P-Toluidine Unveiled: From Precursor to Powerhouse – Exploring its Chemical Wizardry, Ubiquitous Applications, and Safety Triumphs

p-toluidine 106-49-0

p-Toluidine, also known as 4-methylaniline or 4-aminotoluene, is an organic compound belonging to the aromatic amine family. It is a derivative of toluene, with the para-position substituted with an amino group (-NH2). P-toluidine is a colourless to pale yellow liquid at room temperature with a characteristic amine odour.

A. Chemical structure and properties

a. Chemical Structure

p-Toluidine is an aromatic amine compound with the molecular formula C7H9N.Its systematic name is 4-methylaniline or 4-aminotoluene. The chemical structure of p-toluidine consists of a benzene ring with a methyl group (-CH3) and an amino group (-NH2), with the methyl group (-CH3) in the para position (4) and the amino group (-NH2) in the para position (4).

b. Physical Properties

Appearance: colourless to light yellow liquid at room temperature

Odour: Unique amine odour

Molecular weight: 107.16 g/mol

Density: 0.998 g/cm3 at 20°C

Melting point: -15°C (5°F)

Boiling Point: 200.1°C (392.2°F)

Vapour pressure: 0.76 mmHg at 25°C

Refractive index: 1.5725 at 20°C 1.5725 at 20°C

c. Chemical Properties

p-Toluidine is a basic compound because it contains an amino group (-NH2) that accepts protons to form ammonium ions. It is a polar and reactive compound and can participate in a variety of chemical reactions such as electrophilic aromatic substitution, nucleophilic addition and condensation.

p-Toluidine readily forms salts with inorganic and organic acids, such as hydrochloride, sulphate and acetate. It also undergoes oxidation to form azo compounds and other derivatives. P-toluidine has typical aromatic amine properties and can undergo diazotisation, coupling and substitution reactions.

d. Solubility

p-Toluidine is soluble in water, ethanol, ether and various organic solvents such as benzene, toluene and acetone.

It is moderately soluble in water, with a reported solubility of about 20 g/litre at 20 °C.

The solubility of p-toluidine in organic solvents is usually higher than in water due to its aromatic and lipophilic nature.

e. Spectral Properties

UV-Vis Absorption: p-Toluidine shows characteristic absorption bands in the UV and visible regions, with a maximum absorption wavelength of approximately 280-290 nm. Infrared (IR) Spectroscopy: The infrared spectrum of p-toluidine shows characteristic bands associated with amino group (N-H stretching and bending) and aromatic ring vibrations.

Nuclear Magnetic Resonance (NMR) Spectroscopy: The 1H NMR and 13C NMR spectra of p-toluidine provide detailed information about the chemical environment and the number of hydrogen and carbon atoms, respectively, which is useful for structural elucidation and identification.

B. Uses and Applications

a. Dye and pigment production

p-Toluidine is a key intermediate in the synthesis of a wide range of dyes and pigments, especially azo dyes. These dyes are widely used in the textile, leather, plastics and printing ink industries. Its chemical structure produces a wide range of colours, making it valuable in the production of bright and long-lasting colourants.

p-Toluidine is a crucial intermediate in the production of a variety of dyes and pigments. These can be used for:

Textiles: Azo dyes derived from p-toluidine are used to color fabrics because of their vibrant hues and good lightfastness [ChemicalBook].

Plastics: Pigments containing p-toluidine derivatives can color plastics for various applications.

Inks: Printing inks may utilize p-toluidine based dyes for their specific color properties.

b. Pharmaceutical

P-toluidine is used in the synthesis of pharmaceutical compounds and is a basic ingredient in various drugs and active pharmaceutical ingredients (APIs). It is used in drugs such as analgesics, antipyretics and antihistamines.

c. Agrochemicals

In agriculture, p-toluidine is used in the synthesis of herbicides, fungicides and insecticides. It helps develop effective crop protection products that help control pests and diseases and improve crop yield and quality.

d. Rubber Chemicals

P-toluidine is used in the production of rubber chemicals, including accelerators and antioxidants. These additives enhance the properties of rubber products such as elasticity, durability, ageing and degradation resistance.

e. Organic Synthesis

In organic chemistry laboratories, p-toluidine is a versatile building block for the synthesis of a wide variety of organic compounds. P-toluidine can participate in a variety of reactions including acylation, alkylation and condensation to produce complex molecules for research, industrial and pharmaceutical applications.

f. Biological Staining

P-toluidine is used as a biological stain in histology and cytology. It can be used to visualise specific cellular structures or components under a microscope, which aids in the study of tissues and cells in medical and research environments.

g. Agrochemicals

In agriculture, p-toluidine is used in the production of herbicides, fungicides and insecticides.

