Five substances namely Hexane, Eugenol, Unknown hydrocarbon 1, Unknown hydrocarbon 2, and Acetylene gas was used for the selective reactivity of hydrocarbons to functional group tests; Bayer’s test, Bromine test light, Bromine test dark, and Tollen’s test. In Bayer’s test, only Acetylene, Eugenol and Unknown hydrocarbon 2 reacted positively and the rest retain the purplish color of KMnO4. On the other hand, the five compounds in Bromine test with light reacted positively. In Bromine test dark, Hexane and Unknown hydrocarbon 1 did not react because light (UV) is absent. Lastly, in Tollen’s reagent, only Acetylene reacted positively and all the other compounds gave a negative result since they are all hydrocarbons.
Hydrocarbons are simple organic compounds with a distinct character of having Carbon and Hydrogen atoms only. They are divided into three main categories base on the type of orbital hybridization they have. The three main groups of Hydrocarbons are Alkanes, Alkenes, and Alkynes (1). Alkanes are sp3-hybridized hydrocarbons characterized by single bonds between carbon atoms included in their chains. Alkanes are also called saturated hydrocarbons because they contain the highest possible H atoms that they can since their single bonds give the Carbon atoms the opportunity to bond with three or two hydrogen atoms. Alkenes on the other hand are sp2-hybridized substances that are distinguished from other hydrocarbons through the presence of double bond/s between its carbon atoms. The third category of hydrocarbons is sp-hybridized Alkynes that have triple bond/s between C atoms. Alkenes and Alkynes, as contrasted to Alkanes, are classified as unsaturated hydrocarbons (1). In the real world, these single, double, and triple bonding are non-observable since they exist in the molecular level.
The answer to identification of hydrocarbons based on their C atom bonding is the application of functional group tests. These tests utilize active reagents that react with hydrocarbons and gives indicators that are primarily color changes in order to signify that a certain hydrocarbon is a saturated hydrocarbon or an unsaturated one. For hydrocarbons, the level of saturation is determined by using Bayer’s reagent or Bromine test. Bayer’s reactant is an alkaline solution of Potassium permanganate which is considered as a strong oxidizing agent. It will react with triple or double bonded Carbons in order to change the violet color of solution to brown. Tollen’s test on the other hand, is used to identify if a compound is an aldehyde or a ketone. In the experimentation, the three aforementioned tests were tested on the experiment It was assumed that the compounds will react with their corresponding functional group test and therefore the correct reaction mechanism will be established and seen. Furthermore, it was also theorized that through the utilization of the prior tests, the two unknown hydrocarbons will be classified as an alkane, alkene, or alkyne.
MATERIALS AND METHODS
The experimentation was divided into two parts. The first duration dealt with the reaction of four hydrocarbons with different reagents used for the functional group tests that were conducted. 5 drops of Hexane, Eugenol, Unknown hydrocarbon 1, and Unknown hydrocarbon 2 were separately placed in 4 dry clean test tubes one at a time (16 test tubes for all the compounds tested). Each compound was added with Bayer’s reagent, Bromine in Carbon tetrachloride (light and dark), and Tollen’s reagent. The reaction of the four organic compounds was observed and noted down (3). On the second duration of the activity, acetylene, an alkyne, was tested for its reactivity in the previous functional group tests conducted. Four dry clean test tubes with cork were procured. The four reagents used were placed inside these test tubes and was allowed to react with acetylene. Acetylene gas was generated via the reaction of Calcium carbide with water. A set-up was prepared as shown in figure below. Two spoonful of Calcium carbide was placed inside the round bottom flask. This compound was added with small increments of water inside the separatory funnel. The gas that was formed was allowed to enter the previously prepared test tubes with cork. The reactions were observed and noted down (3).
