Preparation of Aniline yellow

Objective 

Preparation of aniline yellow 

 

Theory 

Introduction

Yellow aniline, or para-phenylenediamine, is an organic compound used primarily as a precursor to various dyes. Its synthesis involves several key steps, including aniline nitration, para-nitroaniline reduction, and subsequent isolation of para-phenylenediamine. This synthesis is essential for understanding aromatic compounds' various reactions and transformations. 

Aniline Yellow is an aromatic amine and a yellow azo dye. It is an azobenzene derivative. It appears to be an orange powder. The first azo dye was aniline yellow. C. Mene created it for the first time in 1861. Bismarck Brown was the second azo dye introduced in 1863. Aniline Yellow became the first commercial azo dye in 1864, a year after Aniline Black. It is derived from aniline. 

Aniline yellow is a volatile chemical that serves as a catalyst in the creation of other colours. It is formed through the reaction of benzene diazonium chloride with phenylamine, which is also known as aniline. In the first stage, aniline acts as a nucleophile, interacting with benzene diazonium chloride to form a C-N bond. The azo dye is formed when the product loses its H+ and forms an aromatic ring. The reaction of benzene diazonium chloride with aniline produces aniline yellow (para-amino azobenzene) in an acidic media. Because it is an acid-sensitive dye, it has little industrial significance. 

Step 1: Ar-NH2 + NaNO2 + HCl → Ar-N2 + -Cl-  

Step 2: Ar-N2 + - Cl- + Ar-NH2 → Ar-N=N-NH-Ar 

Step 3: Ar-N=N-NH-Ar → Ar-N=N-Ar-NH2 (Aniline yellow) 

 

Nitration of Aniline

The first step involves the nitration of aniline to produce para-nitroaniline. Aniline is treated with concentrated sulfuric acid and concentrated nitric acid. This reaction introduces a nitro group (-NO2) at the para position of the aniline ring. The reaction is exothermic and requires careful control of temperature to prevent the formation of undesirable by-products. 

Isolation of para-Nitroaniline

After the nitration reaction, para-nitroaniline is isolated from the reaction mixture. This can be achieved through filtration, crystallization, or solvent extraction techniques. The isolated para-nitroaniline is typically in the form of yellow crystals. 

Reduction of para-Nitroaniline

The next step involves the reduction of para-nitroaniline to para-phenylenediamine. This reduction can be achieved through various methods, such as using a reducing agent like iron filings or tin and hydrochloric acid. The reduction process involves the removal of the nitro group, resulting in the formation of the desired para-phenylenediamine. 

Purification of para-Phenylenediamine

The final step is the purification of para-phenylenediamine. This can be done using techniques such as recrystallization or sublimation, depending on the purity requirements. The pure para-phenylenediamine appears as yellow crystals, which can be further characterized using techniques like melting point determination and spectroscopic analysis. 

Uses of yellow aniline 

Yellow aniline, or para-phenylenediamine, has various applications across different industries due to its chemical properties. Some of the common uses of yellow aniline include: 

Dye Synthesis: Yellow aniline is a crucial intermediate in producing various dyes, including azo and sulphur dyes. These dyes find extensive use in the textile industry for colouring fabrics, Fibers, and other materials. 

Hair Dye: Para-phenylenediamine is also a key component in hair dye formulations. It is a primary intermediate in producing permanent hair dyes, incredibly dark shades. Its oxidation with hydrogen peroxide leads to the formation of coloured products that bond strongly to the hair shaft. 

Rubber and Plastic Industries: Yellow aniline produces certain rubber chemicals and antioxidants. It helps enhance the properties of rubber and plastic products, providing increased durability and resistance to degradation. 

Photographic Chemicals: It is used in manufacturing photographic chemicals and developers, particularly in black-and-white photography, due to its role in producing light-sensitive compounds. 

Biomedical Applications: Yellow aniline finds application in the biomedical field, where it is used in the synthesis of certain pharmaceuticals and as an intermediate in the production of various chemicals used in biochemical research and analysis. 

Chemical Intermediates: It is an essential building block for synthesizing various organic compounds, including agrochemicals, pharmaceuticals, and other specialty chemicals. 

Laboratory Reagent: Yellow aniline can be used as a reagent in various laboratory reactions, such as in synthesizing other organic compounds or as a starting material for preparing specific derivatives. 

 

Precautions

  • Handle all chemicals with caution, following the necessary safety protocols. 
  • Wear appropriate personal protective equipment, including lab coats, gloves, and safety goggles. 
  • Perform the reactions in a well-ventilated laboratory to avoid the accumulation of harmful gases. 
  • Keep the diazotization reaction temperature below 5°C to prevent undesirable side reactions. 

 

Learning Outcomes 

Students understand 

  • The concepts of yellow aniline. 
  • They will be able to understand the diazotization mechanism. 
  • The use of yellow aniline. 
  • The essential safety precautions before using hazardous chemicals.