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

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 (λ_max = 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 (λ_max = 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.