Apparatus and Materials
Evaporating dish, test tubes, hexane (C6H14), cyclohexene (C6H10), toluene (C7H8), 5% bromine (Br2 dissolved in either trichloroethane or in methylene chloride; either solvent is fine), 0.5% potassium permanganate (KMnO4) dissolved in water 98.5% H2 SO4).
Experimental Procedure
1. Combustion
1ml of hexane was placed in an evaporating dish and was burned using a lighted match or burning splint. This was then repeated with equal amount of cyclohexene, toluene and with two unknown liquids.
2. Reaction with Bromine
Two sets of hydrocarbons were prepared. For each set, 1ml of hexane, cyclohexene and toluene were placed into three clean test tubes. 3 drops of 5% bromine solution was added to each tube. The first set was placed in the dark while the other was exposed to bright sunlight for 15 min. The above procedures were repeated with the two unknown liquids.
3. Reaction with Potassium Permanganate
Three drops of the potassium permanganate solution was added to 1 mL portions of hexane, cyclohexene and toluene in separate test tubes. A rapid disappearance of the purple colour of the permanganate ion shows that the reaction occurred. The above procedures were repeated with the two unknown liquids.
Result:
- Combustion of hydrocarbon
Table 1.1: Reactions of hydrocarbon upon combustion
Unknown A: hexane
Unknown B: cyclohexene
- Reaction with bromine
Table 1.1: Reactions of hydrocarbon with bromine solution
Unknown A: hexane
Unknown B: cyclohexene
- Reaction with potassium permanganate
Table 1.1: Reactions of hydrocarbon with potassium permanganate
Unknown A: hexane or toluene
Unknown B: cyclohexene
Discussion:
In this experiment the physical properties of hydrocarbon that was studied was the combustion reaction of three different types of hydrocarbon namely, hexane (alkane), cyclohexene (cycloalkene), and toluene (aromatic hydrocarbon). Alkane, as observed, produces orange flame with the least amount of soot among the three hydrocarbons. This saturated hydrocarbon undergoes complete combustion producing two colorless and odorless gasses, which are carbon dioxide, CO2 and water, H2O. Meanwhile for cyclohexene, an unsaturated hydrocarbon, in the absence of abundant oxygen, it will undergo incomplete combustion and produces CO2, H2O, CO and C, which was observed from the amount of soot that was observed. Toluene was observed to combusts with the highest amount of sootiness. This indicates the occurrence of incomplete combustion in which will produce more C. The chemical reaction for the three reactions is an s shown below.
In the reaction involving bromine water, the alkane does not undergo addition reaction with bromine in the absence of light. As observed in this experiment, the hexane undergoes addition reaction in the presence of sunlight and does not react in the absence of it. Alkanes react with bromine in a free radical halogenation reaction. The reaction is initiated with the homolysis of the bromine to produce bromine radical. This was done using the energy from the sunlight. The bromine radical then abstracts hydrogen from the alkane to give an alkyl radical in chain reaction. The termination of this reaction occurs when the radicals react together, or when the alkyl radical reacts together.
Meanwhile for alkene, it will undergo electrophilic halogenation with bromine to alkenes yields vicinal dihalide product in the absence of sunlight. The reaction takes place due to the high electron density at the double bond of cyclohexene. This causes a temporary shift of electrons in the Br-Br bond causing a temporary induced dipole, which makes the Br closest to the double bond slightly positive and hence considered an electrophile. In the presence of sunlight,however, the alkene will produce HBr as a side product.
As for toluene no reaction was observed in neither presence nor absence of sunlight. Bromine will be substituted on the methyl group of toluene only under free-radical conditions with special reagents such as a lewis acid catalyst. This is because the Br2 is electrophilic enough to attack double and triple bonds, but not electrophilic enough to attack the pi bond system of the benzene ring of toluene. The "empty orbital" of lewis acids are much more reactive towards the toluene’s pi electron system.
In the potassium permanganate test, the alkane and aromatic compound is non-reactive towards KMnO4. This is because both the hexane and toluene are both saturated compounds where the single bonds are all filled, hence the purple colour of potassium permanganate does not changes. However, when it is added to an alkene, the purple color slowly disappears and a brown MnO2 precipitate forms. The appearance of the brown precipitate indicates a positive test for unsaturation.
Based on the tests conducted above, the two unknown solution can be deduced. Due to the similarities between the unknown A and the reaction with hexane and unknown B with that of cyclohexene in the combustion and the bromine test, it can be concluded that the unknown A is a hexane and unknown B is a cyclohexene. Although in the KMnO4 test, the reaction of unknown A can be assumed to be either hexane or toluene, however based on the other two tests, it can be concluded that the compound is a hexane.
Few precaution steps should be taken into consideration while conducting this experiment. One of it is while using the bromine water. Glove should be worn at all time because bromine is a
strong oxidizing agent.
Conclusion
The hydrocarbons undergo combustion to produce CO2 and H2O. The alkane will react with bromine in the presence of light while alkenes reacts readily even in the absence of light. Aromatic hydrocarbon will not react with bromine. Alkane and aromatic hydrocarbon does not react with potassium permanganate unlike alkene. Through the series of test, it can be concluded that unknown A is hexane while unknown B is cyclohexene.
Reference
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- D. Spurlock. (1999). Solubility and Reactivity of Alkanes, Alkenes and Aromatic Compounds Course Notes. Indiana University Southeast. Retrieved from http://homepages.ius.edu/dspurloc/c122/sol.htm
- Garcia, C. (ed.). (2006). Laboratory Experiments in Organic Chemistry. (Unpublished manual used by the College of Science, University of Santo Tomas). Pp.31-33.
- Matt. (2011). Properties of Hydrocarbons. MendelSet. Retrieved from http://www.mendelset.com/articles/689/properties-hydrocarbons#3
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