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CHEMISTRY of SYNTHETIC DYES: a REVIEW Dr Journal of Interdisciplinary Cycle Research ISSN NO: 0022-1945 CHEMISTRY OF SYNTHETIC DYES: A REVIEW Dr. Bhavna Chaudhary Associate professor, Department of Chemistry, Smt. Pana Devi Morijawala Govt. Girls College, Kotputli, Rajasthan ABSTRACT: Color chemistry deals with compounds like dyes, pigments, paints, stains, etc. Dyes are the colorants that are employed in various industries including paper, textile, and pharmaceutical. This study focuses on the basics of dyestuff chemistry. It discusses the classification of the dyes concisely based on chemical composition. It also covers various principles and other details which are important to understand the chemistry of dyes. It briefly explains the impact of dyes on the environment. KEYWORDS: Dyestuff, Color Chemistry, Chromophore, Textile, Carcinogenic, Chromogen, Auxochrome INTRODUCTION: Colour is a crucial addition to the world we live in. They are everywhere and are functional as well as aesthetic. Every material is colored and molecules constituting that are quite selective about what energy of the photon they will absorb and what they will not. An object that selectively absorbs red light will not look red. That is the colors that are not absorbed are what is visible.1 The color observed is said to be the complement of the color absorbed. For example, if the color absorbed is blue then the color observed would be orange. Color Chemistry deals with the chemistry associated with colored chemicals or compounds like pigments, dyes, indicators, paints, etc. and it also involves a detailed study of these compounds which includes their physicochemical properties, their synthesis, as well as their applications. All these compounds are of great commercial significance. On the same grounds, dyes are compounds with considerable color imparting capacity and are widely employed in use in textile, food, cosmetic, handmade paper, plastic, pharmaceutical, and photographic industries. These coloring agents can adhere to the suitable surfaces by covalent bond formation or forming complexes with metals or salts, by solution, by mechanical retention or physical adsorption. They are ionizing aromatic and generally heterocyclic compounds that exhibit affinity towards the substrate and are applied in a water-based solution.2 Dyes are generally known composed of a group of atoms called chromophores which are responsible for intensifying the dye color and auxochrome which influence its color.3 According to literature there are over 10,000 different dyes and pigments available which are used in industries and around 7 × 105 tons of synthetic dyes that are produced globally throughout the year. 4,5 These coloring substances are compounds used for coloring paper, plastic, fabric, food items, cosmetics, etc and are of two major types. 6 Dyes have excellent colorant properties, easily processed, and have excellent color strength but apart from all these merits, they have poor solvent and heat stability along with poor durability and high migration. The color imparted to the solution depends on the electronic properties of Volume XII, Issue XI, November/2020 Page No:390 Journal of Interdisciplinary Cycle Research ISSN NO: 0022-1945 the chromophore of the dye molecule. They are used as coloring agents for the substrate which possess affinity towards them.6 Electronic excitation is due to the absorption of radiation in the UV and visible region and leads to the movement of an electron to a higher energy level. This absorption of electromagnetic radiation is caused by a covalently unsaturated group called Chromophore. It may or may not impart color to the substrate depending upon whether it is absorbing light in the visible region or not.7 Another group attached to this molecule is the Auxochrome which is covalently saturated and leads to change in both wavelength and the intensity of the absorption maximum. They are also called color enhancing groups as they tend to increase the values of λmax and εmax by extending the conjugation by resonance.6 Group of molecules acting as auxochrome: OH, Halogens, SH, NH2, etc. while following acts as chromophores in these dyes: • Azo group- Methyl Orange • Nitro group- Martius yellow • Anthraquinone- Turkey red • Indigo- Tyrian purple • Triarylmethane- Ethyl Violet One of the historical theories involving development in dyestuff chemistry was given by Witt which was applied to azo and anthraquinonoid dyes. This theory provided a base to understand the link between color and the molecule structure. The theory states that a dye molecule consists of three constituents- one or more fused benzene connected to a chromophore and a basic auxochrome group.8 In the year 1927, Witt’s work was later extended by Dilthey and Wizinger. They understood that an auxochrome is generally an electron-donating group while a chromophore molecule is a withdrawing group that is linked together in a conjugated system. This particular approach was considered as the initial point of the donor-acceptor