Positive chlorination

Kinetic studies using acidified hypochlorous acid are rather more complicated than these with hypobromous acid. Much higher concentrations of mineral acid are necessary so that the activities of the reacting entities do not correspond closely to their molecular concentrations, and the kinetic order of reaction varies according to the acid concentration and the reactivity of the aromatic. 

Derbyshire and Waters202 carried out the first kinetic study, and showed that the chlorination of sodium toluene-m-sulphonate by hypochlorous acid at 21.5 °C was catalysed more strongly by sulphuric acid than by perchloric acid and that the rate was increased by addition of chloride ion. A more extensive examination by de la Mare et al.203 of the rate of chlorination of the more reactive compounds, anisole, phenol, and / -dimethoxybenzene by hypochlorous acid catalysed by perchloric acid, and with added silver perchlorate to suppress the formation of Cl2 and C120 (which would occur in the presence of Cl and CIO-, respectively), 

The alternative interpretation of the second term in equation (94), i.e. that it represents a slow proton transfer to hypochlorous acid, has been ruled out on two grounds firstly, the rate of this proton transfer has been measured and found to be much higher than the first-order chlorination rate204, and, secondly, the third term should disappear when the reactivity of the aromatic is very high since then the reaction responsible for the second term would be dominant203. 

It follows from the above that, in the reactions of fairly unreactive aromatics, the formation of Cl+ (either from HOC1 or H2OCl+) will be relatively fast compared with the subsequent reaction of this ion with the aromatic so that the kinetics will be governed mainly by the third term in equation (94). Hence de la Mare et al.204 found the rate of chlorination of benzene and toluene by acidified hypochlorous acid to depend on the concentration and nature of the aromatic and to increase with hydrogen ion concentration though (as in the case of positive 

Kinetics studies of acid-catalysed chlorination by hypochlorous acid in aqueous acetic acid have been carried out, and the mechanism of the reactions depends upon the strength of the acetic acid an ( the reactivity of the aromatic. Different groups of workers have also obtained different kinetic results. Stanley and Shorter207 studied the chlorination of anisic acid by hypochlorous acid in 70 % aqueous acetic acid at 20 °C, and found the reaction rate to be apparently independent of the hydrogen ion concentration because added perchloric acid and sodium perchlorate of similar molar concentration (below 0.05 M, however) both produced similar and small rate increases. The kinetics were complicated, initial rates being proportional to aromatic concentration up to 0.01 M, but less so thereafter, and described by 

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Kinetic isotope effects also show a dependence upon the reactivity of the electrophile. Thus some reactions, e.g. positive chlorination, show no isotope effect whereas others, e.g. sulphonation, do show an isotope effect. There are two ways of visualising the reasons for this and they are closely related. Very... 

Several studies338,340-342 show that the chlorination does not proceed, as assumed previously343, by proton abstraction followed by reaction of the carbanion thus formed, with electrophilic chlorine. A mechanism involving a chlorooxosulfonium ion formed by attack of a positive chlorine species on sulfur was shown to be more likely344. 

Clearly, other molecular frameworks exist, besides dibenzo-p-dioxin, that can approximately meet the size and shape requirements, for example, 22-25. These also have four lateral positions chlorinated. It is interesting to look at the levels of biological activity of these analogues, in comparison to TCDD. 22, 2,3,7,8,-tetrachlorodibenzofuran, and 23, 2,3,6,7-tetrachlorobiphenylene, have activities that are, respectively slightly less than and very similar to that of TCDD (Poland, Greenlee, and Kende 1979 Poland and Knutson 1982). On the other hand, 24 and 25 are much less active than is TCDD. 

The naphthalene-like, aromatic stmcture of 1,2,3-benzothiadiazole imparts stability to the system that survives exposure to 20% potassium hydroxide at 150°C or 27% sulfuric acid at 200 °C. It is not oxidized by potassium permanganate, potassium ferricyanide, chromic acid, or dilute nitric acid <1996CHEC-II(4)289>. Electrophilic substitution occurs in the benzo ring, predominantly at the 4-position. Chlorine in the 6-position is displaced by a variety of nucleophiles <1975SST670>. 

