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Auxochrome: Wikis


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Auxochrome - this is a group of atoms attached to a chromophore which modifies the ability of that chromophore to absorb light. Example: -OH , - NH2 , Aldehydes.

An auxochrome is a functional group of atoms with nonbonded electrons which, when attached to a chromophore, alters both the wavelength and intensity of absorption. If these groups are in direct conjugation with the pi-system of the chromophore, they may increase the wavelength at which the light is absorbed and as a result intensify the absorption. A feature of these auxochromes is the presence of at least one lone pair of electrons which can be viewed as extending the conjugated system by resonance.

It increases the color of any organic compound. For example, benzene does not display color as it does not have any chromophore but nitrobenzene is pale yellow color because of the presence of nitro group. Para-hydroxynitrobenzene exhibits a deep yellow color. Here an auxochrome (-OH) is conjugated with the chromophore -NO2. Similar behavior is happens in azo benzene (red color) but para-hydroxy azobenzene is dark red color.

There are mainly two types of auxochromes-

1. Acidic -COOH, -OH, -SO3H

2. Basic -NHR, -NR2, -NH2

The presence of an auxochrome in the chromogen molecule is essential to make a dye. However, if an auxochrome is present in the meta position to the chromophore, it does not affect the colour.

An auxochrome is known as a compound that produces red shift as it increases the wavelength of absorption therefore moving closer to infrared light.


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