chromogen concept.9 Therefore it can be simply concluded that an organic dyestuff molecule contains: a chromogen, chromophore, and an auxochrome. Although the chromophoric group itself is not capable of determining a particular hue and color but is responsible for the chromogen to be colored.10 Gurr classified auxochromes into two types namely colligators and nonobligatory. The former is responsible for ionic or non-ionic and are involved in dye-substrate interaction while the latter is responsible for modifying the color.9 Classification of Dyes: Classes of dyes according to their chemical structure as mentioned in Color Index14,15: • Nitroso dyes: They are formed by reacting phenols with nitrous acid which leads to attaching of NO (nitroso) group at ortho or para position to the phenol (OH) group. The metal-free precursor tautomerizes between nitroso-hydroxy and the quinoneoxime. An important example of this group is sulfonated 1- nitroso-2-naphthol complexed with Iron e.g. CI Acid Green 1 also known as Acid Green 1 is used in paper dyeing. • Nitro Dyes: This category contains ≥ 1 nitro (NO2) group. Both the bonds in the NO2 group are equivalent due to resonance and are conjugated with bonds of the benzene ring. During the early times these dyes we used to dye animal fibers and were the derivatives of phenols or naphthols like Picric acid and CI Acid Yellow 1. Volume XII, Issue XI, November/2020 Page No:391 Journal of Interdisciplinary Cycle Research ISSN NO: 0022-1945 Nitrodipheylamines are also nitro dyes which are ideal for penetrating dense fibers such as polyester. • Azo dyes: these dyes account for over 50% of all dyes used commercially. They contain at least one azo group. An azo group is formed by the reaction of nitrous acid (from NaNO2 and HCl) on aromatic primary amines at about 0°C forming diazonium salt. This group is attached to two radicals which are aromatic and generally exist in the trans form. Diazonium salts are highly unstable and they are used as soon as they are produced. Now while performing azo coupling, the nitrogen atom present at the end of a diazonium ion displaces hydrogen from the aromatic ring of phenol or amine. This coupling reaction generally occurs at either ortho or para position but para is preferred over the former if not hindered by any other substituent. Cationic azo dyes: They have protonated amine side chains in at low pH solutions, or positively charged quaternary nitrogen attached at neutral or alkaline conditions. This charged group to act as a part of the chromogen or attached by a nonconjugated chain of carbon atoms and therefore called delocalized and pendant cationic dyes respectively. Anionic azo dyes: They possess carboxylic or sulphonic acid groups attached to the aromatic rings. Sulfonic acids are strong and so are ionized at any pH therefore the colored ions are negatively charged. Under acidic conditions, these are attracted to protonated amino groups. A carboxylic acid is a weak acid that does not ionize at the pH of the dye bath. Direct azo dyes: These dyes have large molecules containing two or more azo linkages that can assume a coplanar conformation. This type of dyes gets bound to cellulose by non-ionic, noncovalent bonds. Reactive azo dyes: They possess pendant side chains that covalently get bound with the substrate. Mordant azo dyes: This group of dyes possesses a hydroxyl group that is adjacent to a ring carbon that is attached to either azo nitrogen or a carboxyl group. These arrangements can combine with a metal, such as chromium (III), to form a stable chelate ring having 5 or 6 members. The dye-metal complex can bind to the substrate firmly. Solvent azo dyes: An ionized amino or sulfonic acid group is lacking in these dyes which are known to provide solubility in water. Commercially used as coloring agents for hydrophobic materials including waxes, plastics, and inks for ballpoint pens. Azoic dyes: They are formed in/on the substrate by the reaction of separately applied diazonium salts and azoic coupling components. They are colored and insoluble molecules. There are some diazonium salts which can be manufactured as a dry powder, these stable compounds are referred to as azoic diazo compounds. Many of these compounds have two diazonium ions per molecule and are therefore also referred to as tetrazonium salts. Representative azo dyes include Direct Blue 1, Evans Blue, Metanil Yellow, Tartrazine, Trypan Blue, Sudan Red, Aniline Yellow, etc. • Aryl methane dyes: These are so-called because they are derived from methane, but in which some of the hydrogen atoms are replaced with aryl rings. The structure drawn below is the general formula for this large group of dyes: where R and A are either benzene or naphthalene rings while R1 is an amino group in case of diarylmethanes or another ring in case of triarylmethanes. X here is either O or N. Volume XII, Issue XI, November/2020 Page No:392 Journal of Interdisciplinary Cycle Research ISSN NO: 0022-1945 This class of dyes is bright and strong but is not fast.
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