Dyes are colours which is a physical property of an object. In the absence of light we can not perceive the colour.Our brain recognises the quality of light when it falls on retina(the sensitive part of eye which recognises the light).There are three natural colours which are Red, Blue and White. When red light falls on retina, the red receptors are stimulated. Similarly when blue light falls on retina, the blue receptors are stimulated. When white light falls on retina,all the three receptors are stimulated.
The quality of dyes is that they impart colour to the substrate by adsorption or through making bond with. The dyes have particular affinity to the substrate for which they are used. Commercial uses of dyes include textiles, paper , leather, wood, inks, food and metals etc.
History of Dyes
Picric acid
Dyes were practised in Egypt, Persia, China and India thousands of years ago. The first dye was prepared by Peter Woulfe was picric acid from reaction of nitric acid and indigo which dye silk in bright yellow shade. With the passage of time the research in dyes synthesis went on and by 1914 Germans and Swiss had become the leaders in the field of dyestuff but there was a gap in the evolution of dyestuff chemistry during the period of first world war. In this period the supply of Germans to the world cut off and this led the United States to establish its industry.
Classifcation of Dyes
The dyes are classified on the basis of chemical constitution and its application.
The classification on the basis of chemical constitution is as follows. The following groups are chromophoric part of their compounds.
Here "R" means alkyl group, "H" means hydrogen, "N" means nitrogen, "S" means sulphur, "O" means oxygen. The value of alkyl group varies from compound to compound.
1: Nitroso compounds ( N=O-R )
2: Nitro compound ( NO2 -R )
3: Azo compound (R-N=N-R)
4: Azoic compound (H / R-N=N-R)
5: Stilbene (R-CH=CH-R)
6: Carotenoids (C40H56 )
7: Xanthene ( C6H4O.CH2C6H6 )
8: Quinoline ( C9H7N )
9: Aridine ( C5NH5 )
10: Oxazine ( C4NOH8 )
11: Thiazine ( C4NSH8 )
12: Thiazole ( C3NSH3 )
13: Indaphenol ( C12NO2H10 )
14: Organic colouring matter ( include various aromatic conjugated systems such as)
Benzoquinone
Naphthoquinone
Toluquinone
15: Azine ( C5H9N )
Classification on the basis of application
1: Acid dyes
2: Basic dyes
3: Azoic dyes
4: Direct dyes
5: Disperse dyes
6: Fiber reactive dyes
7: Fluorescent agent
8: Food, drug and cosmetidc colours
9: Sulphur dyes
10: Mordant dyes
11: Solvent dyes
12: Vat dyes
Acid dyes
Acid dyes are acidic in nature, solube in bases and they impart colour to the wool, silk and nylon. Example are azo dyes, triaryl methane,anthraquinone etc.
Basic dyes
Basic dyes are basic in nature, soluble in acids and can impart colour to wool and cotton. Examples are amino compounds, substituted amino compounds, triaryl methane, xanthanes etc.
Azoic dyes
These are diazotized material which impart colour to cotton at low temperature therefore they are called ice colours. All azo compounds are azoic in nature.
Direct dyes
These are dyes which are applied directly to the substrate from aqueous solution. Mortise is a typical direct dye. Generally azo dyes are direct dyes. They are applied on Rayon, leather and paper.
Disperse dyes
The disperse dyes are insolube in water and form a colloidal solution. The fabric is immersed in the solution as a result crystals are adsorbed on the stuff. They are used on nylon, orlon, polysester etc.
Fiber reactive dyes
These dyes contain reactive groups such as dichlorotriazine, trichloroazine, vinyl sulphone which react with -OH and -NH group of fibers and impart colour.
Fluorescent Agent
These are colourless compounds which improve the whiteness. These include stilbene, azole, coumarine, naphthole amide etc.
Food, Drug and Cosmetic Colours
These colouring agents contain azo, anthraquinone, carotenoid and triaryl methane compounds.
Mordant Compound
These compounds produce affinity between dye and fiber. These are the salts of Copper, Aluminum, Cobalt, Chromium etc.
Solvent dye
These are organic soluble compounds. These are used to colour oil, waxes, varnishes, shoe polishes, lipsticks etc.
Sulphur dye
These contain sulphur and are used to dye nylon.
