What is Witt's theory of colour and constitution of dye and what are its limitations?
Witt's theory, also called the Chromophore-Auxochrome Theory, was proposed by Otto Witt in 1876 to explain the relationship between molecular structure and colour in organic compounds, especially dyes.
This theory suggests that the colour of a compound is due to specific groups called chromophores and auxochromes.
The colour of a dye arises when chromophores are conjugated with an extensive system of alternating single and double bonds, with auxochromes intensify or deepening the colour and enhancing dyeing ability by increasing the resonance.
The enhanced resonance lowers the energy difference between electronic states, shifting absorption to longer wavelengths (known as a bathochromic or red shift), thereby intensifying and deepening the dye's colour.
Example
Azo dyes: These dyes contain an azo chromophore —N=N— linked to aromatic rings. When auxochromes like —OH or —NH2 groups are present on these rings, the colour shifts from yellow to orange or red due to increased resonance.
Limitations of Witt's Theory
Witt's theory has several limitations, some of which are given below:
✍︎ This theory does not explain the colour of all dyes, e.g., indigo's colour does not fit the chromophore-auxochrome model.
✍︎ Some substances may have the requisite groups but still not act as dyes or may have unexpected colour intensity or depth.
✍︎ Resonance and molecular orbital theory also explain the colour in terms of electronic transitions, not just functional groups.
✍︎ Factors like solvent, pH, and molecular environment can affect colour, but the theory does not account for these.