Why does iodine absorb organic molecules? This question is referring to the effect of the iodine chamber used in TLC.
Iodine has a high vapor pressure, for a solid, plus it is quite reactive to unsaturated compounds. The long and short of it is that iodine has a high affinity for organic compounds and because it vaporizes easily it can stain those spots on a TLC plate that are colorless thus making them visible. Care must be taken to circle the spots on the plate lightly (with a pencil) for the I2 often does not stay put when the compound is removed from the staining vessel.
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Iodine is commonly used in TLC (Thin Layer Chromatography) to visualize the presence of organic molecules on a TLC plate. The reason iodine absorbs organic molecules is due to its chemical properties.
To understand why iodine absorbs organic molecules in TLC, let's start with the composition of iodine. Iodine is a halogen element, represented by the symbol I, and it exists as diatomic molecules (I2). These diatomic molecules have a characteristic brown color.
In TLC, the stationary phase on the TLC plate is usually coated or impregnated with a polar material, such as silica gel or alumina. This polar surface interacts with polar groups of molecules through intermolecular forces like dipole-dipole interactions, hydrogen bonding, and van der Waals forces.
Now, when an iodine chamber is used for TLC, iodine vapors are generated inside the chamber. These iodine vapors readily react with the organic compounds on the TLC plate.
Organic molecules contain different functional groups such as hydrocarbons, alcohols, ketones, aldehydes, and others. These functional groups have varying polarities and reactivities. Iodine is particularly attracted to organic compounds that are electron-rich or have π-electron systems.
When iodine vapors come into contact with the TLC plate, the iodine molecules undergo a chemical reaction with the organic molecules present. This reaction is known as an addition or complex formation reaction. As a result, the originally brown-colored iodine molecules change their color. The degree of color change varies depending on the type and amount of organic molecule present.
The mechanism behind the absorption of organic molecules by iodine involves the formation of a charge-transfer complex between the electron-rich region of the organic molecule and the partially positive iodine atom. This complex is responsible for the observed color change. The absorbed organic molecules reduce the electron density in the iodine molecule, resulting in a shift in the absorption spectrum and a change in color from brown to a range of shades, such as dark brown, purple, or blue.
By examining the TLC plate after exposure to iodine vapor, researchers can determine the presence and approximate quantity of different organic compounds based on the intensity of the color changes. This visual detection technique allows scientists to analyze and separate mixtures of organic compounds using TLC.
Overall, iodine absorbs organic molecules in TLC due to its ability to react with and form charge-transfer complexes with electron-rich functional groups present in those molecules.