Qualitative Analysis of Organic Functional Groups (OCR A Level Chemistry A): Revision Note
Exam code: H432
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

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

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

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

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
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