Vat dye
Thiazole based dyes
These are dyes containing thiazole as a chromophoric part. They are disperse dyes which are not soluble in water and form a colloidal solution. These dyes are coupled with suitable substituent. The amino group of thiazole is diazotized with which substituent is attached. Example is 4-(p-nitrophenyl)-2-(azo-2-naphthyl)thiazole.The characteristics of thiazoles is that it is present in some memeber of several classes of dyes such as direct dyes, sulphurized dyes, cyanide dyes and these dyes are limited in numbers. Since thiazole is very favourable to cellulosic fibers that is why it is used extensively for cotton fabrics coloring. The importance of the given dye is established because it contain nitro and azo group as well.
If we see the history of dyes then it will come to know that Primuline was one of the first dyes which has the great affinity for cotton having a series of different shades and its constitution was improved with respect to fastness by the process of diazotization.
Primuline dye
There is another thiazole based dye called thiaflavin-T which is used to visualize the plaques which are found in the brain of Alzheimer disease patient.
Thioflavin-T
Chloramine yellow is one of the best direct yellow colour known due to its good fastness to light and fastness to bleach is a very important dye as well.
Chloramine
Importance Of Diaztization
The process of formation of diazonium salts on the reaction of aromatic amines with nitrous acid is known as Diazotization. This method was discovered by Peter Greiss in 1860. The process of diazotization is very important in this sense because it is the starting material of various dyes and drugs. Diazotization process enhance the colouring property of thiazole dyes. Secondly the process of diazotization always takes place in a control temperature. The name diazotization is derived from di, meaning two, azo meaning nitrogen and onium signifying those compounds that resemble ammonium salts. The name diazonium is reserved for those which contain linkage (-N=N-) and such compound are called azo or Diazo compounds.
Ar-NH2 + NaNO2 + HCl ----------> Ar-N2Cl + NaCl + H2O
In this process the mineral acid used always in access in order to stabilze the diazonium salt. These mineral acids are hydrochloric acids, sulphuric acids. Nitrous acid always produce in this reaction.
NaNO2 + HCl ----------> NaCl + HNO2
The excess of nitrous acid at the end of diazotization process is normally immaterial in coupling reaction.
Different methods for the preparation of diazonium salt
1: Aryl hydrazine reaction with chlorine or bromine ( Ar-NH-NH2 + Cl2/Br2)
2: Nitroso benzene reaction with hydroxyl amine (C6H5-NO + NH2-OH)
Method of diazotizaton
This method is employed for those compounds which are sparingly soluble in water. In this method sodium nitrite is added as a aqueous solution in amino compounds and the system is well stirred in a ice cooled bath as a result, an azotized compound is obtained.
Coupling Reaction
The next step after the formation of azonium salt is its coupling with a suitable reagent. Normally the reagents used for this purpose are phenols, keto compounds on aliphatic system having active methylene group or keto compounds on ring system either on a homocyclic or heterocyclic system.
The condition for coupling reaction is that it does not take place in a strongly acidic solution. Therefore mild acidic or non acidic condition is favourable for the coupling.
In strongly alkaline medium, the diazonium ion is fairly converted to covalent diazohydroxide, whereas in strongly acidic medium, amine is converted to unreactive conjugate acid.
Synthesis of thiazole
Thiazole is prepared by making reaction of thiourea with acetophenone which is mixed with I2.The mixture is heated on a water bath with occasional shaking. The purpose of I2 is to convert methyl group of acetophenone into active methylene group. In this reaction, the Sulphur of thiourea will invovle for nucleophilic attack on active methylene of acetophenone which will triger cyclization into 5 member heterocyclic ring structure. The solid mixture obtained from this reaction is treated with either to remove unreacted acetophenone which is later washed with aqueous sodium thiosulpate to remove the iodine. The product obtained is dissolved in hot water to remove sulphone which is poured off from the system and the product 2-amino-4-phenyl thiazole is precipated by adding ammonia. Then product is filtered off from it.
Phenyl thiazole synthesis
The quality of dyes is that they impart colour to the substrate by adsorption or through making bond with. The dyes have particular affinity to the substrate for which they are used. Commercial uses of dyes include textiles, paper , leather, wood, inks, food and metals etc.
History of Dyes
Dyes were practised in Egypt, Persia, China and India thousands of years ago. The first dye was prepared by Peter Woulfe was picric acid from reaction of nitric acid and indigo which dye silk in bright yellow shade. With the passage of time the research in dyes synthesis went on and by 1914 Germans and Swiss had become the leaders in the field of dyestuff but there was a gap in the evolution of dyestuff chemistry during the period of first world war. In this period the supply of Germans to the world cut off and this led the United States to establish its industry.
