Procedure C: Reaction with Acidified Potassium Manganate (VII)
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Place 3-4 drops of cyclohexane into a test tube
- Into the tube put 5-6 drops of the acidified Potassium Manganate (VII)
- Fit the bung and shake well
- Record observations of the reaction mixture
Procedure D: Reaction with Concentrated Sulphuric Acid (H2SO4)
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Place 1cm³ of cyclohexane into a test tube
- Into the tube put 1cm³ of concentrated sulphuric acid
- Fit the bung and agitate slightly
- Record observations of the reaction mixture (Do they react? Do they separate? Do they mix but not react?)
Now complete procedures A to D again only this time substituting cyclohexane with cyclohexene. Make the same observations as before and remember to strictly adhere to the amounts stated, especially with regard the reaction with concentrated sulphuric acid.
Results
Alkane (Cyclohexane) Observations:
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Combustion: As with all other fuels the cyclohexane combusted to produce H2O + CO2. The flame burnt a bright orange colour, which appeared quite smokey. The appearance of soot shows a presence of carbon, or un-burnt fuel.
- Bromine: In the presence of light the bromine has reacted with the cyclohexane. The Bromine has lost the yellow-brown pigment it had and the solution now formed is almost completely colourless.
When kept in the dark, however, there is no apparent change to the bromine or cyclohexane. There has been no obvious reaction between the two substances and we can therefore deduce that the previous reaction is photochemical in nature.
- Acidified Potassium Manganate(VII) KMn: There was no apparent reaction, change in temperature or colour. The cyclohexane would not mix at all with the KMn and in fact seemed to settle on top of the KMn.
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Concentrated Sulphuric Acid (H2SO4): The two substances mixed but did not seem to react in any way. Having mixed they then separated to form two distinct layers.
Alkene (Cyclohexene) Observations:
- Combustion: Much like the cyclohexane however the flame did seem a little smokier.
- Bromine: Whether or not in the presence of light when mixed with cyclohexene the Bromine almost instantly decolourised.
- Acidified Potassium Manganate (VII) KMn: An almost instant decolourisation followed by separation into two distinct clear layers.
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Concentrated Sulphuric Acid (H2SO4): There was an extreme exothermic reaction together with an almost instant colour change from clear to cloudy brown. There was the obvious formation of H2O seen in the form of vapour (highlighting the relative exothermic extremity of the reaction.)
Conclusion
We can deduce from our tests that alkanes and alkenes react very differently when placed in the same situations. On the one hand we have an alkane which seems relatively inert when mixed with both acids and oxidising agents (even in this case when a very strong acid and a very strong oxidising agent was used) and on the other the alkene which showed a reaction very quickly and, in the test with H2SO4, extremely violently. I therefore accept my hypothesis on the basis that in at least one of the tests (simple combustion) they reacted almost identically but the other tests proved to highlight their differences. I suspect the contrasts observed are rooted in the chemical differences between the two compounds.
Discussion
Both alkanes and alkenes are referred to by the term hydrocarbon. A hydrocarbon quite simply is a molecule consisting only atoms of hydrogen and atoms of carbon. It is the ratios of these atoms and subsequent arrangement of them within the molecule which give rise to the different chemical properties each individual hydrocarbon will express. An alkane is a hydrocarbon (contains only hydrogen and carbon atoms) in which all of its atoms are joined by single covalent bonds. A typical example of an alkane is Ethane (C2H6). Ethane contains 6 hydrogen atoms to its 2 carbon atoms. The number of bonds an atom of carbon can make with other atoms when the molecule is built up determines these proportions and these proportions remain the same whatever the alkane and can be simplified thus: CnH2n+2, where the number of hydrogen atoms is always “twice plus two” the number of carbon atoms. Below is a diagrammatic representation of the structure of this molecule, which is consistent for all alkanes:
Compare this to Ethene which (although having the same number of carbon atoms as ethane, cannot contain the same number of hydrogen atoms due to the presence of a carbon-carbon double bond (C=C).
It is this double bond that characterises an alkene and due to the double bond the formula for alkenes is as follows: CnH2n. Where with the alkanes there was an extra two hydrogen atoms, the positions available for these hydrogen atoms have been lost as a result of the carbon atoms forming bonds with themselves. Because of this alkenes are referred to as being unsaturated. It is referred to as unsaturated because the presence of a double bond offers a site of reactivity so that other atoms can add to the molecule.
The results from our tests show that the alkenes were more ‘unstable’ and willing to react. With the only difference between the alkane and the alkene being the presence of the double bond I can quite safely state that the reactions observed (but not necessarily combustion test) where as a result of the alkene having a double bond which offered a site of reactivity and the chemical changes to take place.
Bibliography
Access to Chemistry – Alan Jones, Mike Clemment, Avril Higton & Elaine Golding – RSC
Simple Organic Compounds – Perasons Publishing