AQA AS Chemistry

Revision Notes

4.2.4 Testing for Organic Functional Groups

Testing for Organic Functional Groups

REQUIRED PRACTICAL 6

  • For this practical, you need to be able to give the tests and positive results for the following functional groups:
    • Alkenes
    • Alcohols
    • Aldehydes
    • Carboxylic Acids

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 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

Testing for an Aldehyde

Fehling’s solution

  • Fehling’s solution is an alkaline solution containing copper(II) ions which act as the oxidising agent
  • When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Cu2+ ions are reduced to Cu+ ions
    • In the alkaline conditions, the carboxylic acid formed will be neutralised to a carboxylate ion (the -COOH will lose a proton to become -COO )
    • The carboxylate ion (-COO) will form a salt with a positively charged metal ion such as sodium (-COONa+)
  • The clear blue colour of the solution turns opaque red due to the formation of a copper(I) oxide precipitate
  • Ketones cannot be oxidised and therefore give a negative test when warmed with Fehling’s solution

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

The copper(II) ions in Fehling’s solution are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to copper(I) ions in the Cu2O precipitate

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

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

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