Hydrocarbons

Analysis of Hydrocarbons

Jovellanos, Bien Jindrich Johannes; Lacson, Danise Angelica; *Lagula, Nina Francesca; Lañez, Kristine

Department of Psychology
College of Science
University of Santo Tomas
España Manila 1015

Abstract A hydrocarbon is strictly composed of carbon and hydrogen atoms only. Five hydrocarbons were used namely hexane, cyclohexene, toluene, naphthalene (in hexane), and the unknown (which will be known through parallel chemical tests). Three tests, nitration test, bromine test and basic oxidation test were conducted to fully differentiate each type of hydrocarbon from one another. Nitration tests resulted to three positive outcomes and two negative outcomes. A positive outcome in the nitration test shows that the hydrocarbon is aromatic while a negative outcome is aliphatic. In the bromine test, the results showed that there is one saturated aliphatic while the other is unsaturated aliphatic. In the last test (basic oxidation test), all five hydrocarbons resulted to positive products. Through the comparison of results of each hydrocarbon from each test, the unknown was observed to be Toluene.

Introduction Hydrocarbons can probably be considered as the most important class of organic compounds. It is mainly composed of carbon atoms and hydrogen atoms only. Most hydrocarbons are colorless and odorless. It is divided into two categories, aliphatic and aromatic. Aliphatic hydrocarbons have the principal carbon atoms arranged in chains. It can be saturated (wherein carbon-to-carbon bonds are single bonds) or unsaturated (wherein there are one or more carbon-to-carbon multiple bonds). Saturated aliphatic hydrocarbons are alkanes while unsaturated aliphatic hydrocarbons are alkenes and alkynes. The general formula of alkanes, alkenes and alkynes are CnH2n+2, CnH2n, and CnH2n-2, respectively. Alkanes, alkenes and alkynes can be open-chain (acyclic), cyclic, unbranched or branched. Aromatic hydrocarbons are represented by a six-carbon ring. Its difference with cyclic is that it is composed of three double bonds. It can be alkyl-chained or not. Benzene, C6H6, is said to be the most common aromatic hydrocarbon. The hydrocarbons, hexane, cyclohexene, toluene, naphthalene (in hexane) and the unknown (which is toluene) were analyzed in the experiment. Hexane and cyclohexene are aliphatic (saturated) while toluene and naphthalene are aromatic. Hexane is a six-carbon alkane hydrocarbon. Its molecular formula is C6H14. It is colorless and odorless. It is non-polar so it is often used as a solvent. Cyclohexene is a six-carbon ring alkane hydrocarbon. Its molecular formula is C6H12. Its only difference with hexane is that it is in a cyclic form with double bonds while hexane is in a linear form with single bonds only. Also, its hydrogen atoms are two less than that from a hexane’s hydrogen atoms. It is colorless and odorless. It is also used as a solvent because of its non-polarity. Toluene is an aromatic hydrocarbon. Its molecular formula is C6H5CH3. It is related to benzene. It is colorless and has an odor similar to that of a pentel pen’s. It is mostly used as a raw material to produce other chemicals. Benzoic acid is an example that is produced from toluene. Naphthalene is a polycyclic aromatic hydrocarbon. Its molecular formula is C10H8. It is composed of two benzene rings fused together. It is colorless and has an odor close to that of mothballs. It is usually found in crude oils.

Figure 1: Cyclohexane structure Figure 2: Toluene structure Figure 3: Naphthalene structure

These four hydrocarbons have similarities and differences so different tests are conducted to fully differentiate one from the other. Color and miscibility were the common basis of a hydrocarbon’s properties. Nitration test was conducted to test the aromaticity of the compound. Aromaticity of the compound is determined when a yellow globule on the surface or a yellow precipitate appears. Otherwise, it is aliphatic. Bromine test was conducted to test if the compound is saturated or not. Discoloration of the red-orange solution signifies that it is unsaturated. Lastly, the basic oxidation test was conducted to know if the hydrocarbon is alkylated or not. If it resulted to a brown precipitate or a green precipitate, it is alkylated. This experiment simply tends to differentiate various types of hydrocarbons (aromatic, aliphatic, saturated, unsaturated, alkanes, alkenes, and alkynes), to device a scheme in distinguishing hydrocarbon from each type and to characterize an unknown hydrocarbon through parallel chemical tests.

Results and Discussion

Three tests were conducted to easily classify hydrocarbons according to their reactions with specific compounds.

