OCR A Level Chemistry

Revision Notes

1.3.4 Qualitative Analysis of Organic Functional Groups

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PAG 7: Qualitative Analysis of Organic Functional Groups

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

Hydrocarbons Bromine Water, downloadable AS & A Level Chemistry revision notes

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

The-silver-halide-precipitates, IGCSE & GCSE Chemistry revision notes

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

Carbonyl Compounds Positive-Negative Test, downloadable AS & A Level Chemistry revision notes

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

Carbonyl Compounds Tollens Reagent, downloadable AS & A Level Chemistry revision notes

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

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