Organic 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 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 aldehydes using Tollens’ reagent or Fehling’s solution

Tollens' reagent and Fehling's solution can be used to distinguish between aldehydes and ketones. Aldehydes give positive test results with both reagents whereas ketones give negative results with both reagents.

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.

To identify an aldehyde using Fehling's solution:

1. Add blue Fehling's solution containing Cu²⁺ ions to the sample. Gently warm the mixture. Fehling’s solution oxidises aldehydes to carboxylic acids as Cu²⁺ ions ions are reduced to copper(I) oxide.
2. Formation of a red precipitate (of Cu₂O) confirms the presence of an aldehyde.

Ketones cannot be oxidised further by Fehling’s solution so solution remains blue.

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.

Summary of functional group tests

Here is a summary table for the functional group tests: