Industrial Chemistry Past Papers

Concept:

A chromophore is an unsaturated functional group containing delocalized \(\pi\)-electrons that absorbs electromagnetic radiation in the visible region (\(400\text{--}700\text{ nm}\)), thereby imparting color to a molecule.

Formula / Reaction:

$$-\text{N}=\text{N}- \quad (\text{Azo Chromophore})$$

Solution:

• The azo group (\(-\text{N}=\text{N}-\)) contains a double bond with a conjugated \(\pi\)-system capable of \(\pi \rightarrow \pi^*\) transitions in the visible spectrum.



• Azo compounds constitute the largest class of synthetic organic dyes.

Why other options are incorrect:

• Amino Group (\(-\text{NH}_2\)): An auxochrome that intensifies color and shifts absorption wavelength, but cannot impart color on its own.



• Hydroxyl Group (\(-\text{OH}\)): An auxochrome containing lone pairs that enhance the chromophore's absorption.



• Sulfonic Acid Group (\(-\text{SO}_3\text{H}\)): A solubilizing group used to render dye molecules water-soluble.