Dyes are coloured organic compounds that can impart their colour to other substances. To be an effective dye, a compound must have a suitable colour, be capable of firmly binding to fibers, and retain its colour after binding. It should also be stable during dry cleaning. Common dyes include malachite green, orange dyes, alizarin, martius yellow, indigo, congo red, quercetin, and para red. The chromophore is responsible for colour and contains an unsaturated group, while auxochromes intensify colour. Resonance contributes to dye colour by allowing pi electrons to absorb radiation. Dyes find applications in silk, wool, synthetic fibers, biological staining, indicators, and food colouring

1. Coloured organic compounds and impart colour to
other substances

2. REQUIREMENT OF A DYE
 Must have a suitable colour
 Capable of fixing
 After fixing, the colour should
be retained
 Should not react with ordinary
chemicals that are used during
dry cleaning
MALACHITE GREEN
ORANGE-I
ORANGE-II
ALIZARIN
MARTIUS YELLOW
INDIGO
CONGO RED
QUERCETIN
PARA RED

3.  Witt’s theory
 VBT

4.  Structure of a dye
 Chromophore
 Colour producing group (chromogen)
 Unsaturated (nitro, nitroso, azo, azoxy, carbonyl)
 Greater number of chromophores increase colour
intensity
 Auxochrome
 Not itself coloured
 Able to intensify the colour-it is essential in a dye
 NR2 > NHR > NH2 > OH > X > OR

5. CHROMOGEN
-OH
AUXOCHROM
Eg:- p-hydroxyazobenzene

6.  Role of resonance in formation of colour
 Chromophores-π electrons get excited by
absorbing radiation and hence coloured
 Auxochromes
 Colour intensifiers
 Bathochromic or red shift
▪ Shift of absorption towards higher wavelengths
 Hypsochromic or blue shift
▪ Shift of absorption towards lower wavelengths

9. Applications
Dye for silk and wool
Used to dye synthetic fibres
Yellow biological stain
Chemical indicator

10. Indigoid group
Indigotin
Applications
•Dye cotton, wool and silk
•Indigo carmine, a derivative
of indigo is a food colourant

11.  An anthroquinone dye
 Applications
 A mordant dye
 Biological stain/
geological stain
 Red pigment in paint
 Printing ink constituent
 Spot test reagent