CHEM LAB (II) 3

ALLIANZE UNIVERSITY COLLEGE OF MEDICAL SCIENCES FOUNDATION OF MEDICAL STUDIES 2011

CHEMISTRY LABORATORY REPORT

EXPERIMENT 2: IDENTIFICATION OF HYDROCARBONS

NAME

: NURUL HASANAH BINTI NOR IKHSAN

MATRIX NO. : CPM 33/11C GROUP

:A

SEMESTER : II DATE

: 21.12.2011

OBJECTIVE The purpose of carrying this experiment is to identify hydrocarbons and determine their properties.

INTRODUCTION In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls. Aromatic hydrocarbons (arenes), alkanes, alkenes, alcohol, esters and alkyne-based compounds are different types of hydrocarbons. The majority of hydrocarbons found naturally occur in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen which, when bonded, can catenate to form seemingly limitless chains. Hydrocarbons are divided into two classes known as aliphatic compounds and aromatic compounds. Aliphatic compounds are namely alkanes and alkenes. In this experiment, the hydrocarbons that are being used to identify and determine their properties are cycloalkane and cycloalkene which are from group alkane and alkene respectively. Alkanes are the simplest family of hydrocarbons compounds that contain carbon-hydrogen bonds and carbon-carbon single bonds. The carbon-hydrogen bonds are only very slightly polar and so there are no parts of the molecules which carry any significant amount of positive or negative charge which other things might be attracted to. The net effect is that alkanes have a fairly restricted set of reactions. Thus, alkanes can only undergo combustion, halogenations and cracking process. Alkanes are not very reactive and have little biological activity. The formula of alkane is CnH2n+1 in which n started from 1,2,3,4 and so on. For example, the first three are: 

Methane, CH4 where n = 1



Ethane, C2H6 where n = 2



Propane, C3H8 where n = 3

Alkanes with more than three carbon atoms can be arranged in numerous ways, forming different structural isomers. An isomer, in part, similar to a chemical anagram but unlike an anagram, may contain varying number of atoms and components, for which in a chemical compound can be structurally arranged in a multitude of different combinations and permutations. The simplest isomer of an alkane is the carbon atoms are arranged in a single chain with no branches. However the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms. Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond. Alkenes are relatively stable compounds but are more reactive than alkanes due to the presence of a carbon-carbon double bond. The majority of the reactions of alkenes involve the rupture of this carbon-carbon double bond, forming new single bonds. Thus, the main reaction of alkene is addition such as hydrogenation, halogenation and oxidation. The formula of alkene is CnH2n where n starts from 2,3,4,5 and so on. Thus, the first member of alkene family is ethene with a formula of C2H4. As predicted by the VSEPR model of electron pair repulsion, the molecular geometry of alkenes includes bond angles about each carbon in a double bond of about 120°. The angle may vary because of steric strain introduced by non-bonded interactions created by functional groups attached to the carbons of the double bond.

MATERIALS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

10 Test tubes Test tube rack 100 ml beaker Test tube holder (clamp) Cyclohexane Cyclohexene Concentrated sulphuric acid solution, H2SO4 2% Bromine in tetrachloromethane 0.50% potassium permanganate solution, KMnO4 10% sodium hydroxide solution, NaOH 10% sodium carbonate solution, Na2CO3 Distilled water Blue litmus paper

PROCEDURE A. Bromine Test 1. In a clean test tube, 1 ml of cyclohexane is added to 3 ml of 2 % bromine in tetrachloromethane (Br2/CCl4). The test tube is shaken well and is observed after two to three minutes. 2. A second test tube is prepared similarly. The first test tube is placed in a laboratory locker and the second is placed in bright sunlight. 3. Both of the test tubes are allowed to stand for 10-15 minutes and both were then compared. 4. The colour of the solution in each test tube is observed. 5. A blue litmus paper is placed into the test tube to test whether or not hydrogen bromide was evolved. The results are recorded. 6. Steps 1-5 are repeated with cyclohexene, with just one test tube of sample. (There is no need for sunlight reaction.) B. Aqueous Potassium Permanganate (Baeyer’s Test) 1. In a clean test tube, 1 ml of cyclohexane is added to a mixture of 3 ml of dilute potassium permanganate solution (0.5% KMnO4 solution) and 3 ml of dilute sodium carbonate solution (10% Na2CO3 solution). 2. The tube is shaken for 1-2 minutes and the results are noted. 3. Steps 1-2 are repeated with cyclohexene and the results are noted. C. Sulphuric Acid Test 1. In a clean test tube, 1 ml of cyclohexane is added and with gently shaking, 3 ml of concentrated sulphuric acid is added very cautiously. 2. The tubes are shaken well and the results are noted. 3. It is observed whether heat evolved and whether the hydrocarbon dissolves. 4. The contents are discarded by pouring them into a beaker containing at least 50 ml of water. 5. Steps 1-4 are repeated with cyclohexene and the results are noted. D. Sodium Hydroxide Test 1. In a clean test tube, 1 ml cyclohexane is added to 3 ml of dilute sodium hydroxide solution (10% NaOH solution) and is shaken well. 2. The changes are observed and the results are noted. 3. Steps 1-2 are repeated with cyclohexene and the results are noted.

