Qualitative Analysis (OCR A Level Chemistry A): Revision Note

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

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

Each silver halide produces a precipitate of a different colour

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

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 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 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 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