FOUNDATION IN MEDICAL STUDIES (JULY 2013 INTAKE) CHM1024 LABORATORY REPORT PRACTICAL 3 : IDENTIFICATION OF HYDROCARBONS
NAME
:
AKMAL ADIB BIN FADZIL
MATRIX ID
:
CPM0018_2013C CPM0018_2013C
GROUP
:
A
SEMESTER
:
TWO
DATE
:
17th DECEMBER 2013
LECTURER
:
MR. MOHD YUSOFF HUSSAIN
OBJECTIVE The objective of this experiment is to identify hydrocarbons and determine their properties.
INTRODUCTION Hydrocarbons are organic compounds that contain only carbon and hydrogen. Hydrocarbons can be classified into two main types which are aliphatic hydrocarbons and aromatic hydrocarbons. Aliphatic hydrocarbons are further divided into three major groups which are alkanes, alkenes and alkynes. Aromatic hydrocarbons are hydrocarbons that have at least one aromatic ring. They are known as arenes. Some examples of aromatic hydrocarbons include benzene (C 6H6) and methylbenzene (C 6H5CH3). These hydrocarbons can also be classified into saturated and unsaturated hydrocarbons. If a hydrocarbon contains only carbon-carbon single bonds (C – – C) and has no multiple bonds (double or triple bonds), it is said to be saturated because it has the maximum number of bonded hydrogen atoms. If a hydrocarbon contains multiple bonds, it is said to be unsaturated. That means that alkanes and cycloalkanes are classified as saturated hydrocarbons while alkenes, alkynes and aromatic hydrocarbons are known as unsaturated hydrocarbons.
Alkanes are the simplest family of hydrocarbons hydrocarbons that t hat contain carbon – – hydrogen bonds and carbon – – carbon single bonds. The general formula for alkane is C nH2n+1 where n can be either 1, 2, 3 and so on. 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 that makes other things gets attracted. Alkanes have a fairly restricted set of reactions that in which are combustion, halogenations and cracking process. Alkanes are not very reactive and have little biological activity.
Cycloalkanes or cyclic alkanes are a type of hydrocarbon just like alkanes but contain one or more rings. Cycloalkanes with a single ring are named analogously to their normal alkane counterpart of the same carbon count such as cyclopropane, cyclobutane, cyclopentane cyclohexane and so on. The larger cycloalkanes with greater than 20 carbon atoms are typically called paraffins. Cycloalkanes have the same general formula as alkenes (C nH2n). Therefore, cycloalkanes are isomeric with alkenes. Cycloalkanes are similar to alkanes in their general physical properties, but they have higher boiling points, melting points, and densities and densities than alkanes. 2
Alkenes are unsaturated hydrocarbons hydrocarbons containing a carbon – – carbon double bond. The general formula for alkene is C nH2n where n can be either 2, 3, 4 and so on. The f irst member of the alkene family is ethene with a formula of C 2H4. Alkenes are relatively stable compounds but are more reactive than alkanes because of the presence of a carbon – carbon double bond. The majority of the reactions of alkenes involves the break of this double bond into single bonds. The main reaction of alkene is addition such as hydrogenation, halogenation and oxidation.
Cycloalkenes or cyclic alkenes are a type of hydrocarbon related to alkenes but have one or more rings. Cycloalkenes are named in a similar way as cycloalkanes but since there is no chain end to begin from, the cycloalkenes are numbered so that the carbon atoms in the double bond are designated as C1 and C2. Alkenes contain double bonds by definition, and cycloalkenes benefit from enhanced stability if the double bonds are conjugated. Cyclohexene is one such case, where if it contains a total of three double bonds it is no longer called cyclohexene. A six-carbon ring with three alternating (and therefore conjugated) double bonds is calle called d benzene, and benzene ben zene is such a stable molecule that t hat it displays completely different chemistry than other non-aromatic cycloalkenes. Cycloalkenes are perhaps among the most important organic substances for biological and industrial purposes because they are used in the production of molecules essential to a broad spectrum of applications. Understanding cycloalkenes is a critical part of understanding organic chemistry and its applications to biology, medicine, industry and every other relevant field.
For this experiment, cyclohexane (member of cycloalkane family) and cyclohexene (member of cycloalkene family) are to be used for identification and determination of hydrocarbon properties.
