Hexachlorocyclohexane (HCH) is any of several polyhalogenated organic compounds consisting of a six-carbon ring with one chlorine and one hydrogen attached to each carbon. There are many isomers for this structure, differing by the stereochemistry of the individual chlorine substituents on the cyclohexane. 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 the cyclohexane conformation. Some isomers are pesticides.
Common forms are:
- α-HCH (CAS RN: 319-84-6 ), or α-BHC, alpha-hexachlorocyclohexane, the only isomer that is optically active
- β-HCH (CAS RN: 319-85-7 ), or β-BHC, beta-hexachlorocyclohexane
- γ-HCH (CAS RN: 58-89-9 ), or γ-BHC, gamma-hexachlorocyclohexane, or lindane
- δ-HCH (CAS RN: 319-86-8 ), or δ-BHC, delta-hexachlorocyclohexane
- t-HCH (CAS RN: 608-73-1 ), or t-BHC, technical hexachlorocyclohexane
While treatment of benzene under electrophilic aromatic substitution conditions (Cl2/FeCl3 or Cl2/AlCl3) produces chlorobenzene, the substitution product, under radical addition conditions (Cl2, hν or Cl2, Δ, high P) will give hexachlorocyclohexane isomers after three successive radical dichlorination steps:
- electrophilic chlorination: C6H6 + Cl2 → C6H5Cl + HCl
- radical addition: C6H6 + 3Cl2 → C6H6Cl6
Addition rather than substitution takes place, due to the very high C–H bond dissociation energy (112 kcal/mol) that disfavors abstraction of a hydrogen atom. Since mono chloro-de-hydrogenation deactivates the molecule against further electrophilic reactions, the reaction can be halted at one chlorine atom substitution. In contrast, addition of Cl2 destroys the aromaticity of the benzene ring, and the further addition of another two Cl2 molecules is rapid compared to the first. Hence, only thrice dichlorinated product can be isolated.
- Zdravkovski, Zoran (2004). "Theoretical Study of the Stability of Hexachloro- and Hexafluorocyclohexane Isomers" (PDF). Bulletin of the Chemists and Technologists of Macedonia. 23 (2): 131–137. Archived from the original (PDF) on 2005-12-28. Retrieved 2016-04-17.
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