Exp 1 Properties Of Hydrocarbon 4x2c5c

Title

: Properties of Hydrocarbons

Objective : To study the chemical properties of hydrocarbons and to determine the two unknown liquid based on their chemical properties. Theory and background: Hydrocarbons are compounds which contain only carbon and hydrogen, can be classified into several types, depending on their structure. Aliphatic hydrocarbon are divided into three classes, which is alkanes that have only single bonds and are said to be saturated; alkenes and alkynes have carbon-carbon double or triple bonds, and are said ti be unsaturated. Aromatic hydrocarbons are cyclic compounds whose structure is related to that of benzene, with six-electrons in a six-membered ring. In this experiment, there are few ways introduced to identity three classes of the aliphatic hydrocarbon, those are: I.

Combustion - where hydrocarbon is burned in excess oxygen and observation is taken to identity the properties that the particular compound might have. The end product would usually be carbon dioxide and water. There also will be a large amount of heat energy released. For observation, there should be whether soot is produced while burning.

II.

Reaction with Bromine- to have bromine reacts with the hydrocarbon. The reaction is also known as halogenations process. Due to the difference in structure, different hydrocarbon will tend to undergo different reactions. For example, alkane may not react with bromine if there is not in the present of sunlight or not at high temperature.

III.

Reaction with Potassium Permanganate- this process also known as oxidation as potassium permanganate is a well-known strong oxidizing agent. When this reaction occurs, potassium permanganate will be decolorized. Turning from purple liquid to brown precipitate.

Result : Combustion Hydrocarbon Hexane Cyclohexene Toluene Unknown A Unknown B

\Observation No soot. Distinguish very fast. Little or less soot produced. Distinguish moderately. Large amount of soot produced. Distinguish very slow Large amount of soot produced. Distinguish very slow. Little or less soot produced. Distinguish moderately.

Reaction with Bromine (dark) Hydrocarbon Observation Hexane No visible reaction. Colour of solution remain unchanged. Cyclohexene Bromine decolourises. Toluene No visible reaction. Colour of solution remain unchanged. Unknown A No visible reaction. Colour of solution remain unchanged. Unknown B Bromine decolourises. Reaction with Bromine (sunlight) Hydrocarbon Observation Hexane Bromine decolourises and form clear solution. Cyclohexene Bromine decolourises and form clear solution. Toluene Bromine decolourises to pale yellowish solution. Unknown A Bromine decolourises to pale yellowish solution. Unknown B Bromine decolourises and form clear solution. Reaction with Potassium Permanganate Hydrocarbon Observation Hexane No visible reaction. Colour of solution remains unchanged. Cyclohexene KMnO4 decolourises. Colour of solution changes from purple colour and form solution with brown precipitate. Toluene No visible reaction. Colour of solution remains unchanged. Unknown A No visible reaction. Colour of solution remains unchanged. Unknown B KMnO4 decolourises. Colour of solution changes from purple colour and form solution with brown precipitate.

Discussion : In this experiment, hexane, C6H14 is used to represent the alkane group. It is a saturated hydrocarbon which contains no double or triple bond. Besides that, cyclohexene C6H10 which consist of one double bond is used to represent alkene group. Cyclohexene contains six carbons atom, that link together in a ring form. This structure enable the two carbons atom that are linked together by the double bond not fully occupied and are ready to react to form a saturated hydrocarbon, alkane under certain situations. On the other hand, toluene C7H8 which also known as methylbenzene is used to represent the aromatic hydrocarbon. Toluene consists of one benzene ring with a methyl group that attached directly to the benzene ring. During combustion, hexane and cyclohexene burn with yellow flame. Each organic compound is completely burnt and left no soot. Based on the observation, it shows that both of the reaction undergoes a clean combustion which produces only carbon dioxide gas and water vapour as well as heat energy released. However, toluene is burnt and large amount of soot is produced. This indicate that the toluene undergo incomplete combustion causes unburnt carbon appear as soot. Base on this observation, toluene with a benzene ring appears to be stable towards combustion that leads to incomplete combustion. Besides, water vapour and carbon dioxide gas is produced. Unknown compound A undergo incomplete combustion that produced large amount of soot which same as the result of combustion of toluene. Base on this observation, it shows that unknown compound B could be toluene. While unknown compound A undergo clean and complete combustion similar to hexane and cyclohexene. Therefore, unknown compound A is unable to be determining at this stage. Bromine test is usually used to determine whether an alkane, alkene, or aromatic hydrocarbon is present. If the substance reacts with bromine, this indicates that the compound is unsaturated, and it is alkene. When the reddish brown colour of bromine disappears, it indicates that the substance is unsaturated and a reaction has occurred. In the reaction of hexane with bromine in the absence of light, there is no visible reaction occur. The reddish brown colour of bromine remains unchanged in the hexane solution. However, when the mixture is exposed to sunlight, bromine is decolourised forming a clear solution and hydrogen bromide is released as a side product. Hexane undergoes halogenations reaction in the presence of sunlight. Cyclohexene reacts with bromine in the absence of sunlight to form 1,2dibromocyclohexane. The disappearance of the reddish brown bromine colour in cyclohexene indicates reaction has been taken place. In the presence of sunlight, bromine also reacts rapidly with cyclohexene. Toluene does not react with bromine in the absence of sunlight. When it is presence of sunlight, the reddish brown bromine colour turns pale yellow. This indicates that only small amount of toluene is reacted with bromine and little amount of hydrogen bromide which is side product is formed.

For unknown compound A, there is no visible reaction in the absence of sunlight. While under sunlight, bromine decolourises forming a pale yellowish solution and hydrogen bromide gas is formed as. The reaction happened is similar to toluene. For unknown compound B, bromine decolourises to form colourless solution no matter under the sun light or kept in the dark. It shows that unknown compound B most probably would be cyclohexene Potassium permanganate is a well-known strong oxidizing agent and can also oxidize other organic compounds, such as primary and secondary alcohols. Potassium permanganate does not react with hexane because hexane is saturated. In other words, there is no double bond and all single bonds is fully occupied. However, when it is added to cyclohexene, the purple colour slowly disappears and a brown MnO2 precipitate forms. The appearance of the brown precipitate indicates a positive test for unsaturated organic compound. The reaction of permanganate ion (MnO4-) with cyclohexene formed diol. Toluene does not react with potassium permanganate although it consists of double bonds. This indicates toluene which contain benzene ring is stable against oxidising. Unknown compound A does not react with potassium permanganate thus no colour is changes in the solution. Unknown compound B react with potassium permanganate and the purple colour slowly disappears and a brown MnO2 precipitate forms. Conclusion : Saturated hydrocarbons such as hexane can give out a complete combustion and less soot produced. It is not easily oxidised or react with halogen. On the other hand, unsaturated hydrocarbons like cyclohexene are easily react with potassium permanganate and halogen. They also tend to produce more soot compare to saturated hydrocarbons during combustion. While for toluene, toluene tends to produce a large amount of soot during the combustion. Therefore, after the tests on unknown compound A and B, it can be concluded that unknown compound A may be toluene, whereas unknown compound B may be cyclohexene.