CLASSIFICATION TESTS FOR HYDROCARBONS
Margarita Ysabel S. Mangahas, Cara Camille M. Matute, Emilyn D. Millares,
John Acener C. Padua and Kathryn Cheshire P. Pangilinan
Group 5 2A Medical Technology Organic Chemistry Laboratory
ABSTRACT
In the experiment, hydrocarbons were classified by observing their solubility, flammability, active unsaturation, aromaticity, and oxidation to their appropriate and suitable tests. Hexane, heptanes, cyclohexane, cyclohexene, benzene, and toluene were the hydrocarbon compounds studied. First, physical properties of the compounds were observed to identify their physical state or phase, color and odor. All of the compounds were in clear liquid form and are colorless, while they vary in odor. To test for solubility, concentrated H2SO4 or sulfuric acid was used as reagent. Each of the compounds were tested according to their flammability by an ignition test. All of them were flammable and the alkanes produced a yellow flame. In addition to that, only the cyclic compounds produced soot. Their active unsaturation was also tested by using Baeyer’s test and Bromine test. In Baeyer’s test, the compounds reacted with 2% KMnO4 solution. Decolorization from violet to brown was seen only in cyclohexene. As for the Bromine test, the compounds were studied through their reaction with 0.5% Br2 in CCl4 reagent. It can be drawn from the results that there was no change in benzene and toluene, while immediate decolorization occurred in cyclohexene. Moreover, there was a slow reaction in hexane, heptane, and cyclohexane, but produced rapid reactions of turning to a colorless solution under sunlight. Next, aromaticity of the hydrocarbons were tested. This was done by combining a nitrating mixture composed of concentrated HNO3 and concentrated H2SO4 to the hydrocarbons. Heat was also applied to have an apparent reaction. Aromatic compounds were found out to be benzene and toluene, by which both produced a unique yellow oily layer. Lastly, test for basic oxidation was performed. In this test, the compounds reacted with 2% KMnO4 and 10% NaOH. The only distinctive change was seen in cyclohexene which changed from violet to brown, indicating that it is oxidized. The other compounds did not decolorize. [1]
INTRODUCTION
Hydrocarbons are the simplest organic compounds. They are Carbon compounds that can be designated as aliphatic hydrocarbons or as aromatic hydrocarbons. Aliphatic hydrocarbons which are saturated are alkanes, and if found in a closed-chain are named as a cycloalkane. If they are unsaturated, they can be an alkene or
cycloalkene, alkyne or cycloalkyne. [5]
Hydrocarbons may be saturated which means that each carbon is bonded to four other atoms through single covalent bonds, thus it creates the maximum number of hydrogen in a compound due to the absence of any substituent. Hydrogen atoms usually occupy all available bonding positions after the carbons have bonded to each other. While, unsaturated hydrocarbons contain either double or triple bonds. Alkenes as unsaturated compounds are also called olefins and they form oily liquids on reaction with halides like bromine gas. [2] Hydrocarbons may also be aromatic compounds. They follow Huckel’s Rule, their structure is stabilized by resonance, and are all cyclic. They undergo electrophilic substitution reactions and nucleophilic aromatic substitutions. [3]
The objectives of this experiment were to identify and classify the different hydrocarbons. The hydrocarbons were to be tested according to their solubility, flammability, active unsaturation, aromaticity, and basic oxidation. To elaborate, they were categorized and from the results were
inferred whether miscible or immiscible,
saturated or unsaturated, actively unsaturated or
not actively unsaturated, aromatic or not
aromatic, and oxidized or not. This experiment also aim to identify the different physical and chemical properties of hydrocarbon compounds.
EXPERIMENTAL
A. Sample Used
The samples used or compounds tested in the experiment were hexane, heptane, cyclohexane, cyclohexene, benzene, and toluene.
B. Procedure
1. Physical State, Color, Odor
The compounds were first observed to know their physical properties. The state and color of the compounds were observed, and the odor was also sensed by wafting or fanning the hands above the test tube.
One milliliter of of conc. H2SO4 was applied to
a drop of each of the compounds in separate test tubes by using calibrated droppers, afterwhich, change in color and warming effect were observed.
3. Ignition Test
Flammability of the compounds was tested by an ignition test. One by one, 3 drops of each of the compounds were placed in an evaporating dish and lighted with a match.
