Qualitative Analysis of Organic Functional Groups (OCR A Level Chemistry)

Testing for alkenes / saturation

• Halogens can be used to test if a molecule is unsaturated (i.e. contain a double bond)
• Br₂ is an orange or yellow solution, called bromine water
• The unknown compound is shaken with the bromine water
• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

The bromine water test is the standard test for unsaturation in alkenes

Testing for a haloalkane

• The haloalkane must first be warmed with ethanol sodium hydroxide in a mixture of ethanol and water
• The haloalkane will undergo nucleophilic substitution releasing the halide ion into solution, which can then be tested using acidified silver nitrate

Positive test result:

• If a halide is present it forms a silver halide precipitate:

Ag⁺ (aq) + X^– (aq) → AgX (s)

• Depending on the halide present, a different coloured precipitate is formed, allowing for identification of the halide ion
  • Silver chloride forms a white precipitate
  • Silver bromide forms a cream precipitate
  • Silver iodide forms a yellow precipitate

Each silver halide produces a precipitate of a different colour

• It can sometimes be hard to distinguish between the colours of the precipitates
• In this case, ammonia can be added
  • The white silver chloride precipitate will dissolve in dilute ammonia
  • The cream silver bromide precipitate will dissolve in concentrated ammonia 
  • The yellow silver iodide precipitate is insoluble in both dilute and concentrated ammonia

Testing for carbonyls (general)

• The carbonyl group undergoes a condensation reaction with 2,4-dinitrophenylhydrazine

Positive Test Result:

• The product formed when 2,4-DNPH is added to a solution that contains an aldehyde or ketone is a deep-orange precipitate which can be purified by recrystallisation

The test tube on the left shows a negative 2,4-DNPH test and the tube on the right shows a positive test

• The melting point of the formed precipitate can then be measured and compared to literature values to find out which specific aldehyde or ketone had reacted with 2,4-DNPH

Testing for Carbonyls (aldehyde / ketone specific)

• Tollens' reagent, also known as ammoniacal silver nitrate, is a more specific test-tube reaction that can distinguish between aldehydes and ketones 

Positive Test Result:

• When Tollens' reagent is gently warmed with an aldehyde, the silver mirror is formed
  • This is the positive test result

• When Tollens' reagent is gently warmed with a ketone, no silver mirror will be seen, as the ketone cannot be oxidised by Tollens' reagent, so no reaction takes place
  • This is a negative test result

The Ag⁺ ions in Tollens’ reagent are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to silver atoms

Testing for carboxylic acids

• Solid sodium carbonate, Na₂CO₃ (s), or aqueous sodium hydrogen carbonate, NaHCO₃ (aq), can be used to test for the presence of a carboxylic acid

Positive Test Result:

• Effervescence / bubbles of gas are seen as carbon dioxide is evolved