Classifcation of Dyes
The dyes are classified on the basis of chemical constitution and its application.
The classification on the basis of chemical constitution is as follows. The following groups are chromophoric part of their compounds.
Here "R" means alkyl group, "H" means hydrogen, "N" means nitrogen, "S" means sulphur, "O" means oxygen. The value of alkyl group varies from compound to compound.
1: Nitroso compounds ( N=O-R )
2: Nitro compound ( NO2 -R )
3: Azo compound (R-N=N-R)
4: Azoic compound (H / R-N=N-R)
5: Stilbene (R-CH=CH-R)
6: Carotenoids (C40H56 )
9: Aridine ( C5NH5 )
10: Oxazine ( C4NOH8 )
12: Thiazole ( C3NSH3 )
14: Organic colouring matter ( include various aromatic conjugated systems such as)
15: Azine ( C5H9N )
1: Acid dyes
2: Basic dyes
3: Azoic dyes
4: Direct dyes
5: Disperse dyes
6: Fiber reactive dyes
7: Fluorescent agent
8: Food, drug and cosmetidc colours
9: Sulphur dyes
10: Mordant dyes
11: Solvent dyes
12: Vat dyes
Acid dyes
Acid dyes are acidic in nature, solube in bases and they impart colour to the wool, silk and nylon. Example are azo dyes, triaryl methane,anthraquinone etc.
Basic dyes
Basic dyes are basic in nature, soluble in acids and can impart colour to wool and cotton. Examples are amino compounds, substituted amino compounds, triaryl methane, xanthanes etc.
Azoic dyes
These are diazotized material which impart colour to cotton at low temperature therefore they are called ice colours. All azo compounds are azoic in nature.
Direct dyes
These are dyes which are applied directly to the substrate from aqueous solution. Mortise is a typical direct dye. Generally azo dyes are direct dyes. They are applied on Rayon, leather and paper.
Disperse dyes
The disperse dyes are insolube in water and form a colloidal solution. The fabric is immersed in the solution as a result crystals are adsorbed on the stuff. They are used on nylon, orlon, polysester etc.
Fiber reactive dyes
These dyes contain reactive groups such as dichlorotriazine, trichloroazine, vinyl sulphone which react with -OH and -NH group of fibers and impart colour.
Fluorescent Agent
These are colourless compounds which improve the whiteness. These include stilbene, azole, coumarine, naphthole amide etc.
Food, Drug and Cosmetic Colours
These colouring agents contain azo, anthraquinone, carotenoid and triaryl methane compounds.
Mordant Compound
These compounds produce affinity between dye and fiber. These are the salts of Copper, Aluminum, Cobalt, Chromium etc.
Solvent dye
These are organic soluble compounds. These are used to colour oil, waxes, varnishes, shoe polishes, lipsticks etc.
Sulphur dye
These contain sulphur and are used to dye nylon.
Vat dye
These are insolube in water and have great affinity for cotton and other cellulosic fibers. Indigo is a typical vat dye.
Thiazole based dyes
These are dyes containing thiazole as a chromophoric part. They are disperse dyes which are not soluble in water and form a colloidal solution. These dyes are coupled with suitable substituent. The amino group of thiazole is diazotized with which substituent is attached. Example is 4-(p-nitrophenyl)-2-(azo-2-naphthyl)thiazole.The characteristics of thiazoles is that it is present in some memeber of several classes of dyes such as direct dyes, sulphurized dyes, cyanide dyes and these dyes are limited in numbers. Since thiazole is very favourable to cellulosic fibers that is why it is used extensively for cotton fabrics coloring. The importance of the given dye is established because it contain nitro and azo group as well.
4-(p-nitrophenyl)-2-(azo-2-naphthyl)thiazole
There is another thiazole based dye called thiaflavin-T which is used to visualize the plaques which are found in the brain of Alzheimer disease patient.
Chloramine yellow is one of the best direct yellow colour known due to its good fastness to light and fastness to bleach is a very important dye as well.
Importance Of Diaztization
The process of formation of diazonium salts on the reaction of aromatic amines with nitrous acid is known as Diazotization. This method was discovered by Peter Greiss in 1860. The process of diazotization is very important in this sense because it is the starting material of various dyes and drugs. Diazotization process enhance the colouring property of thiazole dyes. Secondly the process of diazotization always takes place in a control temperature. The name diazotization is derived from di, meaning two, azo meaning nitrogen and onium signifying those compounds that resemble ammonium salts. The name diazonium is reserved for those which contain linkage (-N=N-) and such compound are called azo or Diazo compounds.