4. Tests for Active Unsaturation
Two different tests were done to analyze active unsaturation of the compounds. The first test was the Baeyer’s test. This was done by the combination of 5 drops of each of the compounds and 2 drops of 2% KMnO4 solution. Bromine test was performed by adding 10 drops of 0.5% Br2 in CCl4 reagent to 5 drops of each compound. Shaking of the mixture was done. If the mixture did not decolorize, it must be exposed to sunlight.
5. Test for Aromaticity: Nitration
Preparation of the nitrating mixture must first be accomplished by placing 2 ml of conc. HNO3 in anErlenmeyer flask and immersing it into water from an evaporated dish, and conc. H2SO4 was added gradually. Eight drops of the nitrating mixture was added to 5 drops of each compound. The mixture was diluted with water after shaking. If no apparent reaction was seen, heat must be applied by conducting a water bath that is not more than 50°C in 10 mins. Lastly, the mistures was diluted again with water.
6. Basic Oxidation
2% KMnO4 solution and 10% NaOH solution with 8 and 3 drops respectively, were combined to 4 drops of each hydrocarbon compound. The mixture was warmed by a water bath for 2 mins.
RESULTS AND DISCUSSION
Different tests like solubility in conc. H2SO4,
Ignition Test, Baeyer’s Test, Bromine Test, Test for Aromaticity, and Basic Oxidation were conducted to be able to differentiate and classify the different hydrocarbon compounds.
Table 1. Results and Inferences of
classification tests for hydrocarbons
Compounds Studied
Hexane Heptane Cyclohexane
Condensed Structural Formula CH3CH2CH2 CH2CH2CH3 CH3CH2C H2CH2CH 2CH2CH3 A. Physical state at RT Color Odor
Clear liquid Clear liquid Clear liquid
Colorless Colorless Colorless
Plastic
balloon Chlorine-like Alcohol-like B. Solubility in concentr ated H2SO4 Inference
Oily layer Oily layer Oily layer
Immiscible Immiscible Immiscible C. Ignition
Test Inference
Flammable Flammable Flammable Yellow
flame Yellow flame Yellow flame D. Baeyer’s Test Bromine Test Inference No decolorizat ion No decoloriz ation No decoloriza tion Fast change from orange to colorless under sunlight Fast change from orange to colorless under sunlight Fast change from orange to colorless under sunlight Not actively unsaturate d Not actively unsatura ted Not actively unsaturat ed E. Test for Aromatici ty Nitration Inference Clear oily
layer Clear oily layer Clear oily layer Not
aromatic Not aromatic Not aromatic F. Basic Oxidation Inference No decolorizat ion No decoloriz ation No decoloriza tion Not oxidized Not oxidized Not oxidized Compounds Studied Cyclohex e Benzene Toluene
Condensed Structural Formula A. Physical state at RT Color Odor Clear
liquid Clear liquid Clear liquid
Colorless Colorless Colorless
Gas-like Gas-like Rugby-like B. Solubility in concentr ated H2SO4 Inference Orange
oily layer Clear oily layer Clear oily layer
Miscible Miscible Miscible
C. Ignition Test Inference
Flammabl
e Flammable Flammable
With soot With soot With soot
D. Baeyer’s Test Bromine Test Inference Immediat e decoloriza tion from violet to brown No decoloriza tion No decoloriza tion Immediat e decoloriza tion from reddish brown to colorless No decoloriza tion No decoloriza tion Actively Unsaturat ed Not Actively Unsaturat ed Not Actively Unsaturat ed E. Test for Aromatici ty Nitration Inference Yellow
solution Yellow oily layer Yellow oily layer Not
aromatic Not aromatic Not aromatic F. Basic Oxidation Inference Decoloriza tion from violet to brown No decoloriza tion No decoloriza tion Oxidized Not oxidized Not oxidized
1. Physical State, Color, Odor
All of the hydrocarbon compounds tested
were in clear liquid state. They are all colorless but they differ in odor. Hexane smelled like a plastic ballon, heptanes as chlorine-like and cyclohexane as alcohol-like. Cyclohexene and benzene have both a gas-like odor, while toluene was rugby-like.
2. Solubility in Concentrated H2SO4
Clear oily layer was the result in almost all of the compounds except cyclohexene. A clear orange oily layer was produced by cyclohexene with conc. H2SO4. These results indicate that
cyclohexene are miscible and the others are immiscible in conc. H2SO4.
