AbstractPhotochemical demethylation of monuron [3‐(4‐chlorophenyl)‐1, 1‐dimethylurea] and of demethylmonuron [ 1‐(4‐chlorophenyl)‐3‐methylurea]in aqueous solution was investigated. The photoproducts identified in this study were formaldehyde, formic acid and carbon dioxide. These products indicate that demethylation occurs either by oxidation of a methyl group to hydroxymethyl which is readily cleaved to yield formaldehyde, or by further oxidation to anN‐formyl group, which is slowly hydrolysed under weakly acidic conditions to yield formic acid. Samples equilibrated either with air or oxygen (>99 % purity) gave essentially the same yield of oxidation products. Experimental evidence indicated that extensive photodegradation of formaldehyde was occurring during photolysis, causing low recoveries of formaldehyde. Consequently, with [2H3‐methyl]demethylmonuron and deuterated water, only steady state concentrations of formaldehyde could be measured i
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