Ar-NH2 + NaNO2 + HCl ----------> Ar-N2Cl + NaCl + H2O
In this process the mineral acid used always in access in order to stabilze the diazonium salt. These mineral acids are hydrochloric acids, sulphuric acids. Nitrous acid always produce in this reaction.
NaNO2 + HCl ----------> NaCl + HNO2
The excess of nitrous acid at the end of diazotization process is normally immaterial in coupling reaction.
Different methods for the preparation of diazonium salt
1: Aryl hydrazine reaction with chlorine or bromine ( Ar-NH-NH2 + Cl2/Br2)
2: Nitroso benzene reaction with hydroxyl amine (C6H5-NO + NH2-OH)
Method of diazotizaton
This method is employed for those compounds which are sparingly soluble in water. In this method sodium nitrite is added as a aqueous solution in amino compounds and the system is well stirred in a ice cooled bath as a result, an azotized compound is obtained.
Coupling Reaction
The next step after the formation of azonium salt is its coupling with a suitable reagent. Normally the reagents used for this purpose are phenols, keto compounds on aliphatic system having active methylene group or keto compounds on ring system either on a homocyclic or heterocyclic system.
The condition for coupling reaction is that it does not take place in a strongly acidic solution. Therefore mild acidic or non acidic condition is favourable for the coupling.
In strongly alkaline medium, the diazonium ion is fairly converted to covalent diazohydroxide, whereas in strongly acidic medium, amine is converted to unreactive conjugate acid.
Synthesis of thiazole
Thiazole is prepared by making reaction of thiourea with acetophenone which is mixed with I2.The mixture is heated on a water bath with occasional shaking. The purpose of I2 is to convert methyl group of acetophenone into active methylene group. In this reaction, the Sulphur of thiourea will invovle for nucleophilic attack on active methylene of acetophenone which will triger cyclization into 5 member heterocyclic ring structure. The solid mixture obtained from this reaction is treated with either to remove unreacted acetophenone which is later washed with aqueous sodium thiosulpate to remove the iodine. The product obtained is dissolved in hot water to remove sulphone which is poured off from the system and the product 2-amino-4-phenyl thiazole is precipated by adding ammonia. Then product is filtered off from it.
Phenyl thiazole synthesis
Sulphone groups
The process of nitration is also very important because it is also a colouring matter. The nitration of 2-amino-4-(p-nitrophenyl) thiazole result in the nitration of phenyl group and thiazole group. So there is probability of obtaining two kinds of products. One is mononitrated product and second is dinitrated product.
The process of nitration is also very important because it is also a colouring matter. The nitration of 2-amino-4-(p-nitrophenyl) thiazole result in the nitration of phenyl group and thiazole group. So there is probability of obtaining two kinds of products. One is mononitrated product and second is dinitrated product.
Nitration of Thiazole
Diazotization Process
Both mononitrated and dinitrated products can be diazotized. The process of diazotization always takes place in the presence of mineral acid and nitrite salt. It is necessary to maintain the reaction condition i.e. to keep the reaction mixture stirring at cooled temperature.
Coupling Reaction
Coupling of azonium salt keep the compound stabilized then it is necessary to dissolve the suitable reagent such as 2-naphthol in a slightly acidic solution such as acetic acid approximately between 4-5 PH. Then cold solution of azonium salt is added to coupling reagent soluton drop wise until temperature rise to room temperature. Finally the compound obtained is filtered off and washed with warm and cold water until it is acid free. The crystals are dried and their structure is observed through IR.
Coupling Reaction
Coupling Reaction
Dyeing Process
1% dyeing solution : (1g in 100ml solution)
A fixed amount of the dye is taken and mixed with a dispersing agent like Timol which enhance activity of solid particles in the colloidal solution. Both agents are added and grinded to fine powder. Then paste is made by adding water and made up the volume up to 100ml. Then fabric is immersed and keep dyeing at 130C for 1 hour.
2% dyeing solution : (2g in 100ml solution)
The same procedure is adopted for dyeing of 2% solution.
Fastness properties:
These properties are fastness to light, fastness to sublimation and fastness to perspiration which are checked. Fastness preperties are checked to determine the effect of light on the dyed fabric, the effect of heat on the dyed fabric and the effect of perspiration on dyed fabric. The fastness test are performed according to ISO certified standards.
Thiazole dyes contain a wide range of shades ranging from reddish orange to bluish shade. The effect of shade is due to position of substituent on the coupler ring.
