Qualitative Analysis of Organic Functional Groups (OCR A Level Chemistry A): Revision Note

Exam code: H432

Richard Boole

Written by: Richard Boole

Reviewed by: Philippa Platt

Updated on

PAG 7: Qualitative analysis of organic functional groups

Testing for alkenes / saturation

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

  • Br2 (aq) is an orange or yellow solution, called bromine water

  • The unknown compound is shaken with the bromine water

Positive result:

  • If the compound is unsaturated:

    • An addition reaction will take place

    • The coloured solution will decolourise

Ethene reacts with bromine water, forming 1,2-dibromoethane. Bromine water decolourises, indicating an addition reaction at room temperature.
The bromine water test is the standard test for unsaturation in alkenes

Testing for a haloalkane

  • Warm the haloalkane with sodium hydroxide in a mixture of ethanol and water

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

  • The halide ion can then be tested with acidified silver nitrate solution

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 forms a white precipitate

    • Silver bromide forms a cream precipitate

    • Silver iodide forms a yellow precipitate

Three test tubes labelled A to C; A contains white chloride, B has cream bromide, and C has yellow iodide.
Each silver halide produces a precipitate of a different colour
  • It can sometimes be hard to distinguish between the colours of the precipitates

  • In this case, ammonia can be added

    • The white silver chloride precipitate will dissolve in dilute ammonia

    • The cream silver bromide precipitate will dissolve in concentrated ammonia 

    • The yellow silver iodide precipitate is insoluble in both dilute and concentrated ammonia

Testing for carbonyls (general)

  • Carbonyl compounds undergo a condensation reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH)

Positive result:

  • The product formed when 2,4-DNPH is added to a solution that contains an aldehyde or ketone is a deep-orange precipitate 

    • This can be purified by recrystallisation

Two test tubes show results of the 2,4-DNPH test. Left: negative, brown solution, no aldehyde/ketone. Right: positive, orange precipitate, confirms aldehyde/ketone.
The test tube on the left shows a negative 2,4-DNPH test and the tube on the right shows a positive test
  • The melting point of the formed precipitate can then be measured and compared to literature values

    • This allows the specific aldehyde or ketone reacted with 2,4-DNPH to be identified

Testing for carbonyls (aldehyde / ketone specific)

  • Tollens' reagent is a more specific test-tube reaction that can distinguish between aldehydes and ketones

    • Tollens' reagent is also known as ammoniacal silver nitrate  

Positive result:

  • When Tollens' reagent is gently warmed with an aldehyde, the silver mirror is formed

    • This is the positive test result

  • When Tollens' reagent is gently warmed with a ketone, no silver mirror will be seen, as the ketone cannot be oxidised by Tollens' reagent, so no reaction takes place

    • This is a negative test result

Two test tubes show Tollen's solution before and after reaction, forming a silver mirror. Ag+ ions reduce to Ag as aldehyde oxidises to carboxylic acid.
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 carboxylic acids

  • The presence of a carboxylic acid can be tested for using@

    • Solid sodium carbonate, Na2CO3 (s)

    • Aqueous sodium hydrogen carbonate, NaHCO3 (aq)

Positive result:

  • Effervescence / bubbles of gas are seen as carbon dioxide is evolved

Practical skills reminder

  • This practical develops essential qualitative analysis skills, including:

    • Carrying out test-tube reactions to identify unknown organic compounds

    • Observing and interpreting visible changes, such as colour change, gas evolution, and precipitate formation

    • Using reagents safely, including Tollens’ reagent, 2,4-DNPH, and bromine water

    • Applying systematic tests to distinguish between functional groups

    • Linking observations to reaction mechanisms and organic structures

👀 You've read 1 of your 5 free revision notes this week
An illustration of students holding their exam resultsUnlock more revision notes. It’s free!

By signing up you agree to our Terms and Privacy Policy.

Already have an account? Log in

Richard Boole

Author: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

Philippa Platt

Reviewer: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

Download notes on Qualitative Analysis of Organic Functional Groups