Theory of Dyes: A dye consists of a chromophore group and a salt forming group called anchoric group. In 1876, Otto Witt gives forth a theory as to correlate colour with molecular structure (constitution). The theory is named 'The Chromophore Auxochrome Theory' and its main postulates are,
(i) The colour of the organic compounds is due to the presence of certain multiple bonded groups called chromophores. Important kind of chromophores are,
[Chromophore-Greek word, Chroma = colour, Phorein = to bear].
The presence of chromophore is not necessarily sufficient for colour. To make a substance coloured, the chromophore has to be conjugated with an extensive system of alternate single and double bonds as exists in aromatic compounds.
The chromophore part of the coloured substance (dye) absorbs some wavelengths from white light and reflects back the complementary colour. A coloured compound having a chromophore is known as chromogen.
(ii) Certain groups, while not producing colour themselves, when present along with a chromophore in an organic substance, density of the colour. Such colour support groups are called auxochromes (Greek word, Auxanien = to increase; Chrome = colour), i.e. they make the colour deep and fast and fix the dye to the fabric. The auxochromes are basic or acidic functional group sets. The important type of auxochromes are,
However, Otto Witt chromophore-Auxochrome fact fails to explain the colour of certain dye stuffs like indigo.
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