Benzene
Authors: Fruscella William 
Published: August 2004

Benzene, C6H6, is a volatile, colorless, and flammable liquid aromatic hydrocarbon possessing a distinct, characteristic odor. Benzene is used as a chemical intermediate for the production of many important industrial compounds, such as styrene (polystyrene and synthetic rubber), phenol (phenolic resins), cyclohexane (nylon), aniline (dyes), alkylbenzenes (detergents), and chlorobenzenes. These intermediates, in turn, supply numerous sectors of the chemical industry producing pharmaceuticals, specialty chemicals, plastics, resins, dyes, and pesticides. Benzene has also found limited application in medicine for the treatment of certain blood disorders, such as polycythemia and malignant lymphoma, and in veterinary medicine as a disinfectant. Benzene, along with other light high octane aromatic hydrocarbons such as toluene and xylene, is used as a component of motor gasoline. Although this use has been substantially reduced in the United States, benzene is still used extensively in many countries for the production of commercial gasoline. Benzene is no longer used in appreciable quantity as a solvent because of the health hazards associated with it. Since the 1950s, benzene production from petroleum feedstocks has been very successful and accounts for about 95% of all benzene obtained. Benzene is the simplest and most important member of the aromatic hydrocarbons. Benzene forms minimum‐boiling azeotropes with many alcohols and hydrocarbons. Benzene also forms ternary azeotropes. The representation of the benzene molecule has evolved from the Kekule ring formula to the more electronically accurate form, which indicates all carbon–carbon bonds are identical. Benzene has great thermal stability. Benzene undergoes substitution, addition, and cleavage of the ring; substitution reactions are the most important for industrial applications. Petroleum‐derived benzene is commercially produced by reforming and separation, thermal or catalytic dealkylation of toluene, and disproportionation. Benzene is also obtained from pyrolysis gasoline formed in the steam cracking of olefins. The United States is the largest producer of benzene and accounts for about 30% of world production. Several different grades of benzene are commercially available. The most common grades are benzene 535, benzene 485 (nitration grade), benzene 545, and thiophene‐free benzene. Benzene is classified as a flammable liquid and should be stored away from any potential source of ignition. Benzene is shipped by rail tank cars, trucks, barges, and tankers. Because of the flammability, toxicity, and volatility of benzene, transfers from one vessel to another are conducted in closed systems. Benzene should be handled only where adequate ventilation is provided; protective clothing and self‐contained respirators are recommended. Benzene is classified as a hazardous waste by the EPA under subtitle C of the Resource and Recovery Act (RCRA). Effective Sept. 25, 1990, solid wastes containing more than 0.5 mg/mL benzene must be treated in accordance with applicable RCRA regulations. Benzene emissions and effluent streams from petroleum refineries or benzene processing plants are also subject to strict federal regulations. A complete description of the new EPA regulations concerning benzene and other hazardous wastes is found in theFederal Register. At room temperature and atmospheric pressure benzene is sufficiently vaporized to pose an inhalation hazard. Benzene is a toxic substance which can produce both acute and chronic adverse health effects. It is generally recognized that prolonged or repeated exposure to benzene can result in serious damage to the blood‐forming elements. The lethal oral dose for an adult is approximately 15 mL. In chronic benzene intoxication, mild poisoning produces headache, dizziness, nausea, stomach pain, anorexia, and hypothermia. In severe cases, pale skin, weakness, blurred vision, and dyspnea occur on exertion. Bone marrow depression produces a decrease in circulating peripheral erythrocytes and leucocytes. Fatalities from chronic exposure show at autopsy severe bone marrow aplasia, and necrosis or fatty degeneration of the heart, liver, and adrenals. Acute benzene poisoning results in CNS depression and is characterized by an initial euphoria followed by staggered gait, stupor, coma, and convulsions. Treatment for acute exposure to benzene vapor involves removing the subject from the affected area, followed by artificial respiration with oxygen; intubation and cardiac monitors may be necessary for severe acute exposures. Emesis is indicated in alert patients if more than 1 mL of benzene per kg of body weight has been ingested and less than two hours have passed between ingestion and treatment. Because of the potential hazards associated with benzene, exposure to benzene in the workplace has been heavily regulated in the United States. Benzene is considered one of the approximately 40 known human carcinogens.