Synthetic dyes

Smt. A. P. Itkapalle

Head

B. Sc. T. Y.

Introduction

• Dyes are those coloured compounds which when applied to the fabrics imparts a permanent colour to it.
• This colour is not removed by washing with water, soap or on exposure to light.
• All coloured organic compounds are not necessarily dyes.
• For ex. Picric acid and trinitrotoluene are yellow in colour.

Picric acid can fix to a cloth and is a dye while trinitrotoluene does not fix to a cloth and is not a dye.

Characteristics of a dye

• It should be able to fix itself to the material from solution or be

capable of being fixed on it.

• It should resist the action of water, soap and light.
• A dye should be a coloured substance.

Theories of colour and constitution

• White light is the combination of light of all wavelengths in the visible range 400 – 750 nm.
• The different substances show different colours. This is because they absorb and reflect different wavelengths from the white light that falls on them.

The following generalization can be made in respect of colours :

• A substance which totally reflects the white light appears white.
• A substance which absorbs all the wavelengths of white light appears black.

• A substance which reflects a narrow band of wavelength of one colour and absorbs all the other wavelengths has the colour of the reflected light.
• Thus a substance appears blue because it absorbs all the wavelengths of visible light except a narrowband corresponding to blue (around 450 nm) which it reflects.
• A substance which absorbs a single narrow band of one particular colour and reflects the remaining wavelength has the colour due to the combination of remaining wavelengths.

For ex. If a substance absorbs in the wavelength region corresponding to blue and reflects the remaining wavelengths, it will appear yellow and vice versa.

The complementary colours are related to each other as the colour absorbed and colour observed.

The relationship between the complementary colours is shown in the table:

Witt’s theory of colour and constitution:

In 1876, Otto N. Witt observed that, colour in organic compounds is associated with the presence of certain groups in the molecule. According to him, a coloured substance or a dye is essentially composed of two parts namely chromophore and auxochromes.

1. Chromophore

The colour in an organic compound is due to the presence of certain groups with multiple bonds. Such groups are designated as chromophores. The chromophores are the colour bearing groups and their presence produces a colour in the molecule of an organic compound.

Example - The important chromophores are :

The organic compound containing a chromophoric group in its molecule is referred as chromogen. The presence of chromophore in the molecule imparts colour to an organic compound.

It has been observed that the chromogen containing only one chromophoric group is usually coloured (Yellow).

The intensity of colour generally increases with number of chromophoric groups.

A single C = C group as in ethane CH2 = CH2 does not produce any colour, but if a number of these groups are present in conjugation, the colour may develop.

Ex. :- CH3 – ( CH = CH )6 – CH3 is yellow in colour.

In case of weaker chromophores, more than one group is needed to develop a visible colour.

2. Auxochromes

Certain groups (which are not chromophores) when present in the chromogen tend to intensify its colour. Such groups are called as auxochrome.

Ex.: Hydroxyl group (–OH), alkoxy (–OR), amino(–NH2), alkylated amino ( –NHR,–NR2), sulphonic acid (–SO3H), carboxyl (–COOH), phenolic (–OH) etc.

The auxochrome may be acidic or basic in character.

Auxochromes are salt forming groups and perform two functions:

• They deepens the colour of chromogen,
• Its presence is essential to make the chromogen a dye.

The groups which deepens the coloured are called as bathochromic groups. The groups which bring about the opposite effect are known as hypsochromic groups.

The replacement of H in NH₂ group by R or Ar has bathochromic effect while replacement of H in OH group by acetyl group has a hypsochromic effect.

When an auxochrome is introduced in the molecules, a colourless chromogen becomes coloured.

Ex: Benzophenone (colourless) becomes yellow when an auxochrome is introduced in it.

Nitroaniline is deeper in colour than nitrobenzene

Quinonoid theory

According to this theory, all colouring substances may be represented

by quinonoid structures (o or p). If a particular substance can be formulated in a quinonoid form, it is coloured other wise it is colourless.

• According to quinonoid theory, benzene is colourless while benzoquinone is coloured

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• On the basis of this theory, the dye like phenolphthalein is coloured when present in quinonoid structure, but is colourless when p-quinonoid structure is absent.

