why does the FeCl3 test positive for phenols and not other -OH compounds?
http://academics.wellesley.edu/Chemistry/chem211lab/Orgo_Lab_Manual/Appendix/ClassificationTests/phenol_amine_nitro.html#IronChloride
The FeCl3 test is commonly used to detect the presence of phenolic compounds. Phenols are a specific class of organic compounds that contain a hydroxyl (-OH) group attached to a benzene ring. When FeCl3 is added to a solution containing phenols, it forms a colored complex due to a specific reaction called complexation.
The reaction between FeCl3 and phenols involves the complexation of iron(III) ions (Fe3+) with the hydroxyl group of phenols. This complexation results in the formation of a colored precipitate, typically a purple, violet, or green color, depending on the specific phenolic compound. The intensity of the color change can be used to determine the concentration or amount of phenolic compound present.
Why does FeCl3 test positive for phenols and not other -OH compounds?
The specific reaction between FeCl3 and phenols is based on the unique structure of phenolic compounds. Phenols possess an aromatic benzene ring with a hydroxyl group directly attached to it. This structural characteristic makes phenols highly reactive with iron(III) ions.
In contrast, other -OH compounds, such as alcohols and carboxylic acids, do not contain a benzene ring. These compounds may also have a hydroxyl group, but their structures are not suitable for the same complexation reaction that occurs between phenols and FeCl3.
To summarize, the FeCl3 test is specific for phenolic compounds due to their unique structure, which allows them to form a colored complex with FeCl3 through complexation. Other -OH compounds do not exhibit this reaction because they lack the necessary structural elements for complex formation.