6.9.3Organic Chemistry and Analysis
Qualitative Analysis
5 min read 4 parts Lesson 30 of 33 Virtual lab
Section 1 of 4Tests for Functional Groups
Chemical tests give quick evidence for the presence of a particular functional group.
Alkene
- Shaking with bromine water decolourises it from orange to colourless through electrophilic addition.
Alcohol
- Warming a primary or secondary alcohol with acidified K₂Cr₂O₇ turns the orange solution green, while tertiary alcohols give no change.
Aldehyde
- Warming with Tollens' reagent gives a silver mirror, while ketones give no mirror.
Carboxylic acid
- Adding sodium carbonate solution produces effervescence of CO₂, which turns limewater milky.
Phenol
- Bromine water is decolourised and a white precipitate of 2,4,6-tribromophenol forms.
Combining tests
- Two tests are often used together, for example Tollens' reagent and sodium carbonate, to separate an aldehyde from a carboxylic acid.
Section 2 of 4Testing for Halogens in Haloalkanes
The halogen is first converted into a halide ion, then identified with silver nitrate.
1
Warm the haloalkane with aqueous sodium hydroxide, which hydrolyses it and releases halide ions.
2
Acidify with dilute nitric acid so that excess hydroxide does not form a precipitate of silver oxide.
3
Add aqueous silver nitrate and observe the colour of the precipitate formed.
4
Add dilute and then concentrated ammonia to the precipitate to confirm which halide is present.
5
Interpret the combined observations using the table to state the halogen.
Section 3 of 4Worked Example
Question
- An unknown organic liquid is warmed with acidified K₂Cr₂O₇ and the colour changes from orange to green.
- It gives no silver mirror with Tollens' reagent and no effervescence with sodium carbonate.
- Suggest the type of compound.
- Orange to green shows that the compound is oxidised, so it is a primary or secondary alcohol, or an aldehyde.
- No silver mirror rules out an aldehyde.
- No effervescence rules out a carboxylic acid.
- The remaining possibility is a primary or secondary alcohol.
Answer The compound is a primary or secondary alcohol.
Section 4 of 4Exam Checklist & Tips
6.9.3 – Key Points for the Exam
- Qualitative analysis Identifying substances by observations rather than measurements.
- Tollens' reagent Ammoniacal silver nitrate used to detect aldehydes.
- Hydrolysis Breaking a C–X bond using water or hydroxide.
- Precipitate An insoluble solid formed in a solution.
Common Mistakes
- Adding silver nitrate directly to a haloalkane without hydrolysis first.
- Forgetting to acidify with nitric acid before adding silver nitrate.
- Using hydrochloric acid for acidification, which introduces chloride ions.
- Describing the alcohol test as a change to orange rather than from orange to green.