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APPARATUS AND MATERIALS 1. 10 test tubes. 2. Test tube rack. 3. 100 ml beaker. 4. Test tube holder (clamp). 5. Cyclohexane Cyclohexane (C6H12). 6. Cyclohexene Cyclohexene (C6H10). 7. Concentrated sulphuric acid (H 2SO4). 8. 2 % bromine in tetrachloromethane tetrachl oromethane (Br 2/CCl4). 9. Dilute potassium permanganate solution (0.5 % KMnO 4 solution) 10. Dilute Dilute sodium hydroxide solution (10 % NaOH solution). 11. Dilute sodium carbonate solution (10 % Na 2CO3 solution). 12. Distilled water. 13. Blue litmus paper.
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METHOD A. Bromine Test. 1. 1 ml of cyclohexane is added to 5 – – 10 drops of 2 % bromine in tetrachloromethane (Br 2/CCl4) in a clean test tube. The test tube is shaken well and after two or three minutes, any changes are observed. 2. A second similar test tube is prepared. The first test tube is placed in the laboratory locker and the second one is placed in a bright sunlight. 3. Both test tubes are are allowed to stand for 10 – – 15 minutes and are compared after that. 4. The colour of of the solution solution in each test tube is observed. 5. A blue litmus paper is placed into each test tube to test whether hydrogen bromide was evolved or not. 6. Steps 1 – – 5 are repeated with cyclohexene with just one test tube of sample. (Sunlight reaction is not needed) Apparatus set-up for Experiment A
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B. Aqueous Potassium Potassium Permanganate Permanganate Test (Baeyer’s (Baeyer’s Test). 1. 1 ml of cyclohexane is added to a mixture of 5 – 10 – 10 drops of dilute potassium permanganate solution (0.5 % KMnO 4 solution) and 3 drops of dilute sodium carbonate solution (10 % Na 2CO3 solution) in a clean test tube. 2. The test tubes are are shaken shaken for 1 – 2 – 2 minutes and the results are noted.
Apparatus set-up for Experiment B
3. Steps 1 – 2 – 2 are repeated with cyclohexene and the results are noted.
C. Sulphuric Acid Test. 1. 1 ml of cyclohexane is added into into a clean test tube. With gentle shaking, shaking, 5 drops of concentrated sulphuric acid are added very cautiously. 2. The test tubes are shaken shaken well and and the results results are noted. noted. 3. Several aspects are observed which includes whether heat is evolved or not and whether the hydrocarbon dissolves or not. 4. The contents are discarded discarded by pouring them into into a beaker containing containing at least 50 ml of water. 5. Steps 1 – 4 – 4 are repeated with cyclohexene and the results are noted. Apparatus set-up for Experiment C
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D. Sodium Hydroxide Test. 1. 1 ml of of cyclohexane is added to 5 drops of dilute sodium hydroxide solution (10 % NaOH solution). The test tube is shaken well. 2. Any changes are observed observed and the results are noted. 3. Steps 1 – 2 – 2 are repeated with cyclohexene and the results are noted. Apparatus set-up for Experiment D
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RESULTS Observation Reagent Used
2 % Br 2 in CCl4 (in the dark)
Cyclohexane
Cyclohexene
The yellowish-brown
The yellowish-brown
solution remains
solution turns colourless.
unchanged. The colour of
The colour of the blue
the blue litmus paper
litmus paper changes from
remains unchanged.
blue to red.
The yellowish-brown solution turns colourless. 2 % Br 2 in CCl4 (in sunlight)
The colour of the blue
-
litmus paper changes from blue to red. The solution contains The solution contains 0.5 % KMnO4 + 10 % Na2CO3
double layers. The colour of the solution remains purple.
double layers. The colour of the solution changes from purple to brown. There are presence of brown precipitates in the solution.
Concentrated H2SO4
10 % NaOH
The solution contains
The solution contains
double layers. The colour of
double layers. The colour of
the solution remains
the solution changes from
colourless. No heat is
colourless to cloudy. Heat is
evolved.
evolved.
The colour of the solution
The colour of the solution
remains colourless. The
changes from colourless to
solution contains double
yellow. The solution
layers.
contains double layers.
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DISCUSSIONS Molecular and Structural Formula of Cyclohexane and Cyclohexene. As mentioned in the introduction, introduction, cyclohexane cyclohexane is a member of the cycloalkane cycloalkane family. Cyclohexane is produced by reaction between benzene and hydrogen. Cyclohexane has a distinctive detergent-like detergent-like odor, odor, reminiscent of of cleaning products. products. Its molecular molecular formula is C6H12. Its structural formula is shown below : Expanded structural formula
Skeletal structural formula
Cyclohexene is a member of the cycloalkene family. It is produced by partial hydrogenation of benzene. It is a ccolourless liquid with a sharp smell. It is also not very stable for prolonged storage with exposure to light and air because it will form peroxides. Cyclohexene has a molecular formula of C 6H10. Its structural formula is shown below below : Expanded structural formula
Skeletal structural formula
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Experiment 1 : Bromine Test. For this experiment, 1 ml of cyclohexane is added to 5 – – 10 drops of 2 % bromine in tetrachloromethane tetrachloromethane in a test tube and shaken well. well. A similar one is prepared. prepared. One of them is placed in a laboratory locker to represent reaction in dark. The other one is placed under sunlight. This experiment is conducted to detect halogenation of hydrocarbons which is one of the substitution reaction that requires a presence of ultraviolet (UV) light for halogenation to takes place. For the test tube placed in the dark, there are no changes detected. This is because there are no presence of UV light. For the test tube placed under sunlight, it is observed that the yellowish-brown solution turns colourless. This is because the presence of UV light from the Sun causes a hydrogen atom from the cyclohexane to be replaced by a bromine atom from the bromine via the free radical substitution mechanism. The brownish-yellow colour of the bromine fades and the solution becomes colourless. The product of the halogenation reaction is bromocyclohexane and hydrogen bromide. The chemical equation is as follows : C6H12 + Br 2 → C6H11Br + HBr Besides that, the colour of the blue litmus paper changes to red. This is because hydrogen bromide has acidic properties that causes a change in colour of the blue litmus paper from blue to red. The experiment is repeated but this time, cyclohexane is changed with cyclohexene. Unlike cyclohexane, cyclohexene react with bromine in both light and dark conditions. That is why reaction in the dark for cyclohexene is not done as the result will be the same as the reaction under sunlight. For the test tube placed under sunlight, it is observed that the colour of the yellowish-brown solution turns colourless. This is because bromine reacts with cyclohexene to form the product of addition reaction which is dibromocyclohexane. The reaction between them is called an electrophilic addition reaction because the bromine molecule is an electrophilic reagent and it is added to the double bond of cyclohexene during reaction. The yellowish-brown colour of the bromine fades and the solution becomes colourless. The chemical equation is as follows : C6H10 + Br 2 → C6H10Br 2 Besides that, it is observed that the blue litmus paper changes colour from blue to red. This is because the dibromocyclohexane is acidic.
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Experiment 2 : Aqueous Potassium Permanganate Permanganate Test (Baeyer’s Test). For this experiment, 1 ml of cyclohexane is added to a mixture of 5 – – 10 drops of dilute potassium permanganate solution (0.5 % KMnO 4 solution) and 3 drops of dilute sodium carbonate solution (10 % Na 2CO3 solution) in a clean test tube. The test tube is shaken well for 1 – – 2 minutes. There is no reaction or changes occurred except for the presence of double layer forming on the surface of the solution. It is formed because of the presence of oil in the benzene ring of cyclohexane. The experiment is repeated but this time, t he cyclohexane is replaced with cyclohexene. It is observed that the colour of the solution changes from purple to brown. It is also observed that there are presence of brown precipitates in the solution. This is because the cycloalkene is oxidised by the dilute alkaline KMnO 4 solution at room temperature. The purple colour of the KMnO4 solution disappears and a cloudy brown colour appears is caused by the precipitation of manganese (IV) oxide, MnO 2. The decolourisation of the purple solution of MnO4- ions at room temperature is a test for carbon – – carbon double or triple bonds. The presence of double layer on the surface of the solution has the same reason as in cyclohexane. cyclohexane. The chemical equation for the oxidation reaction is as follows :
3C6H10 + KMnO4 + 2H2O → 3C6H10OH + MnO2 + KOH
Experiment 3 : Sulphuric Acid Test. For this experiment, 1 ml of cyclohexane is added to a clean test tube. With gentle shaking, 5 drops of concentrated sulphuric acid are added into the test tube very cautiously. The test tube is shaken well. The test tube is quickly then placed onto the palm to detect whether heat is evolved from the reaction. It is observed that there are no changes and no heat is evolved. This is because cycloalkanes cannot undergo addition reaction with concentrated sulphuric acid as cycloalkanes are not reactive. The experiment is repeated with cyclohexene instead of cyclohexane. It is observed that the colour of the solution changes from colourless to cloudy. This is because cyclohexene reacts with concentrated sulphuric aid to produce alkyl hydrogensulphate which causes the solution to turn cloudy. Besides that, heat is evolved. It is because of the breaking of the carbon – carbon double bond in cyclohexene which causes energy to be released in the form of heat energy. For both experiments, double layer is formed on the surface of the solution. It is caused by the presence of oil on the benzene ring of cyclohexane and cyclohexene. 11
Experiment 4 : Sodium Hydroxide Test For this experiment, 1 ml of cyclohexane is added to 5 drops of dilute sodium hydroxide solution in a clean test tube. The test tube is shaken well. For cyclohexane, there are no changes happens. The experiment is then repeated with cyclohexene. It is observed that the colour of the solution changes from colourless to yellow. The presence of double layer in both test tubes are caused by the presence of oil on the benzene ring of cyclohexane and cyclohexene.
