Analysis of Hydrocarbons Using Parallel Chemical Tests
A laboratory report submitted to the university as one of the course major requirements...
Analysis of Hydrocarbons Using Parallel Chemical Tests xxxxxxxxxxxxxxxxxxxx Department of xxxxxxxxxxxxxxxxx
Abstract This experiment aims to differentiate various types of hydrocarbons and characterize unknown hydrocarbons using parallel tests. Hydrocarbon is a class of organic compound that contains only carbon and hydrogen atoms. Hydrocarbons can be categorized as saturated aliphatic, unsaturated aliphatic, aromatic and substituted aromatic. Different chemical analyses or tests were used to identify the unknown substance based on the reactivities of its functional groups. Nitration Test differentiates Aromatic from Aliphatic. Bromine Test differentiates Unsaturated from Saturated while Basic Oxidation Test differentiates Alkylated from Non-alkylated Aliphatic. Unknown Samples A and B is Cyclohexene and Cyclohexane, respectively.
Introduction Hydrocarbon is an organic compound made of nothing more than carbons and hydrogens. It serves as the skeleton to various forms of organic compounds and is readily abundant in the natural environment (Carey, 2011). There are two main groups of hydrocarbons namely Aliphatic hydrocarbon and Aromatic hydrocarbon. Aliphatic hydrocarbons consist of straight chain and cyclic organic compounds that does not contain conjugated double bonds with delocalized electrons. Aliphatic hydrocarbon can be classified as Saturated or Unsaturated. Examples of Saturated hydrocarbons are Alkanes and Cycloalkanes. Alkanes contain the maximum amount of hydrogen atoms a carbon-carbon skeleton can accommodate and is usually unreactive hard to identify via common chemical analysis. Unsaturated hydrocarbon, on the other hand, is classified as either Alkenes or Alkynes. Alkene contains carbon-carbon double bonds while Alkynes contain carbon-carbon triple bonds. The presence of pi bonds in these compounds renders it nucleophilic. Cyclic hydrocarbons are closed ring structures that may
differ in stability. Aromatic compounds are those which contain benzene ring, a six-carbon ring with alternating or conjugated double bonds and delocalized electrons. Aromatic compounds are inert in most cases except when hydrogen atoms from benzene are substituted with halogens or alkyl groups (Wade, 2010). Several analytic techniques are used to determine the identity of an unknown hydrocarbon. A common and inexpensive way of doing this is through chemical analysis. In this method, a specific chemical reaction will proceed through the interaction of the sample with the given reagent. The result will then be compared to a known standard which will then dictate the identity of the compound. Nitration test can be used to determine if the unknown hydrocarbon is aliphatic (negative result) or aromatic (positive result) because of its selective reaction with aromatic compounds. Bromination reaction is a substitution reaction The first step involves the formation of a cyclic bromonium ion while the second step involves a classic backside attack of an SN2 mechanism and is frequently used as to differentiate Saturated from Unsaturated. Lastly, Basic Oxidation allows the differentiation between an Alkylated Aromatic ring and a Non-Alkylated one. Elemental analysis is another technique employed to determine the identity of the compound based on the reactivities of the elements comprising its functional groups. Modern elemental analysis is done using elemental analyzer. The experiment aims to differentiate various types of hydrocarbons and characterize unknown hydrocarbons using parallel test.
Methodology The necessary equipment for analysis of hydrocarbons was prepared. Two unknown samples were procured as given by the lab instructors with the identity of being Hexane, Cyclohexane, Naphthalene, or Toluene. 10 drops of the unknown samples was used for each
tests namely Nitration Test, Bromine Test and Basic Oxidation Test. The four reference standards (hexane, cyclohexane, naphthalene, or toluene) used in the experiment were also tested so as to provide a basis of comparison on the result of the unknown. For the Nitration Test, eight drops of the nitrating mixture or reagent (1:1 H2SO4-HNO3) was added to the sample. The mixture was shaken and the result observed was noted. As for the Bromine Test, 3 drops of Bromine in DCM was added. The mixture was shaken and the results were recorded immediately. For Basic Oxidation Test, 3 drops of 2% aqueous KMnO4 solution and 2 drops of NaOH were added to the sample. The mixture was shaken for 30 seconds and the results were noted immediately.
