L1 - Qualitative Tests on Organic Compounds

objectives

• conduct qualitative investigations to test for the presence in organic molecules of the following functional groups:
  • carbon–carbon double bonds
  • hydroxyl groups
  • carboxylic acids

Testing for C=C using Br Water

The addition of bromine dissolved in tetrachloromethane (CCl₄) or water (known as bromine water) is used as a test for unsaturation. If the reddish-brown colour is removed from the bromine solution, the substance possesses a C=C bond.

Because the bromine adds to the alkene, it no longer exists as molecular bromine and the typical red-brown colour disappears

Testing for C=C using KMnO₄

Testing for Alcohols

Alcohols all react with sodium metal.

The observations of this reaction are:

• Effervescence of the Hydrogen gas
• A white Precipitate being formed from the sodium alkyloxide formed

The word and symbol equations for the reaction with ethanol are below:

sodium + ethanol → sodium ethoxide + hydrogen

2Na + 2C₂H₅OH → 2C₂H₅ONa + H₂

Testing for Carboxylic Acids

CASH₂OCO₂ - Carbonates/ Hydrogen Carbonates and Acids

Carbonates and Hydrogen Carbonates are basic solids that can react with acids to produce a salt, water and carbon dioxide. This can be observed in the below Equation. Note from the table that we really are only interested in the metal part of the Carbonate/Hydrogen Carbonate for naming salts.

Carbonate + Acid --> Salt + H₂O + CO₂

As we can see that Carbonates (and Hydrogen Carbonates) make the same products we can use the acronym - CASH₂OCO₂ .

So when Carbonates react with Carboxylic Acids Carbon Dioxide is formed this can be bubbled through Limewater and the resultant product would be cloudy.