Why Cyclic Hydrocarbons Are Used Instead Of Chain Hydrocarbons? Cyclic hydrocarbons (cycloalkanes and cycloalkenes) are similar to chain hydrocarbons (alkanes and alkenes) in their general physical property but cyclic hydrocarbons have higher boiling points, melting points and densities than chain hydrocarbons. This is due to the stronger London forces as the ring shape allows for a larger area of contact. Containing only C – C C – H H bonds, unreactivity of cyclic hydrocarbons with little or no ring strain makes – C and C – it preferable for use in the experiment instead of the chain hydrocarbons. when used in the experiments, the results shown have a larger, clearer reaction and easier to detect changes rather than chain hydrocarbons because the chain hydrocarbons’ reactions are too fast for detection in the lab.
Cyclohexane VS Cyclohexene. Based from all of the experiments, it can be seen that cyclohexene is more reactive compared to cyclohexane. Cyclohexene participates in many reactions such as, oxidation and addition reactions mainly due to its C – C – C double bond presence. Cyclohexane does not participate in any addition reaction due to its C – – C single bond. Cyclohexane undergoes halogenation in the first experiment but remain unreactive for the other experiments.
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Precautions. Throughout the experiment, there a few precautions that are taken and noted. One of them is that all of the chemical wastes used during the experiments are disposed in the waste container prepared by the lab instructor. This is to because some of them are too dangerous to be disposed normally. Some of the wastes are also hard to be cleaned if it is disposed normally into the sink. Besides that, all of test tubes used in the experiment needs to be rinsed and cleaned with distilled water before use. This is to avoid any chemical substances that may still contained inside the test tubes to interfere with the results. Another one is that cyclohexane, cyclohexene and potassium permanganate solution are handled in a fume chamber and the ventilation of the lab must be in good condition. This is to avoid or at least minimise the inhalation of these substances among the students.
CONCLUSION In this experiment, experiment, the properties and difference between alkanes and alkenes are identified. In can be seen from the experiments that alkenes (represented by cyclohexene) are more reactive than alkanes (represented by cyclohexane). It is mainly due to the difference of C – – C bonds between alkanes and alkenes. Alkenes shows a lot of reactions including, addition reaction and oxidation reaction. Alkanes shows only substitution reaction in this experiment. Therefore, alkenes are more reactive hydrocarbon than alkanes.
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QUESTIONS 1. Why is there a difference difference in reaction reaction when the bromine test is done in the dark dark and in the presence of sunlight? Explain your answer. The difference is clear when cyclohexane is used. Cyclohexane requires presence of UV light so that halogenation reaction can takes place. Cyclohexane reacts with bromine when the test is done in the presence of sunlight because sunlight contains UV light that is used for the halogenation reaction. Cyclohexane does not reacts with bromine when the test is done in the dark because there are no presence of UV light for the halogenations reaction to take place.
2. Write the equation for the reaction between cyclohexane and sodium hydroxide. Based from the experiment, there is no equation for the reaction between cyclohexane and sodium hydroxide because the cyclohexane did not react with sodium hydroxide.
3. Explain why the hydrocarbons hydrocarbons dissolve in one solvent solvent but not the other. Hydrocarbons are covalent compounds and does not dissolve in ionic solvents. It tends to dissolve in a solvent that has similar properties main in polarity aspect. Hydrocarbons are non-polar. Thus, it cannot dissolve in water which is a polar solvent. 4. Explain why alkenes are are more reactive compared compared to alkanes. Alkenes are unsaturated unsaturated hydrocarbons that consist consist C – C – C double bond. These double bonds have a high tendency to break and combine with other elements to form single bonds which are more stable. Alkanes are saturated compounds that consist of C – C – C C single bonds which are stable. Therefore, alkenes are more reactive than alkanes.
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