Nitration Test Nitration test was the first test that was conducted. Each of the hydrocarbons was mixed with nitrating mixture. The test was done in order to know if a hydrocarbon is aromatic or aliphatic. There can be two potential results, either positive or negative. A positive result yielded either a yellow globule on the surface or a yellow precipitate while a negative result produced a colorless solution. Given the hydrocarbon is positive in the test, it is said to be aromatic. Otherwise, it is aliphatic. As stated in the table 1 below, there are three compounds which are positive in the nitration test. Toluene, Naphthalene and the unknown are positive while Hexane and Cyclohexane are not. Through this, the hydrocarbons, Toluene, Naphthalene and the unknown can now be identified as Aromatics. On the other hand, Hexane and Cyclohexane are the Aliphatic. Since the unknown is also an Aromatic, it can be assume as either Toluene or Naphthalene. Obtaining the aromaticity of the hydrocarbons was done with the help of electrophilic aromatic substitution. Electrophilic aromatic substitution happens when an electrophile (electron-lover) reacts with an aromatic ring and substitutes for one of the hydrogen atoms. Nitrating mixture is composed of HNO3 and H2SO4. H2SO4 acted as a catalyst which helped in the formation of Nitronium Ion (NO2+). Nitronium Ion is an electrophile that substituted one hydrogen atom in the Toluene, Naphthalene and the unknown.

Table 1: Observations in Nitration Test

|Compound |Observations |
|Hexane |Negative ; Colorless solution |
|Cyclohexane |Negative; Dark brown precipitate |
|Toluene |Positive; Yellow globule in solution |
|Naphthalene |Positive; Yellow precipitate |
|Unknown |Positive; Yellow globule in solution |

Bromine Test The second test to be conducted was the bromine test. This test was done to classify the aliphatic hydrocarbons into unsaturated aliphatic or saturated aliphatic. Bromine reagent was the compound added to each hydrocarbon in this test. There can be a positive or negative result as well. A hydrocarbon is positive in bromine test if it resulted to a colorless solution. A negative outcome decolorizes the red-orange solution. If it is positive, it is unsaturated aliphatic. Otherwise, it is saturated aliphatic. As shown below on table 2, only Cyclohexene yielded a positive result on the bromine test. The others produced negative results. It is expected that Toluene, Naphthalene and the unknown will yield negative results since they are not aliphatic hydrocarbons. The negative results for the three are another strong basis that they are indeed aromatic. Hexane which yielded a negative result means that it is a saturated aliphatic while cyclohexene is an unsaturated aliphatic. Unsaturated aliphatic means that there are double bonds or triple bonds. For saturated aliphatic, there are only single bonds. With the observations, it can be said that hexane only contains single bonds while cyclohexene contains double bonds.

Table 2: Observations in Bromine Test

|Compound |Observations |
|Hexane |Negative; Red-orange solution |
|Cyclohexane |Positive; Colorless solution |
|Toluene |Negative; Red-orange solution |
|Naphthalene |Negative; Red-orange solution |
| Unknown |Negative; Red-orange solution |

Basic Oxidation Test Basic oxidation test was the last test to be conducted. Its goal was to know whether a hydrocarbon is alkylated or non-alkylated. KMnO4 and 10% NaOH were added to each hydrocarbon for this test. Again, there are two possible results, either positive or negative. A hydrocarbon positive in this test resulted to a brown precipitate while the negative do not. All of the hydrocarbons yielded to positive results. This only means that they are all alkylated wherein alkyl groups can be added to them. Alkyl groups are not stable compounds themselves; they are simply parts of larger compounds. Alkyl groups are named by replacing the –ane ending of the parent alkane with an –yl ending. Similarly, the removal of a hydrogen atom from the end carbon of any straight-chain alkane gives the series of straight-chain alkyl group.

Table 3: Observations in Basic Oxidation Text

|Compound |Observations |
|Hexane |Positive; Green precipitate |
|Cyclohexane |Positive; Brown precipitate |
|Toluene |Positive; Green precipitate |
|Naphthalene |Positive; Green precipitate |
| Unknown |Positive; Green precipitate |

Some photos of the hydrocarbons

The three test tubes that contain purplish solutions on the figure 4 below are Hexane, Toluene and Naphthalene mixed with KMnO4 and 10% NaOH. They were in purple color before the water bath. The two test tubes behind are the ones which yielded red-orange solution on the Bromine Test. These are Toluene and Naphthalene.
[pic]
Figure 4: Hydrocarbons on Basic Oxidation Test (Before) and on Bromine Test The hydrocarbons on figure 5 are Hexane, Toluene, Naphthalene Cyclohexene and the unknown which have green, green, green, brown and green solutions, respectively. The color purple was decolorized after the water bath. The test tube on the far left (on the 2nd row) has the colorless solution produced by Cyclohexene during the Bromine Test.
[pic]
Figure 5: Hydrocarbons on Basic Oxidation Text (After) and on Bromine Test

