol Melting Point: 240°C Boiling Point: N/A Physical State: Liquid |
2% Potassium permanganate solution is an oxidant and irritant that can cause skin discoloration. Prevent eye, skin, clothing, and combustible material contact. Avoid ingestion as well. |
|
Cyclohexane (C6H12) |
Molar mass: 84.16 g/mol Melting point: 6.47°C Boiling Point: 80.74°C Physical state: Liquid |
Cyclohexane vapor is irritating to the skin, eyes, and respiratory tract. This chemical is highly flammable and can react with other oxidizing materials. It should be used under a fume hood. |
|
1-octene (C8H16) |
Molar Mass: 112.24 g/mol Melting Point: -101.7°C Boiling Point: 121°C Physical State: Liquid |
1-octene is a highly flammable compound whose vapor and air mixtures are explosive. If inhaled, this compound can cause drowsiness and dizziness. Should be used in a fume hood. |
|
Ethyl acetate (C4H8O2) |
Molar Mass: 88.11 g/mol Melting Point: -83.6°C Boiling Point: 77.1°C Physical State: Liquid |
Ethyl acetate is highly flammable and should be handled in a fume hood. It is toxic when ingested or inhaled and can also cause irritation to the skin and eyes. |
|
Cyclohexene (C6H10) |
Molar Mass: 82.143 g/mol Melting Point: −103.5 °C Boiling Point: 83°C Physical State: Liquid |
Cyclohexene is a colorless, flammable compound. If inhaled, it can cause coughs and drowsiness. If ingested, it can cause labored breathing and nausea. Cyclohexene can also irritate the eyes, the skin, and the respiratory tract. |
Procedure:
In this experiment, two different methods were utilized in order to determine the alkene presence in each known and unknown compound. First, a PBP test was carried out and results based on the change in color of the compound after the PBP was added were recorded. The first step in this test was to efficiently label 9 test tubes corresponding with the tested compounds. The rest of the test was carried out in the fume hood, due to the nature of the compounds. Next, 10 drops of each compound were placed within the corresponding test tube. Approximately 2 drops of PBP was added into each tube and the tubes were shook gently to mix the solution. The PBP had a yellowish color, when added so the alkene compounds, a color changed was observed. The color changed to a clear liquid. If there was a color changed, 10 additional drops of PB was added to the solution and if. The solution remained clear after these 12 drops; the test was deemed positive. In some cases, it was difficult to see a color change in our experiment partly because some the glassware was stained from previous usage.
The next test carried out was a KMnO4 test. A total of 9 tubes were used and labeled with each corresponding compound. Approximately 1 mL of acetone was poured into each tube. The, 2 drops of each compound were placed into each corresponding test tube. Lastly, 20 drops of KMnO4 were placed into each test tube. The solution was gently mixed while carrying out this step. The KMnO4 is a purple color. When added to an alkene compound, a color change occurred as the dark purple would change to a brown. The results were recorded.
Results:
|
Compound |
# Drops of PBP |
*Results of the PBP Test |
# Drops of KMnO4 |
Results of the KMnO4 Test |
|
Cyclohexane |
12 |
+ |
20 |
_ |
|
1-octene, |
12 |
+ |
20 |
+ |
|
ethyl acetate |
12 |
+ |
20 |
_ |
|
cyclohexene |
12 |
+ |
20 |
+ |
|
kerosene |
2 |
_ |
20 |
_ |
|
mineral oil |
2 |
_ |
20 |
_ |
|
Q_1 |
12 |
+ |
20 |
_ |
|
Q_2 |
12 |
+ |
20 |
+ |
|
Q_3 |
12 |
+ |
20 |
+ |
*It was difficult to see color change for the PBP test. Despite cleaning it some of the glassware was stained from previous usage and so the slight shift from yellow to clear was difficult to see.
Discussion and Conclusion:
According to this experiment, several of the compounds tested were positive. Of the known compounds, 1-octene and cyclohexene were positive. This demonstrates that these compounds are in fact unsaturated. Of the unknown compounds, Q_2 and Q_3 were positive which means that they too are also unsaturated. This is based off of the results of the KMnO4 test. From the results, it can be seen that the outcome of both tests was not the same for each corresponding compound. This may be is due to the fact that it was difficult to see the color change in the PBP test since the change was from light yellow to clear. Also, some of the glassware was stained due to previous usage, and despite cleaning it, it was still hard to know if the change in color was due to the stained glass or the actual reaction taking place.
IR Spectroscopy
Instrumentation Details and Procedure:
In this procedure, the instrument used was an Attenuated Total Reflection (ATR). An ATR operated by measuring the changes that occur in an IR beam when the beam comes in contact with the sample. To complete this procedure, two unknown samples – on liquid (1R_9) and one solid (IR_13) – were selected. IR_9, a clear liquid sample was loaded into the IR machine and the spectrum was produced. IR_13, a white solid was then loaded into the IR machine and again, the spectrum was produced. Both results are attached at the back of this lab report.
Evaluation of the Spectra:
IR_9: C8H8O
|
Wave numbers (cm-1) |
Functional Group |
|
1681.72 |
C=O |
IR_13: C7H6O2
|
Wave numbers (cm-1) |
Functional Group |
|
1682.58 |
C=O |
|
2500-3200 |
COOH |
References:
National Center for Biotechnology Information. PubChem Database. 1-Octene, CID=8125,
https://pubchem.ncbi.nlm.nih.gov/compound/8125 (accessed on Apr. 14, 2019)
“CDC - NIOSH Pocket Guide to Chemical Hazards - Cyclohexane.” Centers for Disease Control
and Prevention, Centers for Disease Control and Prevention,
www.cdc.gov/niosh/npg/npgd0163.html.
National Center for Biotechnology Information. PubChem Database. Ethyl acetate, CID=8857,
https://pubchem.ncbi.nlm.nih.gov/compound/8857 (accessed on Apr. 14, 2019)
National Center for Biotechnology Information. PubChem Database. Potassium permanganate,
CID=516875, https://pubchem.ncbi.nlm.nih.gov/compound/516875 (accessed on Apr. 14,
2019)
National Center for Biotechnology Information. PubChem Database. Cyclohexene, CID=8079,
https://pubchem.ncbi.nlm.nih.gov/compound/8079 (accessed on Apr. 14, 2019)