RESULTS

Observation

Reagent Used

Cyclohexane

Cyclohexene

There is no reaction. The blue The yellowish brown solution 2% Br2 in CCl4 (in the dark)

litmus paper remains its colour.

turns cloudy. The blue litmus paper turns pink.

The yellowish brown solution 2% Br2 in CCl4 ( in sunlight)

turns colourless. The blue litmus

-

paper turns pink. The solution contains double The solution contains double 0.50% KMnO4 + 10% Na2CO3

layers and remains purple in layers. The purple solution turns colour.

brown colour and there are brown precipitates present.

The solution is clear. It has The solution contains double Concentrated H2SO4

double layers. There is no heat layers. The solution turns cloudy evolved.

10% NaOH

and there is heat released.

The solution remains colourless The colourless solution turns with double layers.

yellow with double layers.

DISCUSSIONS In this experiment, all of the tests conducted are a success. Thus, hydrocarbons that are used are identified and determined their properties. In this experiment, two different hydrocarbons are used which are cyclohexane and cyclohexene. Cyclohexane has chemical equation of C6H12. It is an alkane and is produced by reaction of benzene and hydrogen. It has chemical structure as shown below:

Meanwhile, cyclohexene is an alkene and is produced by partial hydrogenation of benzene. Cyclohexene has chemical formula C6H10 and its chemical structure is as shown below:

The first test conducted in this experiment is bromine test. The test is been done by adding 1 ml cyclohexane to 3 ml of 2 % bromine in tetrachloromethane in a clean test tube. It is done again in another new clean test tube. One of the test tubes is then placed under the sunlight while the other test tube is placed in the dark in a laboratory locker. The process that will take place is called halogenation, a substitution reaction which needs the presence of ultraviolet light or sunlight as it is a photochemical reaction that will produce bromocylohexane and hydrogen bromide. Thus, there will be no reaction in the test tube placed in the dark. The blue litmus paper does not change in colour when put into the test tube placed in the dark. However, there are reactions in test tube placed under the sunlight. The yellowish brown solution in the test tube turns colourless as the hydrogen atom will be replaced with bromine atom and produced bromocyclohexane and hydrogen bromide. The blue litmus paper turns pink as hydrogen bromide is acidic. C6H12 + Br2

C6H11Br + HBr

The experiment is then repeated by replacing cyclohexane with cyclohexene. However, for cyclohexene there is no reaction in the dark as it is much more reactive and undergoes addition reaction. The double bond of the alkene breaks down becomes single bonds which bromine atoms bonded to each of the carbon atoms that shared the double bond. Thus, the yellowish brown solution turns cloudy as it forms dibromocyclohexane and the blue litmus paper turns pink as it is acidic. After conducting this test, the mixture solutions then are discarded in a proper waste container.

C6H10 + Br2

C6H10Br2

The second test conducted is Baeyer’s Test which is aqueous potassium permanganate test. This test is been done by adding 1 ml of cyclohexane to a mixture of 3 ml of dilute potassium permanganate solution and 3 ml of dilute sodium carbonate solution in a clean test tube. The test tube is then shaken for 1 to 2 minutes. There is no reaction in the solution as it remains purple in colour except that a double layer is formed on the surface of the solution. The double layer formed is due to the oil present in the benzene ring of the cyclohexane. The test is then repeated by using cyclohexene instead of cyclohexane. The reaction that takes places is oxidation which causes the purple solution to turn brown colour. There is also brown precipitate formed and the surface of the solution has double layers. The purple solution changes colour to brown because potassium permanganate is reduced to manganese dioxide, which is the brown precipitate. The double layers exist due to the same reason as in the cyclohexane. 3C6H10 + KMnO4 + 2H2O

