Recalcitrant halocarbons

 

•   Common xenobiotic pollutants
•      These compounds contain different numbers of halogen (e.g., CI, Br, F (fluorine), I) atoms in the place of H atoms.
•     C-Halogen bond highly stable-cleavage requires considerable energy.
• Halocarbons are chemically and biologically very stable- persistent compounds, cause environmental pollution
• Two classes-

• Recalcitrant Halocarbons

  • Haloalkyls- Haloalkyl propellants & solvents
  • Haloaromatics- Halobenzenes, halophenols & halobenzoates

 

Haloalkyl propellants & solvents

• ·         C₁-C₂ alkanes in which all or almost all H atoms are replaced by halogens like chlorine, fluorine etc
• ·         They are used as solvents (chloroform, CHCI₃), as propellants in spray cans of cosmetics, paints etc., in condenser units of cooling systems (Freons, CCI₃F, CCl₂F₂, CClF₃, CF₄),
• ·         Chloroform, CCl₄, dichloromethane, dichloro, trichloro, tetra chloro ethenes are industrial and cleaning solvents-  Spilled halocarbon propellants and solvents contaminate ground water
• 
• ·         C₁-C₂ halo alkanes like chloroform, freons etc. are volatile and escape into the stratosphere where they destroy the protective ozone (O₃) layer leading to increased UV radiation-increased incidence of skin cancer and mutagenesis
• ·         Phasing out of halocarbons as aerosol propellants now in practice-research to find out suitable alternatives as refrigerants-Freons gradually being replaced by C₂ based halocarbons containing Cl₂ in place of Fluorine- photo chemically less destructive to the ozone layer, though biodegradability continues to be problem
• ·    Dehalogenation of organic compounds thermodynamically favored under anaerobic conditions by dehalogenases (reductive dehalogenation)
• ·       Tetrachloro/perchloro ethene undergoes stepwise dechlorination and is finally converted to vinyl chloride (toxic) and finally, ethane by methanogenic bacterial consortia
• ·    Aerobically, dichloro methane (DCM) can be utilised as carbon source by Pseudomonas. Dichloro ethene is acted upon by microbial consortium. Tricholoro ethene (TCE) is acted upon by methane monooxygenase to form TCE epoxide which further is converted to formate + CO, glyoxylate and dichloro acetate. TCE can also be converted to unstable formic acid and glyoxylic acid by toluene dioxygenase of Pseudomonas
• ·   Chloro and bromomethane are acted upon by Methylococcus capsulatus to yield formaldehyde, dichloro methane, trichloromethane

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