It contributes to the development of effective crop protection products that help control pests and diseases and improve crop yield and quality.

h. Colour photography chemicals

In colour photography, p-toluidine is used to produce chemicals required for colour film processing.

It helps in the formation of dyes and photographic sensitisers used in colour photographic emulsions.

i. Polymer Additives

P-toluidine can be added as an additive to polymer formulations to give them the desired properties.

It can be used as a stabiliser, antioxidant or UV absorber to improve the performance and durability of polymer materials.

j. Corrosion Inhibitors

In industrial applications, p-toluidine is used as a component of metal corrosion inhibitors.

It helps protect metal surfaces from corrosion and degradation in a variety of environments, extending the life of equipment and infrastructure.

C. Production method

a. Nitration and reduction

One of the main methods of producing p-toluidine consists of two steps: nitration of toluene followed by reduction of nitro.

In the nitration step, toluene (methylbenzene) reacts with a mixture of nitric acid (HNO3) and sulphuric acid (H2SO4) in the presence of a catalyst (usually a metal catalyst such as sulphuric acid) to produce p-nitrotoluene.

The p-nitrotoluene is then reduced to p-toluidine using a reducing agent (e.g., iron filings or metallic tin) and acid.

The reduction reaction is usually carried out under reflux conditions with heating and stirring to ensure complete conversion of p-nitrotoluene to p-toluidine.

Upon completion of the reaction, crude p-toluidine is separated from the reaction mixture, purified and further processed as required.

b. Hydrogenation Reaction

Another method of producing p-toluidine is the direct hydrogenation of p-nitrotoluene using hydrogen (H2) and a catalyst. The hydrogenation reaction is usually carried out in the presence of a metal catalyst, such as palladium (Pd) or platinum (Pt), supported on a suitable carrier material such as carbon or alumina. P-nitrotoluene is dissolved or suspended in a suitable solvent, and hydrogen is passed through the solution at high temperature and pressure in the presence of the catalyst. The hydrogenation reaction converts the nitro group (-NO2) of p-nitrotoluene to the amino group (-NH2) to produce p-toluidine.

Upon completion of the reaction, the p-toluidine product is separated from the reaction mixture, purified and further processed as required.

c. Other Methods

p-Toluidine can be prepared by reduction of other nitro compounds or by reaction of aniline with methylating agents.

D. Health and Safety Considerations

P-toluidine is listed as a potential human carcinogen by several regulatory agencies, including the International Agency for Research on Cancer (IARC) and the U.S. Environmental Protection Agency (EPA).

Exposure to p-toluidine, which may be by inhalation, ingestion, or dermal contact, has been associated with an increased risk of bladder cancer in humans.

Proper handling practices and safety precautions, such as the use of personal protective equipment (PPE) and adequate ventilation, must be taken when working with p-toluidine.

a. Toxicity

P-toluidine is toxic and can cause adverse health effects upon exposure. It may enter the body by inhalation, dermal contact or ingestion. Acute exposure to p-toluidine can irritate the skin, eyes and respiratory tract. Symptoms may include redness, itching, burning sensation, coughing and difficulty in breathing. Long-term or repeated exposure to p-toluidine can cause more serious health effects, including damage to the liver, kidneys and central nervous system. Long-term exposure also increases the risk of certain types of cancer, especially bladder cancer.

b. Safety Precautions

When p-toluidine is used, appropriate safety precautions should be taken to minimise exposure. This includes wearing personal protective equipment (PPE) such as gloves, safety goggles and protective clothing. The work area should be well ventilated to prevent vapour build-up. If ventilation is inadequate, respiratory protection may be required. Avoid skin contact with p-toluidine by wearing impervious gloves and impervious clothing. In case of skin contact, immediately remove contaminated clothing and wash affected area with soap and water. In case of eye contact, flush eyes with water for at least 15 minutes and seek immediate medical attention.

