DyesDyes Prepared by Dr. Khalid Ahmad Shadid Chemistry Department Islamic university in madinah CH-3CH-3

Definition of dye Definition: it’s a stable coloring matter which can be applied on the substrate in the form of solution or dispersion and which can import color to substrate. Prorerties of a good dye: (1) it should have affinity for the fibres. (2) Resistance to the action of water, dilute acids and alkalis, various organic solvents used in dry cleaning. (3) it should have attractive color, once applied to substrate it should be not easily removed. (4) it should be soluble in water or the medium through which it is applied fibres, it should form fine and stable dispersion in that medium. 2

Classification of dyes Dyes are classified in two ways: (1) according to their chemical constitution. (2) by their method of application to the fibre. (1) Chemical classification: It is rather difficult to classify the dyes in distinct groups because the chemical constitution of dyes is very much varied. However, they are classified as : Nitro, Nitroso, Azo, Triaryl methane, Anthraquinone, Indigoid, Phthalocyanine, Acridine, Sulphur dyes etc. 3

2. Classification according to application: (i)Acid dyes: These are sodium salts of sulphonic acid and carboxylic acids. They can be applied to wool, silk, polyamides and acrylic fibres. (ii)Basic dyes: These are salts of the colour bases. (iii)Mordant dyes: These cannot be fixed on the fibre directly but require the help of a mordant. For acidic dyes, metal hydroxides Al(OH) 3, Cr(OH) 3, Fe(OH) 3, etc are used as modants. For basic dyes, tannin ( tannic acid ) is used as a mordant. The metal salts formed are termed as ‘lakes‘. (iv)Vat dyes: These are applied to the fibre in their reduced form (Leuco compounds) After the application to the fibre, the leuco compound is reoxidised to the dye. They are mainly used with the cotton fibres. Indigo is a very well-known example of a vat dye. 4

v.Disperse dyes: These are water-insoluble dyes and are used with synthetic fibres. Before application they are dispersed with suitable reagents. vi.Azoic dyes: These are insoluble azo dyes Hence they are produced on the fibre itself. Cellulose fibres are mainly dyed with them. vii.Reactive dyes: They contain a reactive group which combines directly with cellulosic fibres. viii.Sulphur dyes: These are mainly used with cellulosic fibres. ix.Metal complex dyes: These are different from the mordant dyes. They are prepared as complexes which are then applied to the fibres. x.Organic pigments: pounds and are used as pigments for colouring paints. When made water-soluble, they are used as dyes Classification according to application:

Method of application of dyes: It is already said earlier that the method of application of a dye to a fibre depends upon the nature of the fibre as well as that of the dye. Fibres may be natural or synthetic. Natural fibres may be of vegetable origin ( e.g. cotton, linnen, hemp, Jute etc) or of animal origin (e.g, Wool, Silk, leather, fur etc.) Vegetable fibres are cellulosic in nature while animal fibres are protein in nature. Synthetic fibres are high polymers that are built of various types of monomeric units. They may be polyamides (Nylons), polyesters (Terylene), polyacrylo nitriles (Orlon) etc. Before dyeing, fibres are treated chemically. The dye molecule can bind to the fibre in four ways : (i) by covalent bonds (ii) by hydrogen bonds (iii) by ionic bonds and (iv) by van der Waals forces. The type of bonding of the dye to the fibre depends largely on the nature of the fibre. 6

Synthesis of some typical dyes: (1) Methyl orange (helianthin or Orange III) : Methyl orange is prepared by coupling diazotised sulphanilic acid with dimethylaniline: 7 Dimethylaniline Diazotised sulphanilic acid ( p-sulphonic benzene diazonium chloride) Methyl orange is an acid azo dye. However it is not used as a dye but rather as am acid-base indicator, being orange in alkaline solution, and red in acid solution. Methyl orange Orange Red

(2) Alizarine (1,2-dihydroxy anthraquinone) It is an anthraquinoid dye and one of the most important of the class. It is the chief constituent of the madder root. (Rubia tinctorum). Alizarin was valued for its Turkey Red shade. Preparation: It is prepared by sulphonating anthraquinone with fuming sulphuric acid at elevated temperature and fusing sodium anthraquinone-2-sulphonate with NaOH and the calculated quantity of KCl0 3 at C under pressure. The function of KClO 3 is to provide oxygen for the oxidation of carbon atom one to -C-OH- group. Alizarin forms ruby red crystals (m.p. 290°C). It dissolves in alkali to give a purple solution. It is a mordant dye and the colour of the lake depends upon the mordant used. Thus, with Al(OH) 3 mordant, it gives red lake (Turkey red), with Fe (0H) 3, a violet black and with Cr(OH) 3, a brown violet. Aluminium and Iron lakes a e used for dyeing cotton and for printing while Al and Cr lakes are employed for dyeing of wood. Dyeing with Alizarin is almost obsolete now. 8

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(3) Indigo (Indigotin) It is a vat dye. It is one of the ancient dyes and was formerly obtained from the leaves of the Indigo plant, specially cultivated for the purpose. Indigo does not occur as such in the plant but is derived from the colourless glucoside indican. Natural indigo has been almost completely replaced by synthetic indigo. 10

Preparation: Indigo can be prepared by various methods. Below are given two commercial syntheses. (1) From aniline: Aniline is heated with chloroacetic acid and the phenyl glycine formed is fused with sodamide and a mixture of NaOH and KOH at 200 ˚ C to get indoxyl which on exposure to air (O 2 ) gives indigo. 11 (3) Indigo (Indigotin) Indoxyl

12 Indigo

Indigo is a dark blue powder with a metallic lustre of copper. It is insoluble in water When its paste is agitated with alkaline sodium hydrosulphite in vats, it gets reduced to soluble colourless leuco compound. 13 (3) Indigo (Indigotin)

The cloth to be dyed is soaked in this solution and then exposed to air when the indigo is regenerated on the cloth by atmospheric oxidation. Thus Two geometrial forms, cis and trans, are possible for Indigo. But the trans form is more stable. 14 (3) Indigo (Indigotin) The o-quinonoid charged structure shown below is possibly responsible for the colour of Indigo. Trans form Cis form Intermolecular Hydrogen bond (trans form –stable) Chromophore- conjugated system

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