This document discusses the key properties and components of dyes. It defines chromophores as the part of a molecule responsible for its color, and notes that chromophores undergo π-π* or n-π* electronic transitions. Auxochromes are groups that intensify a dye's color by extending conjugation with the chromophore. Common auxochromes include -OH, -NH2, and -COOH groups. The document also classifies dyes based on their structure, application method, and presence of acidic or basic auxochrome groups.

1. Shri Shivaji Education Society, Amravati
J.D.Patil Sanguldkar Mahavidyalay,
Daryapur
Dr. Aparana. S. Dighade
Department of Chemistry
Topic – Dyes I

2. Contents
• Introduction
• Properties of dyes
• Chromophore
• Types of chromophore
• Auxochrome
• Types of auxochrome
• Classification of dyes

4. Properties of Dyes
Dyes must passes the following properties
• Colour
• Solubility in water
• Ability to be absorbed and retained by fibre
(substantivity) or to be chemically combined with it
(reactivity).
• Ability to withstand washing, dry cleaning and
exposure to light. The dye has a colour due to the
presence of chromophore and it’s fixed property to the
acid and basic groups such as OH, SO3H, NH2,NR2 etc.
The polar auxochrome makes the dye water soluble
and binds the dye to the fabric by interaction with the
oppositely charged groups of fabric structure.

5. What is chromophore
Greak:- chromo-colour and phores- bearer
A chromophore is the part of a molecule
responsible for it’s colour. The colour that is seen by
our eyes
Define as any isolated covalently bonded gp that
shows a electromagnetic radiation in the UV- visible
region.
Compound containing chromophore is chromogen
Eg. C=C, C=O, NO2

6. Type of chromophores
Two types of chromophore are known
1. Chromophores in which the group have 𝜋
electron undergoes 𝜋- 𝜋* transitions for eg.
Ethylene, acetylene etc.
2. Chromophores having both 𝜋 electron and (n-
bonding) electrons undergoes two types of
transitions i,e 𝜋- 𝜋* and n- 𝜋* transitions. For
eg. Carbonyls, nitrtes, azo compds and nitro
compds etc.

7. What is Auxochromes
Greek:- auxo- increase and chroma- colour
Auxochrome is a gp of atoms attached to a chromophore which
modifies the ability of that chromophore to absorb light they
themselves fail to produce the colour, but when present along with
the chromophores in an organic comp intensifies the colour of the
chromogen. Eg. Include the hydroxyl gp, amino gp, and aldehydes.
An auxochrome is a functional gp of atoms with one or more
lone pairs of electons when attached to a chromophore, alters both
the wavelength and intencity of absorption, if these gp are in direct
conjugation with the pi system of the chromophore, they may
increase the wavelength at which which the light is absorbed and as
a result intensify the the absoption. A feature of these auxochrome
is the presence of least one lone pair of electron which can be
viewed as extending the conjugated system by resonance.

8. Types of auxochrome
There are mainly two auxochrome
• Basic auxochrome – positive auxochromic gp
effective in acidic solutions.
eg. -NH2, -NHR, -NR2 etc.
• Acidic auxochrome – negative auxochromic
gp. Effective in alkaline solutions.
eg. -OH, -COOH, -SO3H etc.

9. Effect of Auxochrome
• It increase the colour of any organic compound
eg. Benzene does not display colour as it does
not have a chromophore but nitrobenzene is pale
yellow colour because of the presence of a nitro gp
–NO2 which act as a chromophore. But p-
hydroxynitrobenzene exibits a deep yellow colour,
in which the –OH gp acts as an auxochrome. Here
the auxochrome –OH is conjugated with the
chromophore –NO2

10. Classification of Dyes
• On the basis of structure
• On the basis of mode of application
• Dyes can be classified into acidic and basic
depending on the nature of the auxochrome
present.

12. Thank you