Do not eat, drink or smoke in areas where p-toluidine has been handled to prevent accidental ingestion.

c. Storage and Handling

Store p-toluidine in a cool, dry, well-ventilated area away from incompatible substances. Storage containers should be tightly closed and properly labelled to prevent accidental contact. P-toluidine should be kept away from sources of ignition, heat and direct sunlight. Use appropriate containment measures, such as secondary containment, to prevent spillage and leakage.

d. Disposal

Dispose of p-toluidine and its contaminated materials in accordance with local, state and federal regulations. Do not pour p-toluidine or its solution down the drain or dispose of it in the regular trash. Consult a regulatory agency or waste management professional for proper disposal methods, including incineration or treatment at an authorised facility.

E. Regulatory Status:

Because of the potential health hazards associated with p-toluidine, different regions and countries have different regulations and restrictions on its use and handling.

In the United States, p-toluidine is classified as a hazardous air pollutant under the Clean Air Act and its use is regulated by the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA).

In the EU, p-toluidine is classified as a carcinogen, mutagen and reproductive toxicant (CMR) and its use is restricted or banned in certain applications.

a. Classification and labelling

P-toluidine is classified and labelled according to various regulatory frameworks, including the Globally Harmonised System of Classification and Labelling of Chemicals (GHS), which provides standardised criteria for hazard classification and communication.

Depending on the jurisdiction, p-toluidine may be classified as a hazardous substance or chemical, and appropriate hazard symbols, signal words, and precautionary statements must appear on labels and safety data sheets (SDS).

b. Occupational Exposure Limits

Regulatory agencies such as the U.S. Occupational Safety and Health Administration (OSHA) and the European Agency for Safety and Health at Work (EU-OSHA) in the European Union have established Occupational Exposure Limits (OELs) for p-toluidine to protect workers from potential health risks.

OELs typically specify the maximum permissible concentration of p-toluidine in the air at a workplace over a specific time period, such as an 8-hour time-weighted average (TWA) or a short-term exposure limit (STEL).

c. Environmental regulations

Regulatory agencies may also limit the release of p-toluidine into the environment to minimise its impact on human health and ecosystems.

Emission limits for p-toluidine may be set for wastewater discharges, air emissions, and solid waste disposal, and industries may be required to implement pollution prevention measures and treatment technologies to comply with these regulations.

F. Upstream products:

Toluene (toluene): Toluene is the main raw material for the production of p-toluidine. It undergoes nitration and subsequent reduction to produce p-toluidine.

Nitric acid (HNO3): Nitric acid is used in the nitration step of the synthesis process to convert toluene to p-nitrotoluene.

Sulphuric acid (H2SO4): Sulphuric acid is commonly used as a catalyst in nitration reactions.

Hydrogen (H2): Hydrogen is used in the reduction step of the synthesis to convert p-nitrotoluene to p-toluidine.

Catalyst: Catalysts such as iron filings or metallic tin are used in the reduction reaction.

G. Downstream Products:

Dyestuffs and pigments: p-Toluidine is an important intermediate in the production of various dyestuffs and pigments (especially azo dyestuffs), which can be used in textiles, plastics, printing inks and other industries.

Pharmaceuticals: p-Toluidine is the basic ingredient in the synthesis of analgesics, antipyretics and antihistamines and other pharmaceutical compounds.

Agrochemicals: It is used in the production of herbicides, fungicides and insecticides to protect crops and control pests.

Rubber chemicals: p-toluidine is used in the production of rubber chemicals such as accelerators and antioxidants to improve the performance of rubber products.

Organic Synthesis: p-Toluidine is a versatile building block for the synthesis of various organic compounds in organic chemistry laboratories.

Biological Staining: p-Toluidine is used as a biological stain in histology and cytology for the observation of cell structures under the microscope.

Colour Photography Chemicals: In colour photography, p-toluidine is used to produce chemicals required for colour film processing.

Polymer additives: P-toluidine is used as an additive in polymer formulations to obtain desired properties such as stability and durability.

Corrosion Inhibitors: p-Toluidine can be used as a component in metal corrosion inhibitors to protect metal surfaces from corrosion and degradation.