Propanoic acid (CH₃CH₂CO₂H) and methyl ethanoate (CH₃CO₂CH₃) are classified as functional group isomers due to the difference in their functional groups despite having the same molecular formula, C₄H₈O₂.
In propanoic acid, the functional group is a carboxylic acid (–COOH), which is characterized by the presence of the hydroxyl group (-OH) attached to a carbonyl group (C=O). This structure gives propanoic acid its acidic properties.
On the other hand, methyl ethanoate is an ester, with the functional group being an ester (–COOC–). In this case, the –O– atom connects the carbonyl group (C=O) to a carbon atom from a methyl group (–CH₃), resulting in different chemical properties and behavior compared to propanoic acid.
Despite both compounds having the same molecular formula, their distinct functional groups lead to different physical and chemical properties. This classification as functional group isomers is crucial in organic chemistry, as it showcases how variations in structure can lead to entirely different types of substances.