It has been observed that quinonoid theory fails to explain the colouring characteristics of all the compounds.

eg. Iminiquinone and di-iminoquinone have a quinonoid structure but they are colourless.

Similarly, many coloured compounds like diacetyl and azobenzene are coloured but they can not be represented by quinonoid structures.

Classification of dyes

Dyes are classified in two ways based on the common parent structure ie chemical constitution and their mode of action.

A. The classification of dyes on the basis of their mode of action:

• The classification is concerned mainly with the various different methods of dyeing to various fibres with the different dyes.
• This method is very useful to the different dyers who are concerned with the process of dying.

1. Acid dyes:

• These dyes are essentially the sodium salts of acids which may

contain sulphonic acid or Phenolic acid group.

• The colour of an acid dye is in its negative ion.
• These dyes give very bright colour and have a wide range of

fastness.

• These dyes are also known as anionic dyes.
• The acid dyes are always used in acidic solution.
• The fabric is stirred in hot solution of the dye in the presence of

either an acid or salt till it is smoothly dyes.

• It is then removed and dried.
• Ex. picric acid, orange -II, naphthol yellow etc.

2. Basic dyes:

• These dyes are also called as cationic dyes.
• The basic dyes are those which contain a basic amino group and it

is protonated under the acid conditions of fibres by formation of

salt linkages with anionic or acidic groups in the fibres.

• Examples : crystal violet, methylene blue and methyl violet.

3. Direct dyes:

• These dyes are also known as substantive dyes.
• They are cotton as well as wool and silk.
• The dye is applied to the fibric by immersing it in its hot

boiling solution, removing and then drying the fabric.

• Example:- congo red, naphthol, yellow S, martius yellow etc.

4. Developed dyes:

• These dyes are also called as azotic or ingrain dyes.
• These are the dyes which are produced within the cloth itself as

a result of chemical action between the two reactants producing

the dye.

• For example, the cloth is first dipped in an alkaline solution of

phenol, resorcinol or β- naphthol and then immersed in an

alkaline solution of diazo compound.

• The coupling reaction takes place between the phenols and

diazo compound within the textile fibres giving rise to the

formation of a dye.

5. Mordant dyes:

• These dyes are also called as adjective dyes.
• They cannot directly dye cotton, silk or wool but require the help

of mordant.

• A mordant is a substance which is taken up by the fibres and

which in turns takes up the dye.

• There are acidic and basic mordants.
• If the dye is acidic, the mordant must be basic eg. Salts of Cr, Al,

Sn and Fe.

• On other hand, if the dye is basic, the mordant must be acidic

eg. Tannin or tannic acid containing some amount of tartar emetic.

• The mordanted cloth is dipped into the solution of the dye.
• The dye is absorbed by the mordant forming an insoluble dye compound which gets firmly fixed within the fibres.
• Certain dyes give different colours with different mordants.

6. Vat dyes:

• These dyes are insoluble in water but their reduced forms

are soluble.

• On reduction with alkaline sodium bisulphate, the vat dyes

are converted into water soluble compounds called leuco-

compounds.

• They dye both vegetable and animal fibres directly.
• The vat dyes are mostly used to dye cotton.
• Example indigo and anthraquinone dyes.

B. Classification of dyes on the basis of structure

The chemical classification is based on the common parent structure of the dye.

The number of dyes based on this classification is fairly large. Some important classes of dyes are :

1. Nitro dye

• These dyes contain nitro group ( – NO₂) as the chromophore and

hydroxyl group (–OH) as the auxochrome.

• They are generally nitro derivatives of phenols containing

atleast one nitro group in ortho or para position to the hydroxyl

group.

• They are relatively little importance industrially, because the

colours are not very fast.

2. Azo dyes

• The azo dyes represent the largest and most important group of

dyes.

• These dyes contain one or more azo groups (– N = N – ) which

form bridge between two or more aromatic rings.

• Azo group is the chromophore.
• Ex.:- Aniline yellow, methyl orange, cango red and

Bismark brown etc.

Methyl orange

Methyl orange is anionic dye. It is prepared by coupling diazotized sulphanilic acid with dimethyl aniline.

• Methyl orange is not being used as a dye because it is sensitive to

acids and not sufficiently stable to soap and light.

