Cambridge 9701 · AS Level · Organic only

Organic tests

The observations and conclusions worth recalling for identification questions.
8 of 8 tests

alkene

Bromine water

§ 15.3
Reagent
Aqueous bromine (bromine water).
Conditions
Add at room temperature; no catalyst is required.
Positive observation
Orange bromine water turns colourless.
Negative result
No change for a saturated alkane under these conditions.
Conclusion
An alkene C=C bond is present.

alkene · primary alcohol · secondary alcohol

Acidified manganate(VII)

§ 15.4/17.3
Reagent
Dilute acidified potassium manganate(VII), KMnO4.
Conditions
For an alkene, shake with cold dilute acidified KMnO4 at room temperature; warm or reflux as specified for alcohol oxidation.
Positive observation
The purple acidified manganate(VII) solution turns colourless.
Negative result
Tertiary alcohols do not give oxidation under these conditions.
Conclusion
Oxidisable unsaturation or an oxidisable alcohol is present.

primary alcohol · secondary alcohol · aldehyde

Acidified dichromate(VI)

§ 17.3
Reagent
Acidified potassium dichromate(VI), K2Cr2O7.
Conditions
Warm in a water bath; distil a volatile aldehyde, or reflux for the acid.
Positive observation
Orange acidified dichromate(VI) changes to green.
Negative result
Ketones and tertiary alcohols show no change under these mild conditions.
Conclusion
An oxidisable primary or secondary alcohol, or an aldehyde, is present.

halogenoalkane

Silver nitrate halide test

§ 16.2
Reagent
Aqueous silver nitrate in ethanol.
Conditions
Warm the halogenoalkane with the reagent and compare precipitate colour.
Positive observation
AgCl is white, AgBr is cream and AgI is pale yellow.
Negative result
Fluoroalkanes react too slowly to give a useful result in this test.
Conclusion
The precipitate identifies the halide ion released by hydrolysis.

aldehyde · ketone

2,4-DNPH test

§ 18.5
Reagent
Acidified 2,4-dinitrophenylhydrazine (Brady's reagent).
Conditions
Add at room temperature and allow the derivative to crystallise.
Positive observation
A deep-orange or orange-yellow precipitate forms.
Negative result
Carboxylic acids and esters do not give this precipitate.
Conclusion
An aldehyde or ketone carbonyl group is present.

aldehyde

Tollens' aldehyde test

§ 18.5
Reagent
Fresh Tollens' reagent: aqueous AgNO3 made alkaline with ammonia.
Conditions
Warm gently in a clean test tube or warm water bath.
Positive observation
A silver mirror forms, or black silver precipitate is produced.
Negative result
The solution remains colourless with a ketone.
Conclusion
An aldehyde is present; ketones give no change.

aldehyde

Fehling's aldehyde test

§ 18.5
Reagent
Alkaline Fehling's solution (freshly mixed).
Conditions
Heat the mixture in a boiling water bath.
Positive observation
The blue solution gives a brick-red or red-orange Cu2O precipitate.
Negative result
The solution remains blue with a ketone.
Conclusion
An aldehyde is present; ketones give no change.

methyl ketone · ethanal · propan-2-ol-type alcohol

Iodoform test

§ 18.6
Reagent
Alkaline iodine solution, I2(aq) with aqueous alkali.
Conditions
Warm gently.
Positive observation
A yellow precipitate of tri-iodomethane (iodoform, CHI3) forms.
Negative result
Compounds without the required methyl carbonyl pattern show no yellow precipitate.
Conclusion
A CH3CO– group, ethanal, or CH3CH(OH)– group is present.