Functional Group Analysis

This lesson covers: 

1. Identifying key organic functional groups
2. The positive and negative test results
3. The chemical basis for each functional group test

Test for alkenes using bromine water

To confirm the presence of an alkene:

1. Add orange bromine water to the sample and shake the mixture. Bromine undergoes an addition reaction with the double bond to form a colourless bromoalkane.
2. Decolourisation of the orange bromine water confirms the sample is an alkene.

Test for halogenoalkanes using silver nitrate

To confirm the presence of a halogenoalkane:

1. Add aqueous silver nitrate solution and ethanol to the sample and gently warm the mixture. The halide ion is oxidised to form an insoluble silver halide precipitate.
2. Observe the colour of the precipitate formed and compare to the known precipitate colours of silver halides.

Test for primary and secondary alcohols using acidified potassium dichromate

To confirm the presence of a primary or secondary alcohol:

1. Add acidified orange potassium dichromate solution to the sample. Heat gently for a few minutes. The alcohol undergo an oxidation reaction to form carbonyl compounds.
2. If the orange solution turns green, a primary or secondary alcohol was present in the sample.

Tertiary alcohols cannot be oxidised by potassium dichromate so the solution remains orange.

Test for carbonyl compounds using 2,4-DNPH

To confirm the presence of a carbonyl functional group:

1. Dissolve 2,4-dinitrophenylhydrazine (2,4-DNPH) in sulfuric acid, water and methanol to prepare Brady’s reagent.
2. Add Brady’s reagent to the sample and shake the mixture. 2,4-DNPH reacts with carbonyls to form an orange precipitate.
3. Formation of an orange precipitate confirms the sample is an aldehyde or a ketone.

To identify the specific aldehyde or ketone, you can filter and purify the precipitate by recrystalisation, determine its melting point, and compare to known values.

Test for aldehydes using Tollens’ reagent

Tollens' reagent can be used to distinguish between aldehydes and ketones. Aldehydes give a positive test result whereas ketones give a negative result.

To identify an aldehyde using Tollens’ reagent:

1. Prepare Tollen's reagent, [Ag(NH₃)₂]⁺_(aq), by mixing solutions of silver nitrate, sodium hydroxide and dilute ammonia.
2. Add Tollens’ reagent to the sample and gently warm the mixture. Tollens’ reagent oxidises aldehydes to carboxylic acids as Ag⁺ ions are reduced to silver metal.
3. Formation of a silver mirror inside the test tube confirms the presence of an aldehyde.

Ketones cannot be oxidised further by Tollens’ reagent so solution remains colourless.

Test for carboxylic acids using sodium carbonate

To confirm the presence of a carboxylic acid:

1. Add a small spatula of solid sodium carbonate to the sample. Carboxylic acids react with carbonates to form carbon dioxide gas.
2. Bubble any gas produced through limewater in a second test tube. The limewater will turn cloudy in the presence of carbon dioxide.
3. If the limewater turns cloudy, a carboxylic acid was present in the sample.

Test for phenols using sodium hydroxide and a carbonate

To confirm the presence of a phenol:

1. Add one small spatula of solid sodium hydroxide to a sample. Phenols are weakly acidic and are neutralised by bases to form colourless phenolate salts.
2. If the solid sodium hydroxide dissolves to form a colourless solution, the sample may be a phenol.
3. To confirm, add a small spatula of sodium carbonate to a separate sample. Carbonates are weaker bases than sodium hydroxide so they do not react with weak acids like phenols.
4. A lack of effervescence confirms the sample is a phenol.

Summary of functional group tests

Here is a summary table for the functional group tests: