Here's why:
* Iodine acts as a Lewis acid: It forms a temporary complex with electron-rich regions in molecules, such as the double bonds in alkenes, alkynes, and aromatic rings.
* Complex formation leads to color changes: The iodine-compound complex absorbs light differently than the original compound, resulting in a visible spot on the TLC plate.
Therefore, iodine vapor is a useful tool to visualize unsaturated hydrocarbons, phenols, anilines, and other compounds containing double bonds or aromatic rings.
Note: It's not limited to these specific compound types. Iodine can also react with some other functional groups, but its affinity for unsaturated systems makes it particularly useful for these classes of compounds.
