Identification of Functional Groups by Test Tube Reactions (Oxford AQA International A Level Chemistry)

Test-Tube Reactions

Testing for an alkene

• Halogens can be used to test if a molecule is unsaturated (i.e. contains a double bond)

• Bromine water, Br₂ (aq), is an orange-brown solution

• The unknown compound is shaken with the bromine water

• If the compound is unsaturated, an addition reaction will take place and the orange-brown solution will decolourise

The unsaturation test

Testing for a halogenoalkane

• The halogenoalkane is warmed with sodium hydroxide in a mixture of ethanol and water

• The halogenoalkane will undergo nucleophilic substitution releasing the halide ion into solution

• The released halide ion is tested for using the silver nitrate test:

  • Acidify the sample with dilute nitric acid (HNO₃) to remove carbonate ions

    • Carbonate ions can give a false positive result

  • Add silver nitrate solution, AgNO₃ (aq)

• If a halide ion is present, it forms a silver halide precipitate:

Ag⁺ (aq) + X^– (aq) → AgX (s)

• Different halide ions produce different coloured precipitates, which allows the halide ion to be identified:

  • The chloride ion produces a white precipitate of silver chloride

  • The bromide ion produces a cream precipitate of silver bromide

  • The iodide ion produces a yellow precipitate of silver iodide

Halide ion positive results

Testing for an aldehyde

• The carbonyl group present in an aldehyde can be tested for using:

  • Tollens' reagent

  • 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

• 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 silver mirror test

Fehling's solution

• Fehling's solution is a solution containing Cu²⁺ ions

  • The Cu²⁺ ions are responsible for the initial blue colour of the solution

• When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Cu²⁺ ions are reduced to Cu⁺ ions

• The Cu⁺ ions form an orange / brick-red precipitate

• There is no visible change for a ketone

Fehling's solution test

Testing for an alcohol

• Alcohols can be classified as either primary, secondary or tertiary, depending on the placement of the -OH group

  • Primary alcohols oxidise to aldehydes and carboxylic acids

  • Secondary alcohols oxidise to ketones

  • Tertiary alcohols cannot be oxidised

• To test for the alcohol functional group:

  • Add 1 cm³ of the substance to a test tube using a pipette

  • Add 1 cm³ of a suitable oxidising agent to the sample using a different pipette

  • The most common oxidising agent 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, the solution will remain orange

Potassium dichromate test for alcohols

Testing for a carboxylic acid

• Carboxylic acids in solution have a pH of around 3

  • So, measuring the pH with an indicator or a pH probe is one way of testing for a carboxylic acid

• 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

  • 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

Summary of test tube reactions

Table of test tube reactions