Identification of an organic unknown ferric chloride brady's reagent tollen's reagent bromine water - page 1
Keywords: chemistry identify unknown substance organic
By Jenny on 02/07/2009
Level: A Level (Year 13)
Page Number: 1 of 1
Identification of an organic unknown
Test A (Ferric Chloride)
Method
Add a few drops of iron (III) chloride to a little of unknown sample X in water.
Observations
If sample X is a phenol there will be a colour change from pale yellow to purple, if it isn’t there will be no reaction.
Test B (Brady’s Reagent)
Method
Add a few drops of sample X to Brady’s reagent (2,4-dinitrophenylhydrazine solution).
Observations
If sample X is a carbonyl compound (either an aldehyde or a ketone) an orange precipitate will form, if it isn’t there will be no reaction.
Test C (Tollen’s Reagent)
Method
Warm a few drops of sample X with Tollen’s reagent (ammoniacal silver nitrate).
Observations
If sample X is an aldehyde then a silver solid (‘mirror’) will be deposited on the sides of the test tube. If sample X is a ketone there will be no reaction.
Test D (Bromine Water)
Method
Add a small amount of sample X to a solution of bromine in water.
Observations
If sample X is an alkene then the yellow bromine water will be decolourised if it isn’t then it will remain yellow.
Test E (Sodium hydrogen carbonate)
Method
Mix a small amount of sample X with water and add a little sodium hydrogen carbonate solution.
Observations
If sample X is a carboxylic acid then a fizzing, colourless gas will be given off, which forms a white precipitate with Limewater.
Test F (Phosphorus pentachloride)
Method
Mix a small amount of sample X with a few drops of phosphorus (V) chloride, then test the fumes with moist blue litmus paper.
Observations
If sample X is contains a hydroxy group (R-OH) then steamy white fumes will be given off which turn moist blue litmus paper red. As we have already eliminated all other compounds containing a hydroxy group then if the reaction takes place sample X must be an alcohol, if no reaction takes place then it is an ester.
Test G (Oxidation with potassium dichromate)
Method
Distil the remaining sample X with potassium dichromate (VI) and moderately concentrated sulphuric acid.
Observations
If sample X is a primary alcohol then it will be oxidised and the dichromate (VI) ion will be reduced to the chromium (III) ion, causing a colour change from orange to dark green. If no reaction takes place then sample X is a tertiary alcohol.
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