Abstract
The photochemistry of 6-chloro and 6-bromopicolinate ions (I and II, respectively) was investigated by product studies and ns laser flash photolysis (LFP). In deoxygenated pH 5.4 water, I 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 II leads to 6-hydroxypicolinic acid (78%) and hydroxybipyridines. Oxygen suppresses the photolysis of I but does not affect that of II. By LFP, we detected a short-lived transient at the pulse end from I (λmax = 305 nm, k = (2.8 ± 0.2) × 105 s−1, εϕ = 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 II is detected at pH 1 only (λmax = 320 nm, k > 3 × 107 s−1). The radical ion Cl2−· could be successfully detected by photolysing I in 2-propanol-water (1: 1) in the presence of Cl−. Similarly, Br2−? could be detected by irradiating aqueous II in the presence of Br−. These results show that the photodehalogenation of I is heterolytic in water and mainly homolytic in 2-propanol-water mixtures while that of II 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.
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Rollet, F., Richard, C. & Pilichowski, JF. Photochemistry of 6-chloro and 6-bromopicolinic acids in water. Heterolytic vs. homolytic photodehalogenation. Photochem Photobiol Sci 5, 88–94 (2006). https://doi.org/10.1039/b510001a
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DOI: https://doi.org/10.1039/b510001a