Chlorobenzene

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Chlorobenzene
Image:Chlorobenzene-2D-skeletal.png	Image:Chlorobenzene-3D-vdW.png
IUPAC name	chlorobenzene
Other names	benzene chloride monochlorobenzene Phenyl chloride
Identifiers
CAS number	[108-90-7]
RTECS number	CZ0175000
SMILES	ClC1=CC=CC=C1
Properties
Molecular formula	C₆H₅Cl
Molar mass	112.56 g/mol
Appearance	colorless liquid
Density	1.11 g/cm³, liquid
Melting point	-45 °C (228 K)
Boiling point	131 °C (404 K)
Solubility in water	low
Solubility in other solvents	most organic solvents
Hazards
R-phrases	10 20 51/53
S-phrases	24/25 61
Flash point	29 °C
Related compounds
Related compounds	benzene 1,4-dichlorobenzene
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Chlorobenzene is an aromatic organic compound with the chemical formula C₆H₅Cl. It is a colorless, flammable liquid first made in 1851 by reacting phenol and phosphorus pentachloride.^{[citation needed]}

1 Uses
2 Synthesis
- 2.1 Mechanism
3 References

Uses

Chlorobenzene has been used in the manufacture of certain pesticides, most notably DDT by reaction with chloral (trichloroacetaldehyde). It once found use in the production of phenol. Today the major use of chlorobenzene is as an intermediate in the production of nitrochlorobenzenes and diphenyl oxide, which are important in the production of commodities such as herbicides, dyestuffs, and rubber. Chlorobenzene is also used as a high-boiling solvent in organic synthesis as well as many industrial applications.

Synthesis

Chlorobenzene is prepared by chlorination of benzene, usually in the presence of a catalytic amount of Lewis acid such as ferric chloride:

The catalyst is necessary because Cl₂ does not have enough electrophilicity to react with the stable aromatic benzene.

Because chlorine is electronegative, C₆H₅Cl exhibits decreased susceptibility to attack by other electrophiles. For this reason, when close attention is paid to stoichiometry, the chlorination process produces only small amounts of dichloro- and trichlorobenzenes.

Mechanism

There are two ideas of how this reaction occurs. One understanding is that a molecule of Cl₂ can donate a lone pair of electrons (Lewis base) to the valence of iron in ferric chloride (Lewis acid), forming the electrophilic complex shown:^[1]

Because chlorine is significantly electronegative, the chlorine with positive charge pulls electron density through the sigma bond to the other chlorine, thus making the outer "neutral" chlorine the electrophilic species. This chlorine is the chlorine attacked by a pair of electrons from the pi system of benzene. This in turn forms FeCl₄^-, which then abstracts a hydrogen from the resonance-stabilized sigma complex of benzene to restore aromaticity, regenerate the catalyst, and form HCl.

A second understanding is that Cl₂ can dissociate to form a complex anion with FeCl₃ in which a Cl⁺ species is attracted to the FeCl₄^- anion:^[2]

A pair of pi electrons will bond to the very reactive Cl⁺ species and, as above, FeCl₄^- will abstract the proton, regenerate the catalyst, and form HCl.