# Chlorobenzene

Chlorobenzene (abbreviated PhCl) is an aryl chloride and the simplest of the chlorobenzenes, consisting of a benzene ring substituted with one chlorine atom. Its chemical formula is C$_6$H$_5$Cl. This colorless, flammable liquid is a common solvent and a widely used intermediate in the manufacture of other chemicals.

## Uses

The major use of chlorobenzene is as a precursor for further intermediates such as nitrophenols, nitroanisole, chloroaniline, and phenylenediamines, which are used in the production of herbicides, dyestuffs, chemicals for rubber, and pharmaceuticals.
It is also used as a high-boiling solvent in industrial and laboratory applications, for materials such as oils, waxes, resins, and rubber.
Chlorobenzene is nitrated on a large scale to give a mixture of 2-nitrochlorobenzene and 4-nitrochlorobenzene, which are separated and used as intermediates in production of other chemicals. These mononitrochlorobenzenes are converted to related 2-nitrophenol, 2-nitroanisole, bis(2-nitrophenyl)disulfide, and 2-nitroaniline by nucleophilic displacement of the chloride, with respectively sodium hydroxide, sodium methoxide, sodium disulfide, and ammonia. The conversions of the 4-nitro derivative are similar.

### Niche and former uses

Chlorobenzene once was used in the manufacture of pesticides, most notably DDT, by reaction with chloral (trichloroacetaldehyde), but this application has declined with the diminished use of DDT. At one time, chlorobenzene was the main precursor for the manufacture of phenol:
    C$_6$H$_5$Cl + NaOH → C$_6$H$_5$OH + NaCl
The reaction is known as the Dow process, with the reaction carried out at 350 °C using fused sodium hydroxide without solvent. Labeling experiments show that the reaction proceeds via elimination/addition, through benzyne as the intermediate.

## Production

It was first described in 1851. Chlorobenzene is manufactured by chlorination of benzene in the presence of a catalytic amount of Lewis acid such as ferric chloride, sulfur dichloride, and aluminium chloride:
Industrially the reaction is conducted as a continuous process to minimize the formation of dichlorobenzenes. Because chlorine is electronegative, C$_6$H$_5$Cl exhibits somewhat decreased susceptibility toward further chlorination.

### Laboratory routes

Chlorobenzene could be produced from aniline via benzenediazonium chloride, otherwise known as the Sandmeyer reaction.

## Safety

Chlorobenzene exhibits "low to moderate" toxicity as indicated by its LD$_{50}$ of 2.9 g/kg. The Occupational Safety and Health Administration has set a permissible exposure limit at 75 ppm (350 mg/m$^3$) over an eight-hour time-weighted average for workers handling chlorobenzene.

## Toxicology and biodegradation

Chlorobenzene can persist in soil for several months, in air for about 3.5 days, and in water for less than one day. Humans may be exposed to this agent via breathing contaminated air (primarily via occupational exposure), consuming contaminated food or water, or by coming into contact with contaminated soil (typically near hazardous waste sites). However, because it has only been found at 97 out of 1,177 NPL hazardous waste sites, it is not considered a widespread environmental contaminant. The bacterium Rhodococcus phenolicus degrades chlorobenzene, dichlorobenzene and phenol as sole carbon sources.
Upon entering the body, typically via contaminated air, chlorobenzene is excreted both via the lungs and the urinary system.

## On other planets

Chlorobenzene has been detected in a sedimentary rock on Mars. It was speculated that the chlorobenzene might have been produced when the sample was heated in the instrument sampling chamber. The heating would have triggered a reaction of organics in the Martian soil, which is known to contain perchlorate.