Identification of an unknown organic compound - Sample Essay
Method: Add sodium to the solution of the unknown compound in a test tube. Collect any gas given off, and test it to determine whether the gas collected is hydrogen or not by using the pop test. Pop test- if hydrogen is present, when a lit splint comes in contact with the gas in the test tube; a squeaky noise (pop) is made. Observation: When the lit split enters the tube with the gas, if a squeaky pop noise is made then hydrogen is present. This presence of hydrogen comes from the reaction between the alcohol and sodium. This determines the presence of a tertiary alcohol.
However if no hydrogen is given off, then no squeaky pop noise is made as a lit splint enters the tube. This means that a tertiary alcohol is not present; meaning that the unknown compound is an ester, because this is the only remaining unknown organic compound. Explanation: Hydrogen is produced when an alcohol and alkali metal react. The following reaction: Hydrogen being given off is an indication of sodium and the alcohol reacting. The gas which is collected is tested with a lit splint to determine whether the gas is hydrogen or not. Esters do not react with hydrogen, so no hydrogen is given off.
If the pop test is negative, no pop noise is made; it means that a tertiary alcohol is not present, thus the unknown can only be an ester as all the other organic compounds have been eliminated. Method: add acidified potassium dichromate to the test tube containing the unknown compound using a pipette. As this reaction requires heat, place the test tube in a beaker with hot water. Take note to any colour change. Observation: Colour change from orange to green shows that a primary alcohol is present; however if there is no colour change (stays orange) a primary alcohol is not present.
This leaves two possible unknown compounds either a tertiary alcohol or an ester. Explanation: Primary alcohols are oxidised to aldehydes by using the oxidising agent potassium dichromate. As the primary alcohol oxidises, the orange dichromate ions are reduced to chromium (iii) ions which are green. Therefore the colour change from orange to green clearly indicates the presence of a primary alcohol Tertiary alcohols however react differently, because it would require to break a C-C bond which needs a much higher amount of energy than that required for a C-H bond.