3C6H10OH + MnO2 + KOH

The test that is conducted after that is the sulphuric acid test. 1 ml of cyclohexane is added into a clean test tube and with gently shaking, 3 ml of concentrated sulphuric acid is added cautiously. The tubes are then shaken for a while. Then, quickly the test tube is placed on the palm of my hand to test whether heat evolved. Alkanes are not reactive as this reaction is an addition reaction. Hence the solution remains colourless but there is a double layer that formed on the surface of the solution. For cyclohexene, the solution turns cloudy and there is heat released. Cyclohexene reacts with concentrated sulphuric acid to produce alkyl hydrogensulphates. There is heat released due to the breaking down of the carbon-carbon double bond in cyclohexene which releases energy that produces a net evolution of heat energy. The mixture of solution is then discarded into a beaker containing with at least 50 ml of distilled water to dilute the concentrated sulphuric acid in the mixture as it is too acidic before pouring it into the sink. The last test is sodium hydroxide test that is conducted by adding 1 ml of cyclohexane to 3 ml of dilute sodium hydroxide solution and is then shaken well. There was no change or reaction occurs in the solution except that a double layer is formed on the surface of the solution. The test is then repeated by using cyclohexene to replace cyclohexane. The colourless solution turns yellow and has a double layer on the surface of the solution for cyclohexene. Throughout all the tests, it is noted that cyclohexane is compared with cyclohexene to identify and determine their properties. It can be seen that cyclohexene is much more reactive compared to cyclohexane as it undergoes addition reactions. However, cyclohexane does not participate in any addition reaction due to its carbon-carbon single bond. Instead, cyclohexane undergoes halogenation which is a substitution reaction in the first test but remain unreactive on the other tests.

PRECAUTIONS. 1. Students must always wear lab coats all the time when conducting the experiment. 2. Students must wear gloves when handling with dangerous chemicals such as concentrated sulphuric acid as it is corrosive. 3. Ensure that chemicals such as cyclohexane, cyclohexene and potassium permanganate are handled in the fume chamber. 4. Dispose all waste and chemicals such as bromine water in an appropriate waste container. 5. Ensure that contents containing concentrated sulphuric acid are discarded into a beaker containing at least 50 ml of water before pouring it into the sink.

CONCLUSION In this experiment, the properties and the difference between an alkane and alkene is identified. Hence, this experiment is a success. Alkane is identified from alkene by its properties. It can be seen from the tests conducted in the experiment that alkene is much more reactive than alkane. This is due to its carboncarbon double bond since it is an unsaturated hydrocarbon. Alkenes undergo a lot of addition reaction while alkane does not undergo any addition reaction in the experiments conducted. Instead, alkane only undergo halogenation which is a substitution reaction when react with bromine water. Thus, alkene is more reactive than alkane.

QUESTIONS 1. Why is there a difference in reaction when the bromine test is done in the dark and in the presence of sunlight? Explain your answer. Cyclohexane solution undergoes halogenation in the bromine test which needs the presence of ultraviolet light or sunlight as this is a photochemical reaction. Therefore, cyclohexane solution reacts in bromine test when placed under the bright sunlight to produce carbon and hydrogen bromide but there is no reaction when it is placed in the dark. 2. Write the equation for the reaction between cyclohexane and sodium hydroxide. There is no chemical equation as there is no reaction between cyclohexane and sodium hydroxide. 3. Explain why the hydrocarbons dissolve in one solvent but not in the other? Hydrocarbons are covalent compounds and do not dissolve in ionic solvent. It tends to dissolve in a solvent that has similar properties with it particularly in polarity. Hydrocarbons are non-polar, thus it cannot dissolve in water which is a polar solvent. 4. Explain why alkenes are more reactive compared to alkanes. Alkene is an unsaturated hydrocarbon that contains carbon-carbon double bond in which it has a high tendency to break and combine with other elements to form single bonds as it undergoes addition reaction when react with other substances compared to alkane that consist of carbon-carbon single bond. Hence, alkene is much more reactive than alkane.

REFERENCES Other than the laboratory manual, the sources below has also contributed in making this report :       

Chemistry notes, Chapter 4 : Hydrocarbons Part 1 And 2 by Miss Shyamala A. Personal Book References, Essential Chemistry SPM, Longman Publication, 2009. Personal Book References, Module, Teaching and Learning Chemistry, Form 4, Malindo Publications, 2009 Internet source I : http://en.wikipedia.org/wiki/Hydrocarbon Internet source II : http://en.wikipedia.org/wiki/Alkane Internet source III : http://en.wikipedia.org/wiki/Alkene Internet source IV : http://www.elmhurst.edu/~chm/vchembook/500hydrocarbons.html