Qualitative Analysis (OCR A Level Chemistry)

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Testing for Alkenes & Haloalkanes

Testing for an Alkene

  • Halogens can be used to test if a molecule is unsaturated (i.e. contains a double bond)
  • Br2(aq) is an orange-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 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
  • The released halide ion is tested for using the silver nitrate test:

    • Acidify the sample with dilute nitric acid (HNO3) followed by the addition of silver nitrate solution, AgNO3
      • The acidification is done to remove carbonate ions that might give a false positive 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 is white, silver bromide is cream and silver iodide is yellow

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

Each silver halide produces a precipitate of a different colour

Examiner Tip

The acidification step in the halide ion test must be done with nitric acid rather than hydrochloric acid, as HCl contains chloride ions which would interfere with the results.

Testing for Carbonyls & Alcohols

Testing for a carbonyl

2,4-dinitrophenylhydrazine or Brady's reagent

  • 2,4-dinitrophenylhydrazine (2,4-DNP or 2,4-DNPH) is used to test for the presence of a carbonyl group, C=O
    • 2,4-DNP is usually dissolved in methanol and sulfuric acid 
    • It is a pale orange solution
  • A few drops of the unknown sample are added to 5.0 cm3 of 2,4-DNP and it left to stand
  • A few drops of sulfuric acid are then added to the mixture
    • The formation of an orange / yellow precipitate of 2,4-dinitrophenylhydrazone confirms the presence of an aldehyde or ketone

Examiner Tip

It is more common to use other chemical tests to more specifically check for the presence of an aldehyde or a ketone because:

  1. Brady's reagent / 2,4-DNP only confirms the presence of a carbonyl group, it does not distinguish between an aldehyde and a ketone
  2. Friction or sudden impacts can cause solid 2,4-DNP to explode 

Testing for an Aldehyde

Tollens’ reagent

  • Tollens' reagent is an aqueous alkaline solution of silver nitrate in excess ammonia solution
    • Tollens' reagent is also called ammoniacal silver nitrate solution

  • When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Agions are reduced to Ag atoms
    • In the alkaline conditions, the carboxylic acid will become a carboxylate ion and form a salt

  • The Ag atoms form a silver ‘mirror’ on the inside of the tube
  • Ketones cannot be oxidised and therefore give a negative test when warmed with Tollens’ reagent

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

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

Examiner Tip

  • Fehling’s solution can also be used to test for an aldehyde
  • An aldehyde cause a change from a blue solution to a orange / brick-red prcipitate
  • There is no visible change for a ketone

Carbonyl Compounds Fehlings Solution, downloadable AS & A Level Chemistry revision notes

Testing an Alcohol

  • Alcohols can be classified as either primary, secondary or tertiary, depending on the placement of the -OH group
  • Primary and secondary alcohols can both be oxidised, but tertiary alcohols cannot
  • To test for the alcohol functional group, add a small amount (1 cm3) of the substance to a test tube using a pipette
  • Then, add a small amount (1 cm3) of a suitable oxidising agent to the sample using a different pipette
    • The most commonly used oxidising agent for this test is acidified potassium dichromate solution (K2Cr2O7, acidified with H2SO4)

  • Add a stopper to the test tube and shake well
  • Place in a hot water bath (heated to around 60 oC) for a few minutes
  • If a primary or secondary alcohol are present, then the colour will change from orange to green
    • If a tertiary alcohol is present, then nothing will happen - the solution will remain orange

Hydroxy Compounds Tests Alcohols, downloadable AS & A Level Chemistry revision notes

Positive test results of the oxidation of a primary, secondary and tertiary alcohol

Examiner Tip

When heated with acidified potassium dichromate, an aldehyde will also cause a colour change from orange to green

Testing for Carboxylic Acids & Phenols

Testing for a Carboxylic Acid

  • Carboxylic acids in solution have a pH of around 3, so measuring the pH is a way of testing for the presence of the carboxylic acid functional group in an organic sample
  • The end of a glass rod could be dipped into the solution and then carefully dripped onto indicator paper
  • Or, a pH probe could be used, which would give you an exact pH
  • Since carboxylic acids are acids, they will react with a carbonate solution to produce carbon dioxide gas
  • 1-2 cm3 of sodium carbonate (Na2CO3) or sodium hydrogen carbonate solution (NaHCO3) could be added using a pipette
  • If bubbles of gas are seen, this is a good indicator that the solution is a carboxylic acid
    • If an exam question asks you to simply distinguish between different types of organic compound, and the carboxylic acid is the only organic compound present which would react in this way with a carbonate solution, then this is enough

  • The gas produced could then be bubbled into limewater
  • If the limewater turns milky or cloudy, then this proves that the gas produced was carbon dioxide

Testing for Phenols

  • Bromine water can be used to test if a molecule is phenolic (i.e. contains phenol)
  • The unknown compound is shaken with the bromine water
  • If the compound contains phenol, a substitution reaction will take place and the coloured bromine water will decolourise and then a white precipitate will form

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.