In synthesis and analysis you often need to find which functional groups a compound has. OCR expects you to do this using the chemical reactions you have met in the specification: each group gives a characteristic, observable result with a particular reagent.Key tests:• alkene (C=C): bromine water, orange to colourless
• haloalkane: silver nitrate in ethanol, warm; precipitate of silver halide
• carbonyl (aldehyde or ketone): 2,4-dinitrophenylhydrazine (2,4-DNP), orange precipitate
• aldehyde: Tollens' reagent, silver mirror (ketones do not react)
• primary or secondary alcohol, or aldehyde: acidified potassium dichromate, warm; orange to green
• carboxylic acid: aqueous sodium carbonate, effervescence of CO2
• phenol: bromine water decolourised with a white precipitate; no effervescence with carbonateResults are combined logically. A negative result is useful too: it rules a group out.Worked example: compound Z, C3H6O, gives an orange precipitate with 2,4-DNP but no silver mirror with Tollens' reagent.
2,4-DNP positive: Z has a C=O group.
Tollens' negative: Z is not an aldehyde, so it is a ketone.
The only ketone with three carbons is propanone, CH3COCH3.Chemical tests identify the groups; spectroscopy (IR, mass spectra and NMR) is then used to confirm the full structure.
• haloalkane: silver nitrate in ethanol, warm; precipitate of silver halide
• carbonyl (aldehyde or ketone): 2,4-dinitrophenylhydrazine (2,4-DNP), orange precipitate
• aldehyde: Tollens' reagent, silver mirror (ketones do not react)
• primary or secondary alcohol, or aldehyde: acidified potassium dichromate, warm; orange to green
• carboxylic acid: aqueous sodium carbonate, effervescence of CO2
• phenol: bromine water decolourised with a white precipitate; no effervescence with carbonateResults are combined logically. A negative result is useful too: it rules a group out.Worked example: compound Z, C3H6O, gives an orange precipitate with 2,4-DNP but no silver mirror with Tollens' reagent.
2,4-DNP positive: Z has a C=O group.
Tollens' negative: Z is not an aldehyde, so it is a ketone.
The only ketone with three carbons is propanone, CH3COCH3.Chemical tests identify the groups; spectroscopy (IR, mass spectra and NMR) is then used to confirm the full structure.