Figure 1. Acetylene generation set-up via reaction of Calcium carbide with water
Legend: ++ Fast Reaction; + Slow reaction; – No reaction
Table A shows the reactions of saturated and unsaturated hydrocarbons to other compounds. Identities of Unknown Hydrocarbons
Sample 1: HEXANE
Sample 2: CYCLOHEXENE
TABLE B. Pertinent General Reactions
(1) Bayer’s oxidation
(+) If brown precipitate
(+) with alkenes and alkynes
(-) If still violet
(-) with alkanes, aromatic compounds
(2) Bromination (light)
(+) If disappearance of color
(+) Only with alkanes, alkenes, alkynes and aromatic compound with alkyl group (-) If still brownish orange
(3) Bromination (dark)
(+) If disappearance of color
(+) Only with alkenes and alkynes
(-) If still brownish orange
(-) with alkanes
(4) Tollen’s Test
(+) If white precipitate
(+) Only for terminal alkynes (triple bond at the end)
(-) No change
From the first experiment when bromine water was added to cyclohexane and shaken and was also kept in the dark, no color change was observed which indicated presence of an alkane. But when this same mixture was exposed to the sunlight there was an observable color change and evolution of hydrogen bromide gas which was indicated by a blue litmus paper which turned red. These observable characteristics are very specific of alkanes which undergo substitution reaction under sunlight. When bromine water was added to cyclohexene, the bromine water decolorized rapidly under room temperature with evolution of hydrogen bromide gas and the presence of an alkene was indicated as they undergo addition reaction under these conditions.
Figure 2. Complete mechanism of Halogenation of Hexane to 2-Bromohexane
Figure 3. Formation of Dihalide from Bromination of Eugenol
In the second experiment when drops of KMnO4 solution was added to cyclohexane and shaken there was no observable reaction which indicated the presence of an alkane as alkanes are unreactive towards oxidizing agents. However when the same drops of KMnO4 solution was added to cyclohexene, there was a discoloration of the KMnO4 solution color which is characteristics of alkenes as they readily oxidized at room temperature.
Figure 4. Theoretical reaction of Eugenol with Bayer’s reagent In the final experiment, aqueous potassium permanganate was added to toluene which formed two layers of color. A pale violet color formed above a deep violet color showing the presence of aromatic compound. C. Follow-up Questions and Applications
1. Which of the above chemical tests can be used to distinguish the
following? a. Alkane from alkene
* Bromination (dark)
* Bayer’s oxidation
b. Alkane from alkyne
* Bayer’s oxidation
* Tollen’s Test
* Bromination (dark)
c. Alkene from alkyne
* Tollen’s test
2. Why are unsaturated hydrocarbons more reactive than saturated ones?
Alkanes are less reactive than unsaturated hydrocarbons because of the mere fact that alkanes or saturated hydrocarbons are sp3-hybridized and therefore contain sigma bonds only. Alkenes have the pi bond between the carbon atoms, and during a lot of reactions the pi bond ruptures in order to form a single bond thus they are more reactive than alkanes but relatively stable as compared to alkynes.
3. What is the difference between the mechanism of bromination of saturated and unsaturated carbons?
In saturated hydrocarbons, homolytic cleavage of Halogen occurs (in presence of UV light), meaning two Halogen free radicals are produced. In bromination of alkanes, one of the free radical bromine reacts with the initial hydrocarbon to form HBr and saturated hydrocarbon free radical. Therefore, only one of the bromine free radical is allowed to be attached to the main carbon chain forming an alkyl halide. On the other hand, in unsaturated hydrocarbon bromination, a heterolytic cleavage happens, meaning a cation and an anion are both produced. Both of these ions are attached to the main carbon chain of unsaturated hydrocarbon forming a dihalide.
4. What is the chemical equation involved in the generation of acetylene?
Reaction of Calcium carbide with water to facilitate Acetylene (Ethyne) Generation: CaC2 (s) + 2 H2O (l) C2H2 (g) + Ca(OH)2 (s)
5. What are the economic uses of acetylene?
Calcium carbide generates acetylene when acted upon by water. This process can be a small-scale one to give acetylene suitable for illumination because of its extremely bright flame. Acetylene is also made on a large scale for chemical conversion. Acetylene is also used for oxyacetylene welding because when burned with oxygen it produces an extremely high temperature. Acetylene and ethylene have been in competition for chemical industrial uses. In the 1950s acetylene was widely used as a chemical raw material, and methods were worked out for obtaining it from hydrocarbon sources.
(1) McMurry, J. Simanek, E. Fundamentals of Organic Chemistry 6th edition. 2008. Thomson Brooks/Cole. 5 Shenton Way #01-01 UIC Bldg. Singapore.
(2) Difference Between Alkanes, Alkenes and Alkynes | Difference Between | Alkanes, Alkenes vs Alkynes http://www.differencebetween.net/science/chemistry-science/difference-between-alkanes-alkenes-and-alkynes/#ixzz2FeMdOJsq (3) Experiment no. 3: Reactions of Hydrocarbons hand-out