Testing for Organic Functional Groups (AQA A Level Chemistry)

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

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

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 (-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

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 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

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 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