Degradation of Pesticides in the Environment
83
the nucleophil needed for reaction (Castro, 1964). The most likely source of the
nucleophilic agent in this case is the hydroxide ion (OH-), which can be replaced
by the cyanide ion (CN-) to yield the corresponding p-cyanophenol product.
In aqueous solution the most predominant photochemical reaction on organophosphates and carbamates is hydrolysis. Thus many of the substrates for
hydrolysis cited in Section 3.3.1 would also be expected to be photochemically
attacked.
3.5.2. Deha/ogenation
Dehalogenation is a rather commonly occurring photochemical reaction.
Because of the high energy involved, UV irradiation can directly dehalogenate
even stable halogens attached to aromatic rings-a capability that has not been
found in microorganisms. For example, it has been shown that photolysis of
PCB gives monochlorobiphenyl (Ruzo et ai., 1974). Other examples of aromatic
dechlorination are monuron (Crosby and Leitis, 1969) and pcp (pentachlorophenol). Dechlorination from alkyl carbons takes place with relative ease. The
l-exo-chlorine of heptachlor, for instance, is released by UV irradiation at 300
nm (McGuire et ai., 1970) (Fig. 3.14). Other examples are BHC isomers, mirex,
and chlordane isomers. It must be pointed out here that when there is an abundance of proton donors such as saturated alkanes and ftavoproteins, the end result
of dehalogenation resembles reductive reactions. Whether or not these should
be called reductive reactions is a matter of choice.
3.5.3. Oxidation
Photochemical oxidation usually takes place as a result of reactions between
photoactivated pesticidal molecules and oxygen. Thus the reactions are usually
carried out in atmospheric conditions rather than in aqueous solutions, although
dissolved oxygen in some cases can react with pesticides to give oxidation
products. Perhaps the most frequently encountered photooxidative reaction in
the pesticide field is the conversion of P=S to P=O in the case of organoo
0 " II
~ NH C N
+ HCI + CH20
-
CH3
Figure 3.14. Dechlorination reaction catalized by photochemical reactions.
83
the nucleophil needed for reaction (Castro, 1964). The most likely source of the
nucleophilic agent in this case is the hydroxide ion (OH-), which can be replaced
by the cyanide ion (CN-) to yield the corresponding p-cyanophenol product.
In aqueous solution the most predominant photochemical reaction on organophosphates and carbamates is hydrolysis. Thus many of the substrates for
hydrolysis cited in Section 3.3.1 would also be expected to be photochemically
attacked.
3.5.2. Deha/ogenation
Dehalogenation is a rather commonly occurring photochemical reaction.
Because of the high energy involved, UV irradiation can directly dehalogenate
even stable halogens attached to aromatic rings-a capability that has not been
found in microorganisms. For example, it has been shown that photolysis of
PCB gives monochlorobiphenyl (Ruzo et ai., 1974). Other examples of aromatic
dechlorination are monuron (Crosby and Leitis, 1969) and pcp (pentachlorophenol). Dechlorination from alkyl carbons takes place with relative ease. The
l-exo-chlorine of heptachlor, for instance, is released by UV irradiation at 300
nm (McGuire et ai., 1970) (Fig. 3.14). Other examples are BHC isomers, mirex,
and chlordane isomers. It must be pointed out here that when there is an abundance of proton donors such as saturated alkanes and ftavoproteins, the end result
of dehalogenation resembles reductive reactions. Whether or not these should
be called reductive reactions is a matter of choice.
3.5.3. Oxidation
Photochemical oxidation usually takes place as a result of reactions between
photoactivated pesticidal molecules and oxygen. Thus the reactions are usually
carried out in atmospheric conditions rather than in aqueous solutions, although
dissolved oxygen in some cases can react with pesticides to give oxidation
products. Perhaps the most frequently encountered photooxidative reaction in
the pesticide field is the conversion of P=S to P=O in the case of organoo
0 " II
~ NH C N
+ HCI + CH20
-
CH3
Figure 3.14. Dechlorination reaction catalized by photochemical reactions.