In 1876, a German scientist Otto Witt put forward a theory known as “Chromophore Auxochrome theory” or Witt theory. Some important points of this theory are:
(i) The colour of a substance is mainly due to the presence of unsaturated groups known as chromophores. The important chromophores are:
(ii) The compounds possessing chromophores are referred to as chromogens. The higher the number of chromophores in a chromogen higher is the colour bearing capacity. The chromospheric groups are of the following two types:
1. When a single chromophore is sufficient to impart colour to the compound.
Example: NO, NO2, N = N, = N = N,N = NO, p – quinonoid etc.
2. When more than one chromophores are required to impart the colour.
Example: > C = O, > C < etc.
3. Certain groups, which do not produce colour themselves when present along with a chromophore in an organic substance, intensify the colour. Such colour assisting groups are called auxochromes. They make the colour deep and fast.
