Haloaromatics- Halobenzenes, halophenols and halobenzoates

Recalcitrant Halocarbons

• Haloalkyls
• Haloaromatics

Chlorobenzenes are industrial solvents. Such halo aromatics have low biodegradability under aerobic conditions, with increasing number of halo substituents.

Under anaerobic conditions, such extensively halogenated aromatics are dehalogenated by dehalogenases. Sequential removal of halo substituents occurs, which in turn makes the product less susceptible to further dehalogenation. Thus, Hexachlorobenzene -HCB is most readily dehalogenated to 1,3,5-trichlorobenzene. Further dehalogenation to monochlorobenzene is possible but less readily catalyzed. 

Hexachlorobenzene à1,3,5 trichlorobenzene (dechlorination with ease)

1,3,5 trichlorobenzene à -à  1,4 dichloro benzene (further dechlorination slow and incomplete)

1,4 dichloro benzene -à monochlorobenzene (dechlorination slow and incomplete)

Monochlorobenzene resist dechlorination anaerobically and is stable under anaerobic conditions.

Aerobic degradation of tetra and trichlorobenzene is difficult. Mono and dichloro benzenes degraded with ease aerobically, by the dioxygenases of Pseudomonas and Alcaligenes species forming chlorocatechols.

Degradation of aromatic compounds requires the opening of the aromatic ring. Bacteria must possess enzymes that either

Ø  cleave the ring of the aromatic compound, in spite of the halogen substituents

Ø  catalyse the removal of the halogen substituents prior to the dearomatising reactions.

Chlorophenols

Highly chlorinated pentachlorophenol (PCP) is used as preservatives for wood and canvas. Other chlorophenols find uses in synthesis of pesticides, resins, dyes and pharmaceuticals. They resist degradation since they are highly toxic to microorganisms. In low concentrations, microorganisms degrade them under aerobic and anaerobic conditions.

Aerobically,

Highly halogenated aromatic compounds are particularly resistant to biodegradation by aerobic microorganisms because chlorine substituents interfere with the action of dioxygenase enzymes that oxidatively cleave aromatic rings. Therefore, biodegradation of these compounds requires initial removal of the halogen substituents from the aromatic rings.

The aerobic degradation of PCP proceeds by the action of monooxygenases, to produce tetrachloro-para- hydroquinone; further degradation continues by Pseudomonas, Sphingomonas sp. and Mycobacterium sp. removing chlorine substituents to yield trihydroxybenzene. Further ring opening yields simpler products such as CO₂ and water.

Anaerobically, Reductive dechlorination of PCP occurs in anaerobic sediments and in anaerobic sludges complete degradation of PCP has been observed. The degradation products 3,4,5-trichlorophenol, 2,5-dichlorophenol, and 3-chlorophenol are further subjected to ring opening producing CO₂ and water.

The sulfate-reducing bacterium Desulfomonile tiedjei reductively dehalogenates PCP and other chlorophenols.