Distinguishing Organic Compounds

Suggest one simple chemical test to distinguish them from each other. State the reagents and conditions needed and give the expected observations for each compound.

Analysis:

This pair of compounds are both dienes, meaning they both contain two C=C bonds. 

We will not be able to distinguish them by using chemical tests for alkenes. This is true for any pair of compounds with the same functional group(s). For example, esters, secondary alcohols, amides etc.

The tip is to distinguish them base on their structures. Use structure-specific tests such as iodofoam test and KMnO4.

For the pair of compounds above, note that the double bonds are closer for cyclohexa-1,3-diene. Using hot acidified KMnO4 will yield carboxylic acid for both compounds but one of the product for cyclohexa-1,3-diene is is ethandioic acid which will further oxidize to carbon dioxide and water.

Answer:

Add KMnO₄(aq), H₂SO₄(aq) and heat to both compounds.

Observation: Both solutions will change from purple to colourless but the solution of cyclohexa-1,3-diene will produce effervescence of CO₂ that form white ppt with limewater

Extra practise:

Distinguishing organic compounds is one of the easier types of questions in H2 Chemistry, be sure to score here!

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