Results and Discussion In this experiment, the following procedure for each tests were followed and the results were recorded. The positive and negative results were obtained based on the observations made. Four reference standards (hexane, cyclohexene, naphthalene and toluene) were also tested so as to provide a basis of comparison on the result of the unknown which also aids in faster identification. The desired result for each test to be considered as a positive result is written below while if the desired result did not happen, then it means that there is no reaction and a negative result took place. Tests
(+) Decolorization Basic Oxidation (-) (+) (-) (+) Brown Brown Test precipitate or precipitate or deep violet deep violet solution solution Table 1. Desired Results for the Three Chemical Tests (Positive and Negative Result)
For the Nitration Test, only Aromatic Compounds will have positive results which are Naphthalene and Toluene which differentiates an Aromatic from Aliphatic. Decolorization will be an indication of a positive result for Bromine Test. It is an Addition reaction which differentiates Saturated Aliphatic from Unsaturated Aliphatic. Only Unsaturated Aliphatic will have a positive result. Lastly, for the Basic Oxidation Test, a brown precipitate or a deep violet solution is the indication of a positive result. This differentiates the Non-Alkylated Aromatic which is Naphthalene from Alkylated Aromatic which is Toluene. Cyclohexene and Toluene are the one expected to react or have positive results. Tests
Test Bromine Test Basic
(-) Orangebrown in color (-) Pinkish
(-) Yellow in color with bubbles (+) Decolorizati on
(+) Yellow precipitate/oil
(-) Pink in color
(+) Yellow precipitate/o il (-) Orangebrown in color (+) Brown
(-) Yellow in color with bubbles (-) Orangebrown in color (-) Pink in
(-) Yellow/orang e-brown
precipitate or deep violet solution
precipitate or deep violet solution
precipitat e or deep violet solution
Table 2. Results of the Chemical Tests Performed in the Analysis The following results were obtained in the reaction of the standards and unknown samples with the different chemical analyses performed. First test performed is the Nitration Test. Nitration reaction determines the presence of benzene ring in a compound. The general mechanism in this kind of reaction is an electrophilic aromatic substitution. The first step is the rate determining step while the second step is a proton transfer from the cyclohexadienyl cation.
Figure 1. Mechanism on the Nitration of an Aromatic Ring Step 1 include Nucleophilic Attack of double from hexane to the nitrocation, nitronium ion to form Cyclohexdienylcation intermediate while Step 2 includes proton transfer from Cyclohexadienylcation to water which ultimately yield Nitrobenzene and Hydronium ion. In the mechanism, it is Nitronium ion serves as the Electrophile and therefore plays a critical role in the production of the desired product. The Nitrating reagent ensures that the amount of Nitronium ion in the reaction was optimized to hasten the overall process. A commonly used Nitrating reagent is a mixture of concentrated sulfuric acid and nitric acid which enhances the production of NO2 (Gauthier, 2010).
Basing from Table 2, it can be seen that Naphthalene and Toluene exhibited a positive result in nitration test. Analyzing the structure of the two compounds, it can be inferred that they will be reactive to the Nitronium ion because of the presence of the benzene ring. The other samples such as hexane and cyclohexene were not reactive to the Nitronium ion since they both do not have benzene ring present in their structures. The yellow precipitate or oil formed in the reaction is actually the Nitrobenzene in naphthalene or the Nitrotoluene in toluene by products. Second test used is the Bromination Test. Bromination reaction is the one that detects the presence of unsaturation in a chemical system. It is an example of an Electrophilic Addition Reaction which involves the formation of Cyclic Bromonium ion intermediate and finally a Dibromoalkyl product.
Figure 2. Mechanism of the Bromination of Alkene. The first step includes nucleophilic attack of the double bond from the alkene to the partially positive bromide of the bromine molecule to form cyclic bromonium ion. It is the rate determining step in the reaction and also an Endothermic reaction while the second step involves a classic backside SN2 sequence wherein a Bromide ion attacks a carbon from Bromonium ion to form a Dibromoalkyl product. Bromine molecule is generally symmetric but an induced polarization occurs, as it approaches the electron-rich pi bond of the unsaturated hydrocarbon, which caused it to become an Electrophile (Solomons, & Fryhle, 2000). It is often a selective reaction since only the more stable radical, which is usually the secondary, is formed faster and a
single radical halogenated product predominates. A red bromine solution is usually prepared by dissolving bromine in a carbon tetrachloride solution but because of the toxic nature of this solvent, Methylene chlorideor DCM can be used as a substitute. Only those that are electron rich pi bonds of unsaturated hydrocarbons and are not substituted with electron withdrawing groups will induce the formation of the colorless dibromoalkane product. In the experiment, only Cyclohexene is positive in the Bromination reaction.
