Identification of an organic unknown - Sample Essay
Organic compounds have different functional groups and therefore differ from each other. They also react differently, I will use this method to determine the type of organic compound I have. I will conduct various experiments to narrow down the choices. I will the using the information I have gathered construct an, easy to follow, flow diagram. Prediction While conducting these experiments, I will produce solutions, which I may need for the next step of the flow diagram.
By observing physical changes to the reactants and recording any substance given off by the product, I will be able to follow my flow diagram and therefore determine the unknown compound. For example, if hydrogen gas is released, and the flow diagram gives a choice of hydrogen given off, and no hydrogen given off. I will down the appropriate route and dismiss the other route. Plan To find the organic the unknown, I will need to find the functional group and I will find this by how they react.
I will now describe the experiments that I will conduct, the safety precautions and any other information. Firstly, I will need to figure out how to separate the unknown into 2 different groups. This is done by adding Sodium (Na) to the compound in a test-tube. The Na will sink and a steady stream of hydrogen will be released, this will also happen when NA is added to Phenol and a carboxylic acid. I will collect any gas released and use the ‘pop test’ to test for hydrogen.
I will be wearing safety goggles because the acid and phenol are very corrosive. If there is a ‘pop’ I known that it could be one of, an alcohol, phenol or carboxylic acid. I can now split the route into two and from what I discover from the first experiment I can take either one of the routes. If there isn’t a ‘pop’ then I will take the other route, It could be aldehydes, esters or ketones, I know this because these functional groups do not react with Na. To find out if the either of these groups are the unknown, I will use Fehling’s reagent or Tollens reagent experiment. I chose to use the Fehling’s reagent.
This reagent determine whether the compound contains the aldehyde. The Fehling’s reagent contains copper ions, when added to an aldehyde it is oxidised to a salt of a carboxylic acid and the Fehling’s reagent is reduced. The solution will stay blue if no reaction takes place, but if the slution turn brick red then a reaction has took place and the unknown will identified as an aldehyde PLAN To find out if the unknown is either one of these the second experiment I will do is to do either the Fehling’s or the Tollens reagent experiment.
I have chosen to do the Fehling’s experiment. This experiment is used to discover an aldehyde as Fehling’s solution contains Cu ions. When this is added to aldehyde the aldehyde is oxidised to a salt of a carboxylic acid and the Fehling’s solution is reduced. If there is no reaction and the solution stays blue I will know that the unknown organic compound is not an aldehyde but if the colour changes to brick red there is a reaction and an aldehyde can be identified. The reaction equation is displayed below of an aldehyde reacting with Fehling’s solution.
2Cu (aq) + RCHO(aq) + OH (aq) + H O(l) Cu O + RCOOH(aq) + 3H (aq) I would wear safety goggles during this experiment and handle the solutions with great care. When doing this experiment when adding solutions I should stay clear of the reaction area, as I do not know how the reaction will occur and if it is vigorous. Now that I am left with to other functional groups to identify from I can carry out one more experiment to identify the organic compound. At this stage I would have discovered that it does not react with Na so is not an alcohol, phenol or carboxylic acid.
It also does not react with Fehling’s solution so is not an aldehyde so it has to be either a ketone or an ester. The experiment that I have chosen to see the difference between the two would be the reaction with 2,4 DNPH. I would get the unknown compound and place a bit of it into a test tube, I will then add drop by drop the 2,4 DNPH if I see a change in the solution from clear to a yellow precipitate with yellow crystals I will be able to identify the compound as a ketone if not it will be an ester.
Now that I have the experiments I will do to indicate if the unknown compound was one of the three that did not produce hydrogen when reacted with Na. I will now describe the experiments I will need to carry out to discover if the unknown compound has either the functional group of an alcohol, phenol or a carboxylic acid that would release Hydrogen when reacted with Na. A further experiment I need to carry out to identify if the compound is an alcohol or is it not.
To do this I will do a simple experiment by adding a few drops of universal indicator to a test tube with the unknown organic compound. This will identify to me if there is an alcohol functional group present. If this functional group is present then the solution would go blue/green therefore it would be near neutral although it will turn pink/red if a functional group of either carboxylic acid or phenol is present as they are acidic. After this experiment I will have two functional groups that I would not have identified as I have identified the alcohol.
If I had carried out the first experiments I would know that hydrogen is released by this compound when reacted with Na therefore it would be an alcohol, phenol or a carboxylic acid. From the second experiment in this category I would either discover by adding universal indicator if the unknown is an alcohol or if it is both carboxylic acid and phenol. I can now do another experiment adding sodium carbonate to the compound and discover if there is a reaction. I would have a test tube with the unknown compound and I will add the sodium carbonate.
I know that if a carboxylic acid is present then carbon dioxide gas will be released. I will have a measuring cylinder or larger test tube turned upside down in a water bath where a plastic tube will allow the gas produced to be collected in the measuring cylinder or test tube. This plastic tube will be connected to the reaction test tube via a rubber bung. The gas collected could then be tested to see if carbon dioxide is released. To do this I will add limewater to the test tube and if the solution forms a milky precipitate then carbon dioxide gas is present.
I will then know that the compound is a carboxylic acid. If the precipitate does not go milky then I have identified the unknown organic compound as a phenol. I have discovered that I can do two further experiments on the alcohol. I could reflux the alcohol that I have discovered to discover if this is a tertiary alcohol or one of the primary or secondary alcohols. I will use reflux equipment such as the pear shaped flask and a condenser. I will add it up right and will heat the unknown compound containing anti bumping granules to make it less vigorous.
I will use a Bunsen burner to heat the solution in the pear shaped flask. I will then release drop-by-drop potassium dichromate to the pear shaped flask. If there is no change in the colour of the solution and it stays orange then it is a tertiary alcohol. If there is a reaction and the colour of the solution goes blue/green a primary or secondary alcohol is present. To discover if the alcohol is a primary or secondary I could add 2,4 DNPH to the alcohol discovered and if it goes orange/yellow it is a secondary alcohol and if there is no precipitate it is a primary alcohol.
In all experiments I will need to take great precautions as many of the solutions are corrosive or flammable. As I will not know the functional groups present in the unknown organic compound I will not know if it is flammable or corrosive therefore I have to handle it with care and try to minimise any spillages. I will wear gloves and wear goggles to protect my hands and eyes. I will not have anything close to the table that may be obstructive and may catch fire. I have described all apparatus that I will use in the description of the experiment therefore have not listed them.
In all of the experiments I will add the same amount of unknown compound to the reactant. I will therefore add 2cm of unknown organic compound to 2cm to its reactant. I will however not do these when adding universal indicator, as a few drops will be efficient. Any further reaction equations that may be helpful are written on the sheet with the flow diagram. I have used the chemistry 2 textbooks written by Brian Ratcliff and Helen Eccles. I have also used the Heinemann Advanced Science chemistry textbook written by Ann & Patrick Fullick.