J. A. Santaballa

Reaction pathways and mechanisms of photodegradation of pesticides

The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processes involved, the nature of reactive intermediates and final products. Potential use of photochemical processes in advanced oxidation methods for water treatment is also discussed. Processes considered include direct photolysis leading to homolysis or heterolysis of the pesticide, photosensitized photodegradation by singlet oxygen and a variety of metal complexes, photolysis in heterogeneous media and degradation by reaction with intermediates generated by photolytic or radiolytic means.

Oxidation of aliphatic amines by aqueous chlorine

Abstract The oxidation of aliphatic amines by aqueous chlorine has been studied. The kinetic behaviour is similar for primary, secondary and tertiary aliphatic amines, the elementary step being the transfer of chlorine from the hypochlorous acid molecule to the nitrogen of the free amino group. Chlorination of aliphatic primary and secondary amines involves some water molecules in the transition state. Inductive effects are also discussed.

Aqueous chemistry of N-halo-compounds

Halogens in aqueous solution are still used world-wide as disinfectants. During the process of halogenation, the substances present in water undergo several chemical processes, yielding relatively unstable intermediate species; their life-times in the medium depend on their structure and on the physico-chemical conditions. Several low molecular weight hydrocarbons are formed during water halogenation, some of them potent mutagens and/or carcinogens. Halogenation also takes place in vivo involving the system myeloperoxidase/H2O2/halide, which increases the relevance of such reactions and opens new research fields.

First Steps in the Oxidation of Sulfur-Containing Amino Acids by Hypohalogenation: Very Fast Generation of Intermediate Sulfenyl Halides and Halosulfonium Cations

Abstract Sulfur-containing amino acids show an extraordinary binding towards HOCl/ClO−. During the process, the Cl is transferred from the O to the S of the amino acid. Met reacts with HOCl one order of magnitude faster than the non-S containing amino acids (k (Met+HOCl) =8.7·10 8 mol −1 dm 3 s −1 ). Instead, Cys reacts as its thiolate (RS−), two orders-of-magnitude faster (k ( RS − + HOCl ) =1.2·10 9 mol −1 dm 3 s −1 ). Cys reacts also with ClO− (k ( RS − + ClO − ) =1.9·10 5 mol −1 dm 3 s −1 ). Such processes take place much more readily than the corresponding N-halogenation of the non-sulfur containing amino acids. To our knowledge, these are the first kinetic measurements of the rate of formation of sulfenyl halides and halosulfonium cations in aqueous solution. Sulfenyl chlorides and chlorosulfonium ions derived from amino acids are elusive, and sulfide-type amino acids (Met) eventually yield sulfoxides (MetO), while thiol-type amino acids (Cys) lead to disulfides (Cys^Cys) and sulfonic acids (Cya). The fate of sulfur-containing amino acids upon oxidation with HOCl/ClO− seems to be related to their mutagen-inactivation ability.

α-amino acids chlorination in aqueous media

The reaction of chlorination of α-amino acids for 6 < pH < 11 has been studied. The reaction is an aliphatic electrophilic substitution, the rate determining step being the transfer of the chlorine atom between the HOCl oxygen and the nitrogen of the α-amino acid free amino group.

On the kinetics and energetics of one-electron oxidation of 1,3,5-triazines

One-electron oxidation of 1,3,5-triazines is observed with both excited uranyl ion (*UO2(2+)) and sulfate radical anion (SO4.-) in aqueous solution, but not with Tl2+, indicating that the standard reduction potentials E degree of 1,3,5-triazine radical cations are = 2.3 +/- 0.1 V vs. NHE, consistent with theoretical calculations; this suggests that if triazines inhibit electron transfer during photosynthesis, they would need to act on the reductive part of the electron transport chain.

A B3LYP/6-31G** study on the chlorination of ammonia by hypochlorous acid

B3LYP/6-31G** calculations were performed on the chlorination of NH3 by HOCl, considering explicit participation of zero, one, two, three and four water molecules. Detailed analysis of the free energy profiles shows the mechanism is water-assisted. Cl transfer from the O of HOCl to the N of NH3 is accompanied by proton transfers along a chain of hydrogen-bonded water molecules. Preferential stabilisation of the transition structure arises from cooperative fluctuations involving the hydrogen-bonded chain of water molecules. Calculations show solvent reorganization associated with proton translocations is coupled with Cl transfer in the vicinity of the transition structure.

Acid-base equilibria and decomposition of secondary (N-Cl)-α-amino acids.

Abstract The decomposition of (N-Cl)-Sarcosine, (N-Cl),(N-Me)-Alanine, (N-Cl)-Proline, (N-CI),(N-Me)-Valine, and (N-Cl)-2-piperidine carboxylic acid (Pipecolic acid) was studied under acid conditions. The results suggest the participation of the four possible species of the (N-Cl)-α-amino acid. A reaction mechanism is proposed which allows us to estimate the rate constant for the decomposition of each species, as well as the yet unknown macroscopic, microscopic and tautomeric equilibrum constants. The relation between the different rate and equilibrium constants is also analyzed.

(Re)Greening photochemistry: using light for degrading persistent organic pollutants

Light is a cheap and abundant chemical reagent, capable of inducing highly selective reactions, some of which cannot be reasonably carried out by alternative ways nowadays. Photochemical processes may take place directly from the excited states or short-lived intermediates generated after light absorption or, alternatively, through reaction with species indirectly generated by the action of light on other chemicals present in the medium (catalysts or secondary reagents). As a consequence a range of possibilities are open to transform and/or degrade refractory pollutants or undesired chemicals or microbia in air, water, soil, on surfaces, etc., as well as other applications described below.

Evidence for an intramolecular elimination mechanism in the aqueous decomposition of (N-Cl)-alcoholamines

Abstract (N-Cl)-alcoholamines decompose in aqueous medium through two main reaction paths: an intramolecular elimination and a fragmentation. Both mechanisms take place following a pre-equilibrium deprotonation of the β-hydroxyl group of the (N-Cl)-compound. Evidences for both pathways are given and a very high effective molarity (EM=0.2·10 6 M) is reported for the intramolecular process.