Hexachlorocyclohexane

Hexachlorocyclohexane(HCH),C
₆H
₆Cl
₆,is any of severalpolyhalogenatedorganic compoundsconsisting of asix-carbon ringwith onechlorineand onehydrogenattached to each carbon. This structure has ninestereoisomers(eightdiastereomers,one of which has twoenantiomers), which differ by thestereochemistryof the individual chlorine substituents on thecyclohexane.It is sometimes erroneously called "benzene hexachloride"(BHC). They have been used as models for analyzing the effects of different geometric positions of the large atoms with dipolar bonds on the stability of thecyclohexane conformation.The isomers are poisonous, pesticidal, andpersistent organic pollutants,to varying degrees.

Hexachlorocyclohexane was dimerized to producemirex,a banned pesticide.

Common forms are:

• alpha-hexachlorocyclohexane,α-HCH, or α-BHC (CAS RN:319-84-6), theoptically activeisomer
• beta-hexachlorocyclohexane,β-HCH, or β-BHC (CAS RN:319-85-7)
• gamma-hexachlorocyclohexane, γ-HCH, γ-BHC, orlindane(CAS RN:58-89-9), the most insecticidal isomer
• delta-hexachlorocyclohexane, δ-HCH, or δ-BHC (CAS RN:319-86-8)
• technical hexachlorocyclohexane, t-HCH, or t-BHC (CAS RN:608-73-1), a mixture of isomers

• 
  α-Hexachlorocyclohexane,thedextrorotatoryenantiomer
• 
  α-Hexachlorocyclohexane, thelevorotatoryenantiomer
• 
  β-Hexachlorocyclohexane
• 
  γ-Hexachlorocyclohexane,lindane
• 
  δ-Hexachlorocyclohexane
• 
  ε-Hexachlorocyclohexane
• 
  ζ-Hexachlorocyclohexane
• 
  η-Hexachlorocyclohexane
• 
  θ-Hexachlorocyclohexane

Chlorination ofbenzeneunderelectrophilic aromatic substitutionconditions (Cl₂/FeCl₃or Cl₂/AlCl₃) produceschlorobenzene.Since mono chloro-de-hydrogenation deactivates the molecule against further electrophilic reactions, the reaction can be halted at one chlorine atom substitution.

Electrophilic chlorination:C₆H₆+ Cl₂→ C₆H₅Cl + HCl

In contrast, chlorination of benzene underradical additionconditions (Cl₂,hν (photochlorination) or Cl₂,Δ, highP) yields hexachlorocyclohexane isomers after three successive radical dichlorination steps.Additionrather thansubstitutiontakes place, due to the very high C–H bond dissociation energy (112 kcal/mol) that disfavors abstraction of a hydrogen atom. Addition of Cl₂destroys the aromaticity of the benzene ring, and the addition of two more Cl₂molecules is rapid compared to the first. Hence, only thrice-dichlorinated product can be isolated from this reaction.

Radical addition:C₆H₆+ 3Cl₂→ C₆H₆Cl₆

Hexachlorocyclohexane isomers with more than one chlorine atom per carbon are:

• 1,1,2,3,4,5-hexachlorocyclohexane
• 1,1,2,3,4,6-hexachlorocyclohexane
• 1,1,2,3,5,6-hexachlorocyclohexane
• 1,1,2,2,3,4-hexachlorocyclohexane
• 1,1,2,2,3,5-hexachlorocyclohexane
• 1,1,2,2,3,6-hexachlorocyclohexane
• 1,1,2,2,4,5-hexachlorocyclohexane
• 1,1,2,3,3,4-hexachlorocyclohexane
• 1,1,2,3,3,5-hexachlorocyclohexane
• 1,1,2,3,4,4-hexachlorocyclohexane
• 1,1,2,4,4,5-hexachlorocyclohexane
• 1,1,2,4,4,6-hexachlorocyclohexane
• 1,1,2,4,5,5-hexachlorocyclohexane
• 1,1,2,5,6,6-hexachlorocyclohexane
• 1,1,2,2,3,3-hexachlorocyclohexane
• 1,1,2,2,4,4-hexachlorocyclohexane
• 1,1,3,3,5,5-hexachlorocyclohexane

External links

• Hexachlorocyclohexane (HCH),CDC