Figure 3. Reaction of Cyclohexene with Bromine Reagent Naphthalene and Toluene also contains unsaturated double bonds but is not reactive to the bromine reagent due to their double bonds that are resonance stabilized with their electrons delocalized. Bromine reagent is unreactive to double bonds with electron-withdrawing groups. Lastly, Basic oxidation reaction is the test that determines the presence of substituent, which preferably an alkyl, in an aromatic system. It entails the conversion of alkyl groups in the benzylic carbon into benzoic acid. The first step involves the abstraction of a benzylic hydrogen by the oxidizing agent while the second step involves oxidation of the benzylic carbon to a carboxyl group. This process cleaves off any remaining carbon atoms on the side chain but the case of tert-butylbenzene is an exception. The overall reaction is shown below.
Figure 4. Conversion of alkyl benzene into benzoic acid via side chain oxidation reaction
It is not only limited to alkyl groups but it can also occur on side chains like Alkenyl, Alkynyl and Acyl groups and on rare cases, in benzene ring itself. The most common oxidizing agent used for the process is hot, alkaline potassium permanganate solution. Alhough chromic acid prepared by adding sodium dichromate to sulfuric acid has a strong oxidizing ability; it is not used since it does not react with either benzene or alkyl. The purple colored solution provides a pathway in the conversion of substituted benzene into carboxylic acid through the OH group in the base. However, the one responsible for the formation of brown precipitate is the carboxylic acid product of the oxidation that formed as a potassium salt (Torres, 2015); (Methods of Preparation of Carboxylic Acids, 2017).
Figure 5. Conversion of Alkyl Benzene into Potassium Benzoate Basing from Table 2, Cyclohexene and Toluene yielded a positive result for Basic Oxidation reaction. Although it cannot be easily observed, considering the structure of Toluene, certainly it will form a brown precipitate from potassium salt. The methyl group will be oxidized to carboxyl group with potassium ion attached. Meanwhile, for the result of the Cyclohexene, it is questionable since it does not contain any side chain group. The reason for it having a positive result or reacting is because of the oxidation of the double bond to form a brown precipitate of MnO2.
Figure 6. Oxidation of Unsaturated Hydrocarbon with Permanganate Ion Oxidation reactions can also be used to detect for unsaturated of an unknown compound. It can be seen in Table 2 that only Cyclohexane does not exhibit any reaction to any chemical tests conducted in the analysis. It is because Cyclohexane belongs to Alkane or the saturated aliphatic hydrocarbon group which was supposed to be unreactive. It contains the maximum amount of hydrogen that can be bonded to a carbon-carbon skeleton that makes the formation of any additional bond almost impossible to occur This hydrocarbon is also non-polar as exhibited by the almost close electronegativity values of C—H bonds and by just a fleeting dipole interactions (London forces) that it possess. Alkanes cannot be detected by common chemical analysis because of these characteristics. The results obtained from Unknown Samples A and B matched with the desired result for Cyclohexene and Cyclohexane, respectively making both unknown samples an Aliphatic. The Bromine Test serves as the defining factor for the identification of the unknowns since it differentiates Saturated from unsaturated. Conclusion Based from the results of the parallel, sample without reactivity to any test procedures or the one that yielded a negative reaction is a Saturated Aliphatic Hydrocarbon like Cyclohexane. On the other hand, a sample with a positive result in the Bromination Test and Basic Oxidation Test but renders negative result in the Nitration reaction is an Unsaturated Aliphatic hydrocarbon like Cyclohexene. Sample with a positive result in the Nitration reaction but with negative results
to both Bromination and Basic Oxidation reactions is a Nonalkylated Aromatic hydrocarbon like Naphthalene while Alkylated Aromatic hydrocarbon like Toluene is positive to both Nitration and Basic Oxidation reactions but is negative on Bromination reaction. From the observation or results obtained, Unknown Sample A is a Cyclohexene which is an Unsaturated Aliphatic because it yielded a negative result in Nitration but a positive result in both Bromination and basic Oxidation test. Meanwhile, Unknown Sample B is a Cyclohexane which is a Saturated Aliphatic because of a negative result obtained from all three types of test. References Carey, F. (2011). Organic Chemistry. USA: McGraw-Hill Companies, Inc Gauthier, N. (2010). What is Hyrocarbon?- Definition, Formula & Compounds. Retrieved April 24,2017, from http://study.com/academy/lesson/what-is-hydrocarbon-definition formula-compounds.html Methods of Preparation of Carboxylic Acids. (2017). Retreived April 24,2017, from http://www.tutorvista.com/content/chemistry/chemistry-iv/oxygen-ii/carboxylic-acids preparation.php Solomons, T.W.G. & Fryhle, C. (2000). Organic Chemistry, 7th edition. USA: John Wiley & Sons, Inc Torres, L. (2015). What are The Different Types of Hydrocarbons. Retrieved April 23,2017, from https://www.quora.com/What-are-the-different-types-of-hydrocarbons Wade, L.G. (2010). Organic Chemistry, 7th edition, Pearson Education Inc.