Planning and Designing Lab
Download as DOCX, PDF9 likes29,475 views
This experiment aims to identify alkanes and alkenes by their reactions with different substances and determine their properties. Cyclohexane and cyclohexene are used as representative alkanes and alkenes, respectively. A series of tests are conducted including bromine, potassium permanganate, sulfuric acid and sodium hydroxide tests. The results show that cyclohexene is more reactive than cyclohexane due to the presence of a carbon-carbon double bond, undergoing addition reactions, while cyclohexane only undergoes substitution reactions. This supports the hypothesis that alkenes are much more reactive than alkanes.
Planning and Designing Lab
- 1. 1 PLANNING AND DESIGNING LAB Title Alkanes and Alkenes Hypothesis Alkenes are much more reactive than alkanes. Aim The purpose of carrying out this experiment is to identify Alkanes and Alkenes by their reactions with different substances 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 are found naturally occurring 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,
- 2. 2 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.
- 3. 3 Materials 10 Test tubes Test tube rack 100 ml beaker Test tube holder (clamp) Alkane Alkene 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 In a clean test tube, 1 ml of the alkane 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. 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. Both of the test tubes are allowed to stand for 10-15 minutes and both were then compared. The color of the solution in each test tube is observed.
- 4. 4 A blue litmus paper is placed into the test tube to test whether or not hydrogen bromide was evolved. The results are recorded. Steps 1-5 are repeated with alkene, with just one test tube of sample. (There is no need for sunlight reaction.) B. Aqueous Potassium Permanganate (Baeyer’s Test) In a clean test tube, 1 ml of alkane 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). The tube is shaken for 1-2 minutes and the results are noted. Steps 1-2 are repeated with alkene and the results are noted. C. Sulphuric Acid Test 1. In a clean test tube, 1 ml of alkane 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 alkene and the results are noted. D. Sodium Hydroxide Test 1. In a clean test tube, 1 ml alkane 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 alkene and the results are noted.
- 5. 5 Results Table Reagent Used Observation Alkane Alkene 2% Br2 in CCl4 (in the dark) 2% Br2 in CCl4 (in sunlight) 0.50% KMnO4 + 10% Na2CO3 Concentrated H2SO4 10% NaOH
- 6. 6 Expected Results Reagent Used Observation Alkane Alkene 2% Br2 in CCl4 (in the dark) There is no reaction. The blue litmus paper remains its color. The yellowish brown solution turns cloudy. The blue litmus paper turns pink. 2% Br2 in CCl4 (in sunlight) The yellowish brown solution turns colorless. The blue litmus paper turns pink 0.50% KMnO4 + 10% Na2CO3 The solution contains double layers and remains purple in color. The solution contains double layers. The purple solution turns brown color and there are brown precipitates present. Concentrated H2SO4 The solution is clear. It has double layers. There is no heat evolved. The solution contains layers. The solution turns cloudy and there is heat released. 10% NaOH The solution remains colorless with double layers. The colorless solution turns yellow with double layers.
- 7. 7 Discussions In this experiment, hopefully all of the tests conducted will be successful. The hydrocarbons that are used will be identified and determined by their properties. In this experiment, two different hydrocarbons are used which are alkane and alkene. If however it is assumed that the alkane is Cyclohexane which has chemical equation of C6H12 and is produced by reaction of benzene and hydrogen. It has chemical structure as shown below: Meanwhile, the alkene is cyclohexene and is produced by partial hydrogenation of benzene. Cyclohexene has chemical formula C6H10 and its chemical structure is as shown below: The following discussion could be made for the expected results of the reactions of these hydrocarbons: 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
- 8. 8 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 color 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 colorless 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. 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. 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 color except that a double layer is formed on the surface of the solution. The double layer formed
- 9. 9 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 color. There is also brown precipitate formed and the surface of the solution has double layers. The purple solution changes color 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. 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 colorless 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 hydrogen sulphates. 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
- 10. 10 is then repeated by using cyclohexene to replace cyclohexane. The colorless solution turns yellow and has a double layer on the surface of the solution for cyclohexene. Throughout all the tests, it can be 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 halogenations which is a substitution reaction in the first test but remain unreactive on the other tests.
- 11. 11 Variables The dependent variable is the factor that is measured in order to determine how it responds to changes made to the independent variable (i.e. the value of the dependent variable depends on the independent variable). In this experiment the dependent variable is the reactivity of the molecules (which we measured by the rate at which they decolored bromine water). It is important to control all variables other than the dependent variable and independent variable to make the experiment valid – ie so that measured changes to the dependent variable are only due to the changes made to the independent variable. Some examples from this experiment include temperature, time of measurement, light intensity, and the fact that we used corresponding molecules (same # of carbons with the same shape). A control is a sample that is left unchanged so that all other results can be compared to it (ie to determine if the independent variable really had any effect on the dependent variable). In this case samples of bromine water were left, cyclohexene and cyclohexane unmixed over the same time period as the experiment. The fact that they did not change colour indicates that the observed color changes in the experiment were due to the reaction between cyclohexene and bromine water.
- 12. 12 Limitations/Sources of Error Human errors, such as measuring incorrectly, inadvertently contaminating a solution by dropping another substance into it, or using dirty instruments, are examples of how making a simple mistake affects the experiment. Equipment limitations also cause errors if instruments are not calibrated properly or if an instrument is unable to take a measurement because of calibration limitations. Taking measurements during an experiment is another source of observation errors. Measurement values may not be well defined, which means that some items may have a range of values rather than a single value. Finally, inconsistent sampling techniques also cause errors. Every time an experiment is done, each step must be repeated the same way as it was previously. If this does not happen, different results are likely. 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.
- 13. 13 REFERENCES Fesseden R.J. and Fesseden, J.S. Organic Chemistry, 4th Edition, California, U.S.A, Pacific Grove, 1990, Pages 103, 431, 477. Hornback, J.M. 1998. Organic Chemistry. Brooks/ Cole Publishing Company, Pacificgrove. p.p 816-817 Maitland, J.J. 1997. Organic Chemistry.w.w Norton and Company, USA. pp. 616 Michelle, J. Sienko, Robert A. Plane, Chemistry, 5th Edition, London, McGraw-Hill Book Company, 1981, Pages 455-465 Personal Book References, Essential Chemistry SPM, Longman Publication, 2009. Personal Book References, Module, Teaching and Learning Chemistry, Form 4, Malindo Publications, 2009 Shriner, Hermann, Morill, et. Al. 2004. The Systematic Identification of Organic Compounds. John Wiley and Sons Inc.USA. p. 130