Experiment Properties of Alkenes

February 5, 2018 | Author: Mei Hui | Category: Alkene, Alkane, Chemical Reactions, Combustion, Hydrocarbons
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Discussion part for experiment properties of alkenes...


Experiment Properties of Alkenes Discussion Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond which is known as olefinic bond or ethylenic bond. Alkenes have the fewer number of hydrogen per carbon and their general formula is CnH2n. alkene is also known as olefin. Alkanes which is also known as paraffin are saturated aliphatic hydrocarbons with general formula of CnH2n+2. They have only carboncarbon single bond and carbon hydrogen bond with the maximum possible number of hydrogen per carbon. Alkenes are more reactive than alkanes due to the presence of  electrons in the ethylenic bond which is a weak bond that can easily undergo additional reaction such as electrophilic addition. Alkenes are connected to each other by ethylenic bond so they are having free electrons in the outer shell. Alkenes have more bond energy compared to alkanes because of the presence of  bond in addition to sigma bond. The presence of the  bond does not allow free movement of molecules. Hence a strain builds up in the alkene molecules which makes it more reactive. The  bond of alkenes is a very weak bond that make the electrons of  bond are more exposed to be attacked by an electrophile. Therefore, alkenes are more reactive than alkanes. In the combustion reaction, cyclohexene was burnt with a yellow-orange and very sooty flame. Alkenes burn completely in excess oxygen to produce carbon dioxide and water. However, the incomplete burning of alkenes will produce carbon monoxide and carbon. The carbon produced are the soots that can be observed during the experiment. For example, the thermochemical equation of the combustion of cyclohexene are (i) (ii)

2C6H10 (l) + 17O2(g)  12CO2(g) + 10H2O(l) C6H10 (l) + 4O2(g)  CO2(g) + CO(g) + 4C(g) + 5H2O(l)

(complete combustion) (incomplete combustion)

Alkenes (cyclohexene) burn with more sooty flame compared to alkanes (cyclohexane). This is because alkenes have higher percentage of carbon in their molecules than alkanes. The reaction by adding bromine to cyclohexene is an additional reaction which is known as bromination (halogenation). When an alkene is mixed with bromine, brown colour of bromine is rapidly decolourised. whenever the reaction is carried in dark or light, there are no differences because reaction with bromine is very reactive. The bromination starts when the electron cloud of the  bond in the hexane molecule first polarises an approaching bromine molecule. Electrophilic addition the occurs when a weak sigma bond is formed between the positive end of the polarised molecule, Br+ and one of the carbon atoms in the double bond. The carbocation or carbonium ion formed then react rapidly with the nucleophile. The example of mechanism of bromination is shown as below :

The example of chemical equation of bromination of cyclohexene is :

diethyl ether

C6H10Br2 (l)

C6H10 (l) + Br2(l) The product formed is only in single layer because both organic solvent which are bromine and cyclohexene are mixed. The reaction of adding bromine water to cyclohexene is an additional reaction of alkene. The brown colour of bromine water decolourised and two colourless layer of solution are formed. The solution formed two layer because the bromine water are inorganic solvent while cyclohexene is organic solvent which cannot mix together. The chemical equation of this experiment is : C6H10 (l) + Br2(aq)  C6H10Br2 (aq) Moreover, the reaction between cyclohexene and potassium manganate(VII) is known as hydroxylation. The purple colour of potassium manganate(VII) decolourised and 2 layer of solution are formed. In reaction with potassium manganate (VII), cyclohexene is oxidized and produce diol which two hydroxyl groups are added to the double bond. The chemical reaction of this reaction is : C6H10 + KMnO4  C6H10(OH)2 + MnO2 Hexene react with potassium manganate (VII) will produce diol. However, hexane will not react with potassium manganate (VII) because hexane does not contain any carbon-carbon double bond and it has reached maximum number of hydrogen per carbon. In addition, when cycloalkene react with concentrated sulphuric acid, 3 layer of different colour of solution are formed. the colour of the solution formed are dark brown, yellow and colourless. The dark brown colour of solution is due to the presence of the precipitate which is dark in colour known as cyclohexane hydrogen sulphate. This reaction is an oxidatihexn reaction. The chemical equation for this reaction is C6H10 + H2SO4  C6H11OSO2OH Hexane react with concentrated sulphuric acid will produce alkyl hydrogensulphates while hexane will not react with concentrated sulphuric acid. This is because alkane is non-polar molecule while sulphuric acid is polar molecule. There are no interaction between non-polar and polar molecule. Thus, hexane will not react with concentrated sulphuric acid. There are some tests can be used to distinguish between cyclohexene and cyclohexane. Firstly, react these two organic compound with concentrated sulphuric acid. Cyclohexene will react with concentrated sulphuric acid and produce three layer of colour solution while there is no reaction for the cyclohexane. Secondly, react these two organic compound with bromine. Cyclohexene will decolourise the reddish-brown bromine but cyclohexane will not decolourise the reddish-brown. Besides, reaction with acidified potassium manganate (VII) solution also can be used to distinguish between cyclohexene and cyclohexane. Cyclohexene will decolourise purple potassium manganate (VII) solution but cyclohexane will not have any reaction. During the experiment, there are some safety and precaution steps that must be done to minimize the danger and error that will occur. During the experiment do not handle the chemical with bare hand because the chemical solution such as concentrated sulphuric acid and bromine dissolved in diethyl ether are corrosive. A pair of gloves must be worn. When shaking the solution, a cork is used to

close the mouth of the test tube before shaking. The solutions such as cyclohexene and concentrated sulphuric acid must be handle in the fume chamber. After using the cyclohexene and bromine, the reagent bottle needed to close quickly to avoid the organic vapors come out. Do not shake the test tube when concentrated sulphuric acid is added to the cyclohexene. After finishing the experiment, the solutions cannot be pour into the sink, it must be pour into a reagent bottle which is used to keep the chemical waste.

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