Testing for Organic Functional Groups (AQA A Level Chemistry): Revision Note

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

Testing for 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 is 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

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 (-COO^–Na⁺)

• The clear blue solution turns opaque due to the formation of a red precipitate, copper(I) oxide

• Ketones cannot be oxidised and therefore give a negative test when warmed with Fehling’s solution

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 Ag⁺ ions 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

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 effervescence/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

Related topics

• Reactions of Alkenes

• Oxidation of Alcohols

• Test-Tube Reactions