• It is used as an indicator in acid base titrations.
• It is orange in alkaline solution and red in acid solution.
• The colour change takes place in the pH range 3.1 – 4.5.

Congo red

Congo red is a member of diazo dyes. It is prepared by the coupling of reaction between tetrazotised benzidine with two molecules of naphthionic acid.

• Congo red is not a good dye for textiles because the colour changes when acid is added.
• It was once an important stuff for dyeing paper.
• It is red in alkaline medium (pH above 3).
• Its sodium salt dyes cotton perfectly.
• It is also used as an indicator.

3. Nitroso dye

These dyes contain a nitroso group ( – NO ) as the chromophore and hydroxyl group as auxochrome.

Ex. : Fast Green O (Dinitroso resorcinol),

Gambine Y (α- nitro-β-naphthol)

4. Phthaleins

These dyes are obtained by the condensation of phthalic anhydride with phenols in presence of some dehydrating agents like conc. H₂SO₄ or anhydrous ZnCl₂

Ex.: Phenolphthalein.

Phenolphthalein is prepared by heating phthalic anhydride (1 molecule) with phenol (2 molecules) in presence of conc. H₂SO₄.

• Phenolphthalein is a colourless solid with M.P. 261 °C
• It is insoluble in water, but dissolves in alkalies to form deep red

solution. This is due to the formation of a disodium salt, the ion

of which is coloured because if resonance.

• When excess of strong alkali is added, the solution of phenolphthalein becomes colourless. This is attributed to the formation of a trisodium salt, the ion of which is colourless because of loss of resonance and quinonoid structure.

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Because of such colour changes, phenolphthalein is used as indicator rather than dyes.

It is extremely powerful laxative and this accounts for its wide spread use as a denaturant for laboratory alcohol.

5. Xanthene dyes

These dyes are obtained by condensing phenols with phthalic anhydride in the presence of ZnCl₂, H₂SO₄ or anhydrous oxalic acids etc.

Ex. :- Fluorescein, eosin, rhodamine B.

Synthesis Of Alizarin

• When phthalic anhydride is treated with benzene in presence of AlCl₃, then o- benzoyl benzoic acid is formed which on treatment with conc. Sulphuric acid undergoes cyclization to form anthraquinone.
• When anthraquinone is heated with fuming sulphuric acid at 180 ºC, then anthraquinone -2-Sulphonic acid is formed which is converted to its sodium salt by treatment with sodium hydroxide.
• Sodium salt of anthraquinone -2-Sulphonic acid is fused with sodium hydroxide in the presence of potassium chlorate at 200 ºC under pressure followed by the treatment with sulphuric acid gives alizarin.

• Alizarin can also be prepared by the condensation of phthalic anhydride with catechol in the presence of sulphuric acid at 180 ºC

Uses:

• Alizarin is used to dye cotton and wool.
• It is also used for making printing ink.

Synthesis Of Diamond black-F

Diamond black-F is a disazo mordant dye. It is one of the earliest chrome dyes.

The coupling of diazotized 5 – amino salicylic acid with 1 – naphthylamine gives amino azo dye struff which is then diazotized and coupled with 1- naphthol-5- sulphonic acid to give Diamond black-F.

Synthesis Of Orange – II

Orange – II is prepared by the coupling of diazotized sulphanilic acid

With β-naphthol in basic medium.

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Orange – II is to dye wool, silk, nylon, paper and leather.

Synthesis Of Indigo

The condensation of aniline with chloroacetic acid gives N-phenyl glycine which is then fused with sodium hydroxide and sodamide at 250 ºC and forms indoxyl, which on oxidation by air gives indigo.

The condensation of anthranilic acid with chloroacetic acid gives N-phenyl glycine- o- carboxylic acid. N-phenyl glycine- o- carboxylic acid is then fused with sodium hydroxide and sodamide to give unstable indoxylic acid, which on decarboxylation gives indoxyl. Then indoxyl on oxidation by air gives indigo.

Synthesis Of Malachite green

The condensation of benzaldehyde with two molecules of N, N – Dimethyl aniline in presence of conc. Sulphuric acid gives leuco base. Oxidation of the leuco base with lead peroxide followed by the treatment of HCl gives malachite green

Malachite green is used as a dye for acrylic fibres, leather, paper and lacquers

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