Auxochrome

An auxochrome (Greek αὐξάνειν auxánein: "to increase" and χρῶμα chrōma: "colour") is a group of atoms attached to a chromophore which modifies the ability of that chromophore to absorb light. They themselves fail to produce the colour; but when present along with the chromophores in an organic compound intensifies the colour of the chromogen. Examples include the hydroxyl group (-OH), the amino group (-NH₂), the aldehyde group (-CHO), and the methyl mercaptan group (-SMe).

An auxochrome is a functional group of atoms with nonbonded electrons 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 a chromophore; but nitrobenzene is pale yellow color because of the presence of a nitro group (-NO₂) which acts as a chromophore. But Para-hydroxynitrobenzene exhibits a deep yellow color, in which -OH group acts as an auxochrome. Here the auxochrome (-OH) is conjugated with the chromophore -NO₂. Similar behavior is seen in azobenzene which has a red color, but para-hydroxy azobenzene is dark red in color.

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