Why is para red dye red in colour? Explain with a structure.

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ncchemist eNotes educator| Certified Educator

I cannot draw the structure here but I will explain it.  Para red is a chemical dye.  It is composed of a naphthyl ring (two benzene rings fused together) connected to a benzene ring via an azo group (nitrogen nitrogen double bond).  The naphthyl ring has a hydroxyl group (OH) attached to it and the benzene ring has a nitro group (NO2) attached to it.  All of these structural features have lots of pi electrons interacting with each other that give the molecule interesting light absorbance properties.  Para red absorbs all visible wavelengths of light except for the red band.  It reflects these, so that gives it a red appearance.  The structure can be found in the first link below.