Photochemistry of 6-chloro and 6-bromopicolinic acids in water. Heterolytic vs. homolytic photodehalogenation.

The photochemistry of 6-chloro and 6-bromopicolinate ions ( and , respectively) was investigated by product studies and ns laser flash photolysis (LFP). In deoxygenated pH 5.4 water, yields 6-hydroxypicolinic acid (70%) and a substituted pyrrole. In 2-propanol-water (1 : 1) mixture, the reaction yields, very unselectively, 6-hydroxypicolinic acid, 2-carboxypyridine, pyridine and bipyridines. Photolysis of aqueous leads to 6-hydroxypicolinic acid (78%) and hydroxybipyridines. Oxygen suppresses the photolysis of but does not affect that of . By LFP, we detected a short-lived transient at the pulse end from (lambda(max)= 305 nm, k=(2.8 +/- 0.2)x 10(5) s(-1), epsilonphi= 2200 +/- 200 dm3 mol(-1) cm(-1)). This is quenched either by oxygen or methyl acrylate and thus assigned to the triplet excited state. The triplet excited state of is detected at pH 1 only (lambda(max)= 320 nm, k > 3 x 10(7) s(-1)). The radical ion Cl2- could be successfully detected by photolysing in 2-propanol-water (1 : 1) in the presence of Cl-. Similarly, Br2- could be detected by irradiating aqueous in the presence of Br-. These results show that the photodehalogenation of is heterolytic in water and mainly homolytic in 2-propanol-water mixtures while that of is both heterolytic and homolytic in water. A mechanism in which the triplet excited state undergoes homolysis of the C-X bond and subsequent electron transfer from the carboxypyridyl radical to the halogen atom to form an ion pair may account for these observations.