**3. Nomenclature**

The chemical names of dyes are very complicated hence trade names are more popular instead of their chemical names. From the ancient time humans have tried to extract the dyes from plants and other natural sources to colour their clothes and other belongings, such dyes are known as Natural Dyes. Indigo and Alizarin are two examples of natural dyes. As the natural dyes have very few colours and shades so now a day's most of the used dyes are synthetic dyes having several colours and shades. Almost all synthetic dyes are aromatic and obtained from coal-tar [22]. Hence synthetic dyes are also called coal-tar dyes. Dyes can be classified by the following ways;

	- Covalent Bonds
	- Hydrogen Bonds
	- Ionic Bonds
	- Van Der Waals Forces

### **Based on application dyes can be categorised as follows**;

I.**Acid dyes:** The sodium salt of azo dyes containing sulphonic acid (-SO3H) and carboxylic acid(-COOH) groups are called acid dyes. To colour the

fabric from these dyes the acidic solution of these dyes is used. These can be used to colour wool, silk, nylon and polyurethane fibres. The affinity of acid dyes for nylon is very high due to the higher protonation of the free amino group present in polycaprolactam fibres. Orange-I (**Figure 3**), orange-II, methyl orange and congo red are some examples of it [22, 23].

The dye orange I and orange II can be obtained by coupling diazotised sulphonic acid with α and β- Naphthol respectively.

Diazotisedsulphonic β-Naphthol Orange-I acid sodium salt


**Figure 3.** *Orange-I.*

I.**Valence bond theory (VBT):** According to VBT, in the ground state the electron pairs of a molecule are in a state of oscillation and absorb a photon of appropriate energy and get excited when placed in the path of a beam of light. The wavelength of a photon of light adsorbed depends upon the energy difference between the ground state and the excited state.

II.**Molecular orbital theory (MOT):** According to MOT, whenever a molecule absorbs a photon of light, one electron is transferred from bonding (nonbonding) orbital to an anti-bonding orbital. Based on different type of electron present in a molecule, different types of electronic transitions are

The chemical names of dyes are very complicated hence trade names are more popular instead of their chemical names. From the ancient time humans have tried to extract the dyes from plants and other natural sources to colour their clothes and other belongings, such dyes are known as Natural Dyes. Indigo and Alizarin are two examples of natural dyes. As the natural dyes have very few colours and shades so now a day's most of the used dyes are synthetic dyes having several colours and shades. Almost all synthetic dyes are aromatic and obtained from coal-tar [22]. Hence synthetic dyes are also called coal-tar dyes. Dyes can be classified by the following ways;

I.**Based on their constitution:** In this classification dyes are classified based on the functional group to which the dyes owe their colour. Some examples are *azo dyes, nitro dyes, nitroso dyes, triphenylmethane dyes, indigoid dyes,*

II.**Based on their application:** The colouring of dye on a particular fibre depends on the nature of both the dye and the fibre. A dye molecule can be

I.**Acid dyes:** The sodium salt of azo dyes containing sulphonic acid (-SO3H) and carboxylic acid(-COOH) groups are called acid dyes. To colour the

possible.

*phthalein dyes, acridine dyes etc.*

• Covalent Bonds

• Hydrogen Bonds

• Van Der Waals Forces

• Ionic Bonds

**134**

attached to fibre by following methods;

**Based on application dyes can be categorised as follows**;

**3. Nomenclature**

**Figure 2.**

*2,4-Dinitro-1 naphthol.*

*Dyes and Pigments - Novel Applications and Waste Treatment*

**Figure 4.** *Aniline yellow.*

*Dyes and Pigments - Novel Applications and Waste Treatment*

**Figure 6.** *Chrysodine G.*

**Figure 7.** *Martius yellow.*


lakes [22, 23]. So, the metal ions first get attached to the fabric and then the dye molecules are linked to the metal ion through covalent and or coordinate bond

These dyes constitute the largest part of the synthetic dyes. The chromophore of the azo dyes is aromatic system joined to the azo group and auxochromes are NH2,

It is obtained by coupling of N, N-dimethylaniline with diazotised sulphanilic acid. **Properties:** Methyl orange is a colouring dye for the wool and silk but its colour fades on the exposure to the light and washing. Usually, it is not used as a dye but

Azo dyes are classified as the number of azo group in the molecule such as monoazo, diazo and triazole etc. The characteristic of two important azo dyes

methyl orange (**Figure 11)** and congo red can be described as follows:

**4. Classification of dyes based on their constitution**

*Alizarin-aluminium fibre complex (rose red lake).*

(**Figure 10)**.

**Figure 10.**

**Figure 9.**

*Indigo or Indigotin (Leuco form).*

*Structure and Properties of Dyes and Pigments DOI: http://dx.doi.org/10.5772/intechopen.97104*

**4.1 Azo dyes**

NR2, OH.

**Figure 11.** *Methyl Orange.*

**137**

*4.1.1 Methyl orange*

These dyes are also used in foodstuffs, cosmetics, drugs and as an indicator in chemical analysis.


Metal ions are used as mordants for the acid dyes while tannic acid is used as the mordant for basic dyes. The desired fabric is first soaked in the suitable metal salt and then this soaked fibre is dipped in the solution of dye when insoluble coloured complexes formed on the fabric. These insoluble coloured complexes are called

**Figure 8.** *Para red.*

*Structure and Properties of Dyes and Pigments DOI: http://dx.doi.org/10.5772/intechopen.97104*

**Figure 9.** *Indigo or Indigotin (Leuco form).*

**Figure 10.** *Alizarin-aluminium fibre complex (rose red lake).*

lakes [22, 23]. So, the metal ions first get attached to the fabric and then the dye molecules are linked to the metal ion through covalent and or coordinate bond (**Figure 10)**.