3. Ignition Test
All of the compounds tested were flammable. Thus, it can be inferred that all hydrocarbons are combustible. The only difference seen was the production of soot during the process. Only the cyclohexene, benzene, and toluene had a soot wiye.
4. Tests for Active Unsaturation
Baeyer’s Test
Potassium permanganate did not react with the alkane compounds tested because they are saturated. When the reagent was added to alkanes, the purple color does not change. [4] Only cyclohexene gave out a reaction that changed its color from violet to brown. Then it may be concluded that cyclohexene was the only actively unsasturated compound due to its positive result against the reagents in Baeyer’s Test.
Figure 1. Chemical reaction of alkene
in Baeyer’s Test
When KMnO4 solution was added to an alkene,
the purple color slowly disappeared and a brown MnO2 precipitate formed. The appearance of the
brown precipitate indicate a positive test for unsaturation.
Figure 2. Decolorization of alkene in
Baeyer’s Test
C C C R HO R OH R R (purple) (brown) + 2KMnO4 + 2CH3OH + 2MnO2 + 2KOCH3 R C R R RA purple result was first produced by the Baeyer’s Test for Unsaturation, this diluted into light purple until it produced a brown precipitate which indicated that the compound was already unsaturated.
The bromine test was used to determine if the colorless organic compounds contain any double Carbon to Carbon bonds. Bromine did not react with any alkane because the alkane has only single C-C bonds which cannot add the bromine. Alkanes merely dilute the red-brown bromine color to an orange or yellow color in the absence of a strong catalyst which was visible in the result in hexane, heptane and cyclohexane. Due to their C=C double bonds in cyclohexene, which can be broken, alkenes react readily with bromine to produce saturated dibromoalkanes. When an alkene is reacted with bromine, the red-brown color of the bromine is immediately lost due to the reaction of the bromine. [4]
Figure 3. Chemical reaction of alkene
in Bromine Test
Due to their C=C double bonds in cyclohexene, which can be broken, alkenes react readily with bromine to produce saturated dibromoalkanes.
Figure 4. Decolorization of alkene in
Bromine Test
The red aqueous layer as bromine, was used to test the organic chemicals for a reactive carbon double bond to decolourize the liquid. The
organic layer formed by cyclohexene,
decolourized the liquid slowly into orange because cyclohexene contains a reactive double bond. The reddish brown color of Bromine that slowly diluted to orange indicated that there was no reaction between the reagents, thus, the compound is saturated. But, when the mixture
was exposed to sunlight, immediate
decolorization took place, wherein the orange solution became colorless, which presented that the mixture was already unsaturated.
Benzene and toluene are also alkenes but, they are both aromatic that make them very stable but also less reactive than cyclohexene because their atoms are less available to be given off. [6]
5. Test for Aromaticity: Nitration
Aromatic compounds are extremely stable and will not react without a catalyst so they act like alkanes but are still unsaturated compounds.
Figure
4.
Chemical reaction of
aromatic compounds in nitration
A nitrite compound attached to benzene was produced as seen in the chemical reaction of benzene from a nitrating mixture composed of conc. HNO3 and conc. H2SO4, and with aid of heat. Nitrobenzene as well as p-nitrotoluene both gave a yellow oily layer above a colorless solution. This indicated that both compounds possess aromaticity.
6. Basic Oxidation
This test was done by using an inorganic chemical compound, KMnO4 which is a strong
oxidizing reagent. An oxidation reaction occurred in this test. Furthermore, NaOH had been used to test if precipitate was to be formed. From the results, only cyclohexene produced a change UV light C C R R R R C C R Br R Br R
(reddish brown) (colorless)
+ Br2
wherein it decolorized from violet to brown. That occurrence perceived that it is the only oxidized compound.
REFERENCES
[1] Bayguen, A.V., et al. (2009). Laboratory Manual in Organic Chemistry. Pearson Education, Inc. pp 81-93 [2] http://www.elmhurst.edu/~chm/vchem book/500hy drocarbons.html [3] http://www.encyclopedia.com/topic/aromatic _com pound.aspx#0-1E1:aromatic-full [4] http://homepages.ius.edu/dspurloc/c122/hy d.htm [5] http://hyperphysics.phyastr.gsu.edu/hbase/ organic/hydrocarbon.html [6] http://sciencephoto.com/media/4388/enla rge
