Photoinduced degradation of chlorophenols in water represents both a tool for the destruction of resistant, widespread environmental pollutants and a concrete possibility to exploit the relevant reactive intermediates for synthetic purposes by reacting them with suitable nucleophiles. Thus the photochemical dehalogenation of chlorophenols in aqueous solution and in other media has been the subject of much attention as regards both the final products and the mechanism of the reaction. In air equilibrated aqueous solutions the prevailing photoproducts are hydroquinone, benzo-1,4-quinone and coupling derivatives; phenol is also formed in presence of H-donors. Inclusion of chlorophenols in cyclodextrins has been considered to be useful to extract these compounds from water and provide a modified environment affecting their photoreactivity. In this contribution we present a steady state and laser flash photolysis investigation of the photodechlorination of 4-chlorophenol in water in presence of cyclodextrins. In the CD cavity the only photoproduct is phenol. To rationalize such a selectivity effect, the reactivity of the photochemical intermediates with the ?-CD cavity was examined. In pure water the key species is the carbene 4-oxocyclohexa-2,5-dienyliden formed by heterolytic photoelimination of the halogen and fast deprotonation of the 4-OH-phenyl cation; this species has microsecond lifetime and high quantum yield (0.75)[1]. In presence of ?-CD the carbene is quenched with bimolecular rate constant kq = 5.8x 108 M-1 s-1 and in presence of D-glucose with kq = 8.5x 106 M-1 s-1 The high quenching constant observed with cyclodextrin is rationalized on the basis of a high affinity of this intermediate for the CD cavity. Photoexcitation of the preformed 4-chlorophenol-cyclodextrin complex shows that within the 20 ns duration of the laser pulse the phenoxyl radical is formed with a quantum yield of ca. 0.42 ± 0.06. Oxygen does not affect the course of the reaction in agreement with conversion of the phenoxyl radical to phenol within the cavity itself. Fast deprotonation of the 4-phenyl cation within the cavity and H-abstraction from the CD by the carbene intermediate may be hypothesized.
Photophysical and photochemical study of cyclodextrin inclusion complexes of 4-chlorophenol
IManet;S Monti;
2003
Abstract
Photoinduced degradation of chlorophenols in water represents both a tool for the destruction of resistant, widespread environmental pollutants and a concrete possibility to exploit the relevant reactive intermediates for synthetic purposes by reacting them with suitable nucleophiles. Thus the photochemical dehalogenation of chlorophenols in aqueous solution and in other media has been the subject of much attention as regards both the final products and the mechanism of the reaction. In air equilibrated aqueous solutions the prevailing photoproducts are hydroquinone, benzo-1,4-quinone and coupling derivatives; phenol is also formed in presence of H-donors. Inclusion of chlorophenols in cyclodextrins has been considered to be useful to extract these compounds from water and provide a modified environment affecting their photoreactivity. In this contribution we present a steady state and laser flash photolysis investigation of the photodechlorination of 4-chlorophenol in water in presence of cyclodextrins. In the CD cavity the only photoproduct is phenol. To rationalize such a selectivity effect, the reactivity of the photochemical intermediates with the ?-CD cavity was examined. In pure water the key species is the carbene 4-oxocyclohexa-2,5-dienyliden formed by heterolytic photoelimination of the halogen and fast deprotonation of the 4-OH-phenyl cation; this species has microsecond lifetime and high quantum yield (0.75)[1]. In presence of ?-CD the carbene is quenched with bimolecular rate constant kq = 5.8x 108 M-1 s-1 and in presence of D-glucose with kq = 8.5x 106 M-1 s-1 The high quenching constant observed with cyclodextrin is rationalized on the basis of a high affinity of this intermediate for the CD cavity. Photoexcitation of the preformed 4-chlorophenol-cyclodextrin complex shows that within the 20 ns duration of the laser pulse the phenoxyl radical is formed with a quantum yield of ca. 0.42 ± 0.06. Oxygen does not affect the course of the reaction in agreement with conversion of the phenoxyl radical to phenol within the cavity itself. Fast deprotonation of the 4-phenyl cation within the cavity and H-abstraction from the CD by the carbene intermediate may be hypothesized.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
