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M.H. Mayura

Dinendrasingh

e

Assignment 5.

ID #

883380290V

Direct

Dyes

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Table of Contents

Table of Contents ....................................................................................................... 2

Introduction ............................................................................................................... 1

Direct Dyes ................................................................................................................. 1

Structural Requirements ............................................................................................. 2

Conjugated Double Bonds .......................................................................................... 2

Linear molecule with co-planarity ............................................................................... 4

Higher molar mass for better Van der waal forces ..................................................... 5

Sufficient number of Hydrogen bondable groups ....................................................... 7Presence of optimum number of solubilising groups .................................................. 7

Assignment

What are the essential structural requirement for a colorant to be a direct

dye?

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Introduction

A colorant is a substance, which is either a pigment or a dye thatcombines with other substances with different means to give out colour.

There are many different type of dyes. Some of them are

Acid Dyes

Basic Dyes

Direct Dyes

Reactive Dyes

Vat Dyes

Direct Dyes

A Direct dye which is also called as a substantive dye can be directly

dyed with cotton fibre without the help of any additional chemical and so thename direct dye is used. Direct dyes are combined together with fibres with

the help of week van der waal forces and hydrogen bond. The bond so

formed is not strong. Therefore direct dyes are moderate light and wash

fastness. Although direct dyes are moderate fastness they are still widely

used in the textile industry due to its low cost and the easy procedure of

dying.

The society of Dyers and colourists has divided direct dyes into three classes

according to recommendations. They are as follows,

1.Moderately salt controllable level dyes which also easily migrates, as

Class A.

2.Highly salt dependent level dying as, Class B.

3.Salt not controllable, only temperature controllable dyes as Class C

dyes.

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Although direct dyes are cheep in price and easy to dye, only about 60% of

the dyes are used today. This is due to the health issuers occurred after

using cloths dyed with direct dyes.

Structural Requirements

All colorant must have the following structural requirements to be a directdye. They are as follows

1. Conjugated double bonds

2. Linear molecule with co-planarity

3. Higher molar mass for better van der waal forces

4. Sufficient number of Hydrogen bondable groups, and their distribution

to coincide with primary OH group of the cellulose chain.

5. Presence of optimum number of solubilising groups, preferably on the

opposite side of the chain, where H-bondable groups are present.

Conjugated Double Bonds

Colour in dyes is invariably explained as a consequence of the

presence of a Chromophore. Since, by definition, dyes are aromatic

compounds their structure includes aryl rings which have delocalised

electron systems. These are responsible for the absorption of

electromagnetic radiation of varying wavelengths, depending on the energy

of the electron clouds. For this reason, chromophores do not make dyes


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coloured in the sense that they confer on them the ability to absorb

radiation. Rather, chromophores function by altering the energy in the

delocalised electron cloud of the dye, and this alteration results in the

compound absorbing radiation from within the visible range instead of

outside it. Our eyes detect that absorption, and respond to the lack of acomplete range of wavelengths by seeing colour.

Chromophores are atomic configurations which can alter the energy in

delocalised systems. They are composed of atoms joined in a sequence

composed of alternating single and double bonds. Double bonds in organic

compounds can be of two types. If the atoms with double bonds are not

adjacent, they are termed isolated double bonds, and exist independently of

other double bonds in the same molecule. If adjacent atoms have double

bonds they are termed conjugated double bonds and the bonds interact with

each other. Chromophore configurations often exist as multiple units, having

conjugated double bonds, and are more effective when they do so. This is

due to the interaction between the double bonds, which causes partial

delocalisation of the electrons involved in the bonds. In this case, although

specific atoms are involved in the bonds, the electrons are distributed over a

larger area than the specific atoms and also involve adjacent atoms that

have double bonds.

The point of this is that conjugated systems have partially delocalised

electrons, and the energy in these delocalised electrons can impact on the

energy of the delocalised electrons of the parent aromatic compound by

extending the number of electrons involved in the system and the energy

needed to keep the whole system in place.

Conjugated double bonds with carbon


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Another common chromophore is the nitro group. This chromophore is

a nitrogen with two oxygen atoms attached. One oxygen is shown attached

with a single bond, the other with a double bond. In fact, like the carbon

atoms in benzene, these two oxygen atoms are attached to the nitrogen with

bonds of equal strength. The extra electrons are delocalised between the

three atoms.

Nitro group

Linear molecule with co-planarity

All the direct dyes should have a coplanar structure are a requirement

to bond with the fibre. Direct dyes are combined with the fibre using

hydrogen bonds and van der waal forces. Hydrogen bonds and van der waal

forces are very week attractions. Without co-planarity the hydrogen bonds

and the van der waal forces cannot take place as they cannot come close to

the fibre molecule.


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Higher molar mass for better Van der waal forces

Van der waal forces occur due to the attraction of the masses of the

molecules. That means the strength of the van der waal forces increases

with the increase of the molar mass of the molecules. Therefore to increase

the attraction the dye molecule should have a bigger molar mass. As a resultall the direct dyes have long chain of atoms.

For example, some of the dyes are given below


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Sufficient number of Hydrogen bondable groups

Hydrogen bonds usually have a better strength of bonding than van der waal

force. There fore hydrogen bonds are the main bondable groups in directdyes. Due to this reason a sufficient number of hydrogen bond must be

present in the dye to properly combine with the fibre. Just having the

sufficient number of hydrogen bondable groups doesnt make a hydrogen

bond. The hydrogen bondable groups must have a proper distribution to

coincide with primary OH group of the cellulose chain. Then only a proper

hydrogen bond is formed

Presence of

optimum

number

of

solubilising groups

To be a dye it must first be soluble in water. To be soluble specific

groups like sulphonates should be present in the dye. Each dye should have

an optimum number of solubilising groups to be totally soluble. But thesegroups are a barrier for the dye to come close to the fibre molecule as these

groups repel the fibre molecule due to the same charge. To avoid repelling

the dye should

be formed in

such a way that

the hydrogen


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bondable groups and the sulphonates groups are attached to the either side

of the dye molecule. By doing this we can reduce the repelling force.

For example


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