- Aqueous chlorination kinetics and mechanism of substituted dihydroxybenzenes
-
The initial chlorination kinetics of several substituted dihydroxybenzenes, including chlorinated resorcinol compounds, was studied over the pH range of 2-12 at 22 °C. For each of the resorcinol substrates, the apparent chlorination rates are a minimum in the pH range of 3-6 and a maximum at pH values between 8-11. A mechanism that involves the reaction of HOCl with ArX(OH)2, ArX(OH)O-, and ArX(O-)2, and an acid-catalyzed pathway at pH 2, ArX(OH)O-, and ArX(O-)2, and an acid-catalyzed pathway at pH 4 was proposed to explain this pH dependence. Over natural water pH conditions, the reactions of HOCl with the anion and dianion forms of resorcinol groups are the most important. Although the intrinsic reactivity of HOCl with resorcinol substrates decreases with the extent of chlorine substitution on aromatic ring, the apparent reactivity of HOCl increases for more chlorinated resorcinols. In the presence of excess HOCl, monochloro- and dichloro resorcinol intermediates, therefore, should not accumulate when resorcinol groups undergo chlorine substitution. Linear free energy relationships for the reactivity of HOCl with resorcinols and phenols were developed. The sequential chlorination kinetics of resorcinol up to trichlororesorcinol can now be modeled.
- Rebenne, Laurence M.,Gonzalez, Alicia G.,Olson, Terese M.
-
-
Read Online
- Ammonium Salt-Catalyzed Highly Practical Ortho-Selective Monohalogenation and Phenylselenation of Phenols: Scope and Applications
-
An ortho-selective ammonium chloride salt-catalyzed direct C-H monohalogenation of phenols and 1,1′-bi-2-naphthol (BINOL) with 1,3-dichloro-5,5-dimethylhydantoin (DCDMH) as the chlorinating agent has been developed. The catalyst loading was low (down to 0.01 mol %) and the reaction conditions were very mild. A wide range of substrates including BINOLs were compatible with this catalytic protocol. Chlorinated BINOLs are useful synthons for the synthesis of a wide range of unsymmetrical 3-aryl BINOLs that are not easily accessible. In addition, the same catalytic system can facilitate the ortho-selective selenylation of phenols.
- Xiong, Xiaodong,Yeung, Ying-Yeung
-
p. 4033 - 4043
(2018/05/22)
-
- Photocatalytic degradation of lindane by polyoxometalates: Intermediates and mechanistic aspects
-
The photocatalytic degradation of lindane (γ-1,2,3,4,5,6-hexachlorocyclohexane) has been studied in the presence of the polyoxometalate PW12O403- in aqueous solutions. Lindane is fully decomposed to CO2, Cl- and H2O, while a great variety of intermediates has been detected using GC-MS, including aromatic compounds (dichlorophenol, trichlorophenols, tetrachlorophenol, hexachlorobenzene, di- and trichloro-benzenodiol), non-aromatic cyclic compounds (penta-, tetrachlorocyclohexene, heptachlorocyclohexane), aliphatic compounds (tetrachloroethane) and condensation products (polychlorinated biphenyls). The number and nature of the intermediates implies that the mechanism of decomposition of lindane is based on both oxidative and reductive processes. Common intermediates have been reported during photolysis of lindane in the presence of titanium dioxide. A similar overall mechanism of polyoxometalates and TiO2 photocatalysis through the formation of common reactive species is suggested.
- Antonaraki,Triantis,Papaconstantinou,Hiskia
-
experimental part
p. 119 - 124
(2010/08/22)
-
- Influence of activated carbons on the kinetics and mechanisms of aromatic molecules ozonation
-
Companies have been looking for new methods for treating toxic or refractory wastewaters; which can mainly be used prior to or after or in connexion with biological treatment processes. This paper compares conventional ozone oxidation with activated carbon (AC) promoted ozone oxidation, which helps developing a mechanism involving HO{radical dot} radical. For a compound which is quite easy to oxidise, like 2,4-dichlorophenol (2,4-DCP) conventional ozonation is efficient enough to remove the initial molecule. The mechanism involved mainly consists of an electrophilic attack on the aromatic ring, which is activated by the donor effect of the -OH group, then followed by a 1,3 dipolar cycloaddition (Criegee mechanism) that leads to aliphatic species, mainly carboxylic acids. Yet, the addition of AC, through the presence of HO{radical dot} radical, enhances the removal of these species which are more refractory. For a refractory compound like nitrobenzene (NB), with a de-activated aromatic ring because of the attractive effect of -NO2, conventional ozonation is inefficient. On the contrary, this molecule can be quite easily removed with AC promoted oxidation and it is found that the mechanism (electrophilic attack followed by a 1,3 dipolar cycloaddition) is quite similar to the one corresponding to conventional ozonation, but with less selectivity. For both molecules, a mass balance has established that the by-products accounting for more than 75% of the remaining COD can be quantified. A significant part is composed of carboxylic acids (acetic, oxalic, etc.), which could afterwards be easily removed in an industrial wastewater treatment process followed by a final biological treatment step.
- Merle,Pic,Manero,Mathé,Debellefontaine
-
experimental part
p. 166 - 172
(2010/08/20)
-
- Selective halogenation of aromatics by dimethyldioxirane and halogen ions
-
The oxidation of halogen anions by dimethyldioxirane (DMD) produced reactive species which led, in acidic media, to the halogenation of activated aromatic rings. The reaction can be efficiently controlled to obtain selective and mixed halogenated species.
- Bovicelli,Mincione,Antonioletti,Bernini,Colombari
-
p. 2955 - 2963
(2007/10/03)
-