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Aniline Dye

We know that all organic compounds can be classified as aliphatic and aromatic compounds. Aliphatic compounds are open chain or branched compounds with single or multiple covalent bonds between carbon atoms. Alkanes, alkenes and alkynes are most common aliphatic compounds. Aromatic compounds are well known for their aroma and extra stability due to delocalization of electrons. Benzene is the simplest aromatic compound. Aromatic compounds play important role in organic chemistry and act as precursor for several other organic compounds. 

For example aromatic compounds involve in preparation of many drugs and dyes. Dyes are colored, ionizing and aromatic organic compounds. They can be defined as the substance which bond with given substrate in an aqueous solution. Some of the dyes require a mordant to enhance the rate of binding reaction of dye. On the basis of origin, dyes can be classified as natural and synthetic dyes. Natural dyes are found naturally in roots, wood, lichens, leaves, berries and bark. On the contrary, synthetic dyes are prepared synthetically in laboratory. Today we have a large number of synthetic dyes which are used for different purposes. 

 

Aniline Dye Definition

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Aniline dye can be defined as the coloring substance that is mainly prepares from aniline. It is mainly used to color fabric, leather, and wood.  Aniline dyes are synthetic organic compounds which are available in a variety of colors. They are well known for their clarity. First aniline dye was discovered by William Henry Perk during the preparation of a synthetic version of the anti-malaria drug quinine and named it as Mauvine. Later this dye was used in textile industries. Aniline dyes are mainly used in the leather and woodworking industries. Aniline wood dyes provide clarity and saturation over wood surface as these dyes do not leave pigmentation on the surface due to more absorption into porous. The aniline dyes which are used to dye leather are considered to be of a higher quality dyes as these dyes provide a unique texture to leather surface.

Preparation of Aniline Dye

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Today we have a large number of aniline dyes which are commercially available in the form of powders. These dyes can be dissolved in different solvents like water, alcohol or oil. Aniline Yellow is one of the most common aniline dyes. It is a yellow azo dye which is mainly prepared by aromatic amine (aniline). It is an orange color powdered derivate of azobenzene. 

Let’s discuss the preparation method for aniline dye. Aniline reacts with cold solution of benzenediazonium chloride and form a yellow solid product. 

Preparation of Aniline Dye

These yellow solid compounds are also called as azo dyes and this reaction is called as azo coupling reactions. The azo compound that is formed from aniline is called as aniline yellow.

Uses of Aniline Dye

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Aniline yellow, Aniline black, Chrysoidine-G are some common examples of aniline dyes. Some common uses of aniline yellow are listed below.
  • In microscopy for vital staining
  • In pyrotechnics
  • For yellow pigments and inks
  • In inkjet printers
  • In insecticides, lacquers, varnishes, waxes, oil stains, and styrene resins
  • As an intermediate and precursor for other dyes like acid, solid yellow etc.
Another common aniline dye is Chrysoidine-G. Chemical name of Chrysoidine-G is 4-phenyldiazenylbenzene-1, 3-diamine;hydrochloride. Chrysoidine-G is a red-brown powder with large black shiny crystals. The boiling point of Chrysoidine-G is 2262°C and melting point is around 118-118.5 °C. It is widely used in different industries. Some common uses of aniline blue are listed below;
  • Used in fluorescence microscopy
  • Used to stain collagen in plant tissues
  • Used in the staining of Mallory's connective tissue

Chemical Formula of Aniline Dye

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Aniline dyes are azo compounds with amino group on aromatic ring. For example the structure of Chrysoidine-G is given below. 

Chrysoidine-G

Azo compounds can be easily recognized with highly delocalized electron system with benzene rings and the N=N (azo group) in the molecule. The delocalization of electrons makes the molecule stable. In the presence of white light, it absorbs some wavelengths and causes color due to non-absorbed wavelengths. The group which is responsible for color in the compound is called as a chromophore. A little modification in the molecule can change the wavelength of light absorbed as well as color of compound. The structure of aniline yellow and aniline black are given below. Aniline yellow is formed by reaction of aniline with diazonium compound at low temperature. 

Aniline yellow

Azo Compound
 
The structure difference in aniline yellow and aniline black affects the color of these aniline dyes. Another example of aniline dye is Mauveine A. 

Mauveine A
 
Another example of aniline dye is Aniline Blue which is also known as China blue, or Soluble blue. It is a mixture of methyl blue and water blue. The structural formula for Aniline blue is given below. 

Aniline Blue

Preparation of Azo Aye From Aniline

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We know that aniline is an aromatic amine in which one –NH2 groups is directly bonded to benzene ring. Azo dyes are also aromatic compounds with N=N bond in the molecule which is called as azo linkage. Azo dyes are also used as pH indicators. Some common examples of azo dyes are methyl yellow, methyl orange, methyl red, congo red and alizarine yellow. The preparation method azo dye consists of two steps. 
  • Preparation of an aromatic diazonium ion from an aniline or aniline derivative.
  • Coupling of the diazonium salt with an aromatic compound to get desired azo dye.
For example methyl yellow is prepared by the reaction of aniline with N, N-dimethylaniline.

Aniline reacts with NaNO2 and HCl at 273 K temperature to form diazonium compounds. This step is called as diazotization.

Diazotization
 
In the next step, diazonium compound reacts with N, N-dimethylaniline to form methyl yellow. 

N,N-Dimethylaniline
 
Another example is formation of azo dye from 2-naphthol and 4-aminobenzenesulfonic acid. It forms Acid orange 7 dye which is also called as 2-naphthol orange. 
2-Naphthol Orange

Whereas the bonding of 1-naphthol with 4-aminobenzenesulfonic acid results the formation of another azo dye. 

Bonding of 1-Naphthol

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