Chemical tests Flashcards
Test for alkenes
Alkenes decolorize bromine water because they undergo an electrophilic addition reaction with bromine:
Add a few drops of bromine water to the sample and shake. If the bromine decolorizes, an alkene is present. If not, no alkene is present.
Test for haloalkanes
When haloalkanes are heated with dilute sodium hydroxide, a nucleophilic substitution reaction occurs and halide ions are produced. The halide ions can be identified using the tests described in AS Unit 2:
Add aqueous sodium hydroxide to the sample and heat. Then allow to cool, add dilute nitric acid and then aqueous silver nitrate. A white precipitate soluble in dilute ammonia indicates that a chloroalkane was present, a cream precipitate soluble in concentrated ammonia indicates that a bromoalkane was present, and a yellow precipitate insoluble in ammonia indicates that an iodoalkane was present.
Test for aldehydes
Aldehydes can be oxidized by mild oxidizing agents such as Fehling’s solution or Tollen’s reagent.
Add Tollen’s reagent to the sample and heat. A silver mirror indicates the presence of an aldehyde.
Or
Add Fehling’s solution to the sample and heat. A brick-red precipitate indicates the presence of an aldehyde.
Test for carboxylic acids
Carboxylic acids are acids and can liberate carbon dioxide from carbonates:
Add sodium carbonate solution to the sample. If effervescence is seen, and the gas produced turns limewater milky, a carboxylic acid is present.
Test for acyl chlorides
Acyl chlorides release chloride ions very readily to give hydrogen chloride gas.
Add water to the sample slowly. If white misty fumes are given off, an acyl chloride is present.
Test for amines
Amines are basic.
Add universal indicator to the sample. If it turns blue/purple an amine is present. It will also have a fishy smell.
Tests for alcohols
Alcohols react with carboxylic acids in the presence of sulphuric acid to make esters.
Add ethanoic acid to the sample, followed by sulphuric acid and heat. If a the mixture starts smelling sweet and fruity an alcohol was present.
Distinguishing tests for primary, secondary and tertiary alcohols
Primary alcohols can be oxidized to form aldehydes. Secondary alcohols are oxidized to form ketones. Tertiary alcohols cannot be oxidized.
Add potassium dichromate and dilute sulphuric acid to the mixture and warm gently.
If the mixture goes green a primary or secondary alcohol is present. If it does not go green a tertiary alcohol is present. If the mixture does go green, add Fehling’s solution to the mixture and heat. If a brick-red precipitate if formed then a primary alcohol was present. If no brick-red precipitate is formed then a secondary alcohol was present.
