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analysing the unknown compound A

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Introduction The aim of this coursework is to identify two unknown organic compounds which are mono-functional using wet tests we deem suitable and the spectroscopy of the unknown compounds. Background Wet Tests Reaction with bromine To test for alkenes bromine water can be used. If the unknown compound is an alkene it will decolourise the bromine water. This is because the ?-bond which makes up part of the double bond in alkenes is relatively easily to break making alkenes react in electrophilic addition reactions. Although most of this information may seem relevant to the investigation, the only important part is that bromine water can be used to test for alkenes as alkenes decolourise the bromine water. Reaction with acidified potassium dichromate Acidified potassium dichromate is an oxidising agent which can be used to test for primary and secondary alcohols. This is because primary alcohols can be oxidised into aldehydes and secondary into ketones whereas tertiary alcohols cannot be oxidised at all. "In the reaction, the orange dichromate(VI) ions are reduced to green chromium(III) ions" as said by the Essential AS Chemistry book for OCR. In addition to this because aldehydes can be further oxidised into carboxylic acids, acidified potassium dichromate can also be used distinguish aldehydes from ketones as ketones cannot be further oxidised. The most important part of this information I need to consider for my investigation is to recognise that because primary alcohols can be oxidised and tertiary alcohols cannot, we can use acidified potassium dichromate to distinguish between the two. This is because if the unknown compound is a primary alcohol there will be a colour change from orange to green, and if it is a tertiary there will be no colour change. Reaction with Fehling's solution "Fehling's solution is a solution used to differentiate between aldehyde or ketone functional groups" as said by http://en.wikipedia.org/wiki/Fehling's_solution. This is because it is an oxdising agent, which oxidises aldehydes but not ketones as they cannot be oxidised. ...read more.


If the mixture remains brown i.e. the colour of the bromine water then no reaction has taken place 8. If the mixture decolourises or forms a white precipitate then a reaction has taken place 9. To dispose of this mixture pour it into a beaker and add with stirring 1ml amounts of 500ml of 10% sodium carbonate solution and then pour down the foul water drain Reaction with Brady's Reagent 1. Pour approximately 5cm3 of the unknown compound from the beaker into a measuring cylinder 2. Pour this 5cm3 from the measuring cylinder into a test tube 3. Put the test tube into a test tube rack in your working area 4. Using a pipette suck into the pipette a small amount of Brady's reagent (2,4-dinitrophenylhydrazine) and add approximately 5cm3 of the solution into the test tube 5. Put a bong on the test tube and shake vigorously 6. If the unknown compound is a ketone or aldehyde then an orange precipitate will be formed 7. If there is no formation of an orange precipitate then no reaction has occurred 8. To dispose of the Brady's reagent you have to dilute it with 10litres of water and pour it down the foul water drain Reaction with Fehling Reagent 1. Pour water into a kettle and heat the water 2. Once the water as been heated pour it into a into a large beaker and let it cool 3. Pour approximately 5cm3 of the unknown compound from the beaker with the unknown compound into a measuring cylinder 4. Pour this 5cm3 from the measuring cylinder into a boiling tube 5. Using a pipette suck into the pipette a small amount Fehling's reagent and add approximately 5cm3 of the solution into the boiling tube 6. Put the boiling tube into the beaker and warm the unknown compound in a water bath 7. If there is a colour change from blue to red then the unknown compound is an aldehyde 8. ...read more.


Again because of the 13C isotope this molecular ion has an increased mass. Finally the third largest fragment has a mass/charge ratio of 15 which tells us that the single bond has split hetrolytically because for this molecular ion occur both electrons must remain with the larger fragment because it has the positive ion. Therefore by splitting hetrolytically the smaller will have a positive charge and the large a negative. In addition to this the mass/charge ratio of 15 tells us the molecular ion has 1 carbon and 3 hydrogens which means that this fragment does not have the 13C isotope. In conclusion the mass spectrum has shown me that the compound has a structure of CH3COCH3 because this is the only possible structure for a ketone with 3 carbons as ketones cannot be on the end of the chain. Additionally this agrees with the iodoform test I did which told me the ketone I had was a methyl ketone as this is a methyl ketone. Furthermore for base peak to occur and form the molecular ion CH3CO+ the C-C bond must have split and for the CH3+ molecular ion to occur the same bond must have but hetrolytically. NMR The NMR tell us the environments the hydrogens in the compound are in and I believe I can use this to tell me whether the structure I proposed for the ketone I believe to have is accurate. The chemical shift of 2.1 shows that the type of hydrogen in that environment is R-COCH3 this means that there will be no splitting of the peak because there is no hydrogens on a carbon adjacent to the hydrogens on the CH3. In addition to this because the nmr tells use that there are 6 hydrogens in this environment the Are group must be another CH3. Therefore the structure of the ketone I have is CH3COCH3 as I proposed above. Consequently unknown compound A is the ketone propanone otherwise better known as acetone. ...read more.

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