Isotope Fractionation Associated with the Photochemical Dechlorination of Chloroanilines

Abstract

Isotope fractionation associated with the photochemical transformation of organic contaminants is not well understood and can arise not only from bond cleavage reactions but also from photophysical processes. In this work, we investigated the photolytic dechlorination of 2-Cl- and 3-Cl-aniline to aminophenols to obtain insights into the impact of the substituent position on the apparent (13)C and (15)N kinetic isotope effects (AKIEs). Laboratory experiments were performed in aerated aqueous solutions at an irradiation wavelength of 254 nm over the pH range 2.0 to 7.0 in the absence and presence of Cs(+) used as an excited singlet state quencher. Photolysis of 2-Cl-anilinium cations exhibits normal C and inverse N isotope fractionation, while neutral 2-Cl-aniline species shows inverse C and normal N isotope fractionation. In contrast, the photolysis of 3-Cl-aniline was almost insensitive to C isotope composition and the moderate N isotope fractionation points to rate-limiting photophysical processes. (13)C- and (15)N-AKIE-values of 2-Cl-aniline decreased in the presence of Cs(+), whereas those for 3-Cl-aniline were not systematically affected by Cs(+). Our current and previous work illustrates that photolytic dechlorinations of 2-Cl-, 3-Cl-, and 4-Cl-aniline isomers are each accompanied by distinctly different and highly variable C and N isotope fractionation due to spin selective isotope effects.

Pesticide degradation and export losses at the catchment scale: Insights from compound-specific isotope analysis (CSIA)

“…Based on a multiple lines of evidence approach derived from the combined CSIA, mass balance (MB) and predictive calculations in the following section, attribution of degradation extent to photochemical degradation is likely to be small ( < 5%), if not negligible. Indeed, carbon isotope effects associated to photochemical degradation in aqueous solutions of aniline substructures have been reported to be negligible under indirect photolysis (Ratti et al, 2015a) and nearly insensitive to inverse at environmental pH (Ratti et al, 2015b). If inverse fractionation where to be of relevance for this case, CSIA biodegradation estimations would therefore be regarded as conservative.…”

Photodegradation of pesticides using compound-specific isotope analysis (CSIA): a review

“…The results indicated that the fractionation of C and N isotopes is affected by the excited state of 4-Cl-aniline dechlorination, which may determine the reactivity factor. The 3 C for 2-Cl-aniline photolysis (1.3 AE 0.1&) notably differs from 4-Cl-anline, 31 which may be interpreted as evidence for increased bonding to C in the ratelimiting step of the reaction. In conclusion, by comparing the AKIE isotopic values at different substitution sites, we can use the AKIE values to identify the isomers in which photodegradation occurs, and we can speculate on the mechanism of degradation for different species.…”

“…Chloroanilines can directly synthesize the insecticide diubenzuron and can synthesize parachlorophenylurea or p-chlorobenzene isocyanate, which are two types of diubenzuron insecticide intermediates. Ratti et al 31 observed chloroaniline with different substituents for the variation of C and N isotope enrichment factors, 3 C and 3 N , between À1.2 AE 0.2& to À2.7 AE 0.2& for C, and À0.6 AE 0.2& to À9.1 AE 1.6& for N. The direct photodegradation of chloroaniline was very sensitive to changes in solution pH and ionic composition. The results also showed that the initial concentration of 4-Cl-aniline did not signicantly affect the carbon isotope fractionation process during photolysis, but the carbon isotope enrichment factor changed from À1.2 AE 0.2& to +1.0 AE 0.3& with increased initial concentration.…”

Tracking photodegradation products and bond-cleavage reaction pathways of triclosan using ultra-high resolution mass spectrometry and stable carbon isotope analysis

“…Carbon-based CSIA has been applied to characterize biotic and abiotic reactions for a wide spectrum of organic contaminants, including chlorinated solvents (Blessing et al, 2009;Buchner et al, 2017;Sherwood Lollar et al, 2001), BTEX compounds (Jin and Rolle, 2014;Vogt et al, 2008;Vogt et al, 2016) and micropollutants (Elsner and Imfeld, 2016;Jin and Rolle, 2016). There is also an increasing number of recent CSIA studies focusing on the photolytic transformation of organic contaminants such as organophosphorus compounds (Wu et al, 2018), chlorobenzenes (Passeport et al, 2018), methyl-tert-butyl ether (MTBE, (Zhang et al, 2015), 4-chloroaniline (Ratti et al, 2015) and atrazine (Hartenbach et al, 2008). However, to the best of our knowledge, CSIA has not yet been used to elucidate the reaction mechanisms of TCS photodegradation.…”