From the observations made and noted, various types of hydrocarbons were differentiated. There are aromatics and aliphatic. Aromatic have six-ring carbon chain with double or triple bonds while aliphatic have the principal carbon atoms arranged in chains. Under aromatics, there are alkylated in which alkyl groups can be added to the hydrocarbon and non-alkylated wherein alkyl groups cannot be added to the hydrocarbon. For aliphatic, there are saturated which contains only single carbon bonds and unsaturated which contains double or triple carbon bonds. The differences in each result made each type of hydrocarbon distinctive from one another and because of that, the hydrocarbons can be distinguished from one another in an easier way. Color is the most basic difference of all hydrocarbons from one another. Each hydrocarbon has its own specific characteristics when the tests were conducted. Through parallel chemical tests, it was concluded that the unknown was Toluene since the unknown has the same results with Toluene. Hexane is a saturated aliphatic. Cyclohexene is an unsaturated aliphatic. Toluene and Naphthalene are alkylated aromatics.

Methodology

The analysis of hydrocarbons was made possible by conducting nitration test, bromine test and basic oxidation test. In each test, there were five test tubes used labeled 1, 2, 3, 4 and 5 for each of the five hydrocarbons namely hexane, cyclohexane, toluene, naphthalene (in hexane) and the unknown (which will be known through parallel chemical tests), respectively. Five drops of each hydrocarbon was placed in its corresponding test tube.

Nitration Test Eight drops of nitrating mixture was placed in each of the five test tubes and it was shaken well. Observations were noted after two minutes. For the test tube that does not have visible change, it was heated in a water bath for eight minutes then it was diluted with twenty drops of water. Again, changes were observed.

Bromine Test Three drops of bromine reagent were placed in each test tube. Changes in color were observed for each test tube.

Basic Oxidation Test Three drops of KMnO4 reagent and two drops of 10% NaOH were added in each test tube. All test tubes were warmed in a water bath for two minutes then the changes in color were observed.

References

Books:

Boyd, R. N. & Morrison, R. T. (2002). Organic Chemistry 6th Edition. Connaught Circus, New Delhi: Prentice-Hall of India Private Limited.

Hoffman, R. V. (2004). Organic Chemistry: An Intermediate Text Second Edition. Hoboken, New Jersey: John Wiley & Sons, Inc.

McMurry, J. (2008). Organic Chemistry. Belmont, CA, USA: Brooks/Cole, Cengage Learning.

Stoker, H. S. (2010). General, Organic and Biological Chemistry 5th Edition. Belmont, CA, USA: Brooks/Cole, Cengage Learning.

World Book. (1988). The World Book Encyclopedia Volume 9: H. USA: World Book, Inc.

World Book. (1988). The World Book Encyclopedia Volume 19: T. USA: World Book, Inc.

Internet sources:

Figure 2: http://www.lib.utexas.edu/chem/tutorials/data/name_problem.html - Retrieved on December 11, 2012

Figure 3: http://www.pherobase.com/database/compound/compounds-detail-naphthalene.php - Retrieved on December 11, 2012

References: Books: Boyd, R. N. & Morrison, R. T. (2002). Organic Chemistry 6th Edition. Connaught Circus, New Delhi: Prentice-Hall of India Private Limited. Hoffman, R. V. (2004). Organic Chemistry: An Intermediate Text Second Edition. Hoboken, New Jersey: John Wiley & Sons, Inc. McMurry, J. (2008). Organic Chemistry. Belmont, CA, USA: Brooks/Cole, Cengage Learning. Stoker, H. S. (2010). General, Organic and Biological Chemistry 5th Edition. Belmont, CA, USA: Brooks/Cole, Cengage Learning. World Book. (1988). The World Book Encyclopedia Volume 9: H. USA: World Book, Inc. World Book. (1988). The World Book Encyclopedia Volume 19: T. USA: World Book, Inc. Internet sources: Figure 2: http://www.lib.utexas.edu/chem/tutorials/data/name_problem.html - Retrieved on December 11, 2012 Figure 3: http://www.pherobase.com/database/compound/compounds-detail-naphthalene.php - Retrieved on December 11, 2012