Chlorine dioxide has a positive chlorine oxidation state of four (+4), which is intermediate between chlorite (+3) and chlorate (+5) ions. Oxidation of chlorine dioxide usually results in the formation of chlorite ions (e.g., CIO2 + e C102 =0.95 volts). Chlorite ions (CIO2 ) are also effective oxidizing... 

Chlorination of 9-acetylcarbazole in acetic acid produced values of 4,300 (C-1), 8,600 (C-2), 122,000 (C-3), and 8,600 (C-4) for the partial rate factors. Chlorination of carbazole itself gave values of > 10 and > 10 for the 1- and 3-positions chlorination of carbazole was found to be six times slower than that of diphenylamine. ... 

C-rings of the benzofuran system. Most of these isomers had all their lateral positions chlorinated. Contrary, all the PCDF isomers apparently excreted had two vicinal hydrogenated C-atoms in at least one of the two rings, and these unblocked positions are involved in the metabolism by forming epoxides, see Figure 3 (23) The data discussed here show a striking similarity between the most toxic PCDD and PCDF isomers and the isomers most efficiently retained. 

J. Stieglitz has pointed out that in A. A. Jakowkin s reaction Cl2+H2O HOCl -f-H +Cr, part of the hypochlorous acid must form positive chlorine and negative chlorine ions in order to produce the neutral molecules Cl2 and H20. The ionization of hypochlorous acid—as an acid—will be HOCW H+OCl and as a base, HOCl =iHO,+Cl . There is also evidence that not only can hypochlorous act as a weak acid, but it can also act as a weak base. The basic properties of hypochlorous acid are illustrated by the iodoso- and iodonium compounds.16... 

A convenient synthesis of chlorohydrins is based on the use of Chlorine T (TosNCINa) as the positive chlorine source in water-acetone.147 It adds to a variety of alkenes to form Markovnikov and anti-Markovnikov products in a ratio of 4 1. 

The 2-chloroallyl cation generated by matrix isolation has been studied by FT-IR spectroscopy and calculations [MP2(fc)/6-311G(d,p) level].235 Structure 78 of Cs symmetry with bridging chlorine, proposed earlier,162 was found to be less stable by 7.5 kcal mol 1 than cation 79 of C2v symmetry and could not be found by spectroscopy. Furthermore, in contrast to 1- or 3-chloroallyl cations, the centrally positioned chlorine does not contribute to charge delocalization through back-donation as a consequence of the Ji-orbital noninteraction between the n electrons of Cl and the LUMO of the allyl cation.236... 

Halogenation of the 3-hydroxypyridines follows the pattern set by the 3-aminopyridines in that substitution takes place at the 2-position. Chlorination with hydrochloric acid and hydrogen peroxide,102 bromination with one equivalent of bromine in pyridine,192 and iodination with iodine and sodium carbonate102 all give the 2-halo-3-hydroxypyridine. Chelation and formation of a cyclic transition state does not appear a likely possibility in these reactions. Under more vigorous conditions or with an excess of the halogen, 2,6-disubstitu-tion162,183 and 2,4,6-trisubstitution can occur.162... 

But the most versatile entry into the whole ynamine chemistry starts with cheap and readily available trichloroacetamides. Phosphines and phosphites are capable of abstracting a positive chlorine atom thereby leading to trichloroenamines. The conversion is smooth regardless of substituents on the amide nitrogen (53). 

Chloramine T ((H ) is a powerful positive chlorine releasing reagent toward alkenes in acetic acid, giving mainly a ira/u-chlorohydrin acetate (73) as shown in Scheme 7. The similar reaction in an acetone/H20(l l)/H2S04 system under reflux produces chlorohydrins in moderate yields. - 7... 

Early attempts to add chlorine fluoride to alkenes led only to chlorine addition however, control of temperature and the correct choice of solvent allow addition of a positive chlorine to give a chloronium ion followed by fluoride ion attack, so that overall CIF addition takes place giving a-chloro-jS-fluoroalkanes. 

In analogy to chloramines, dichloramine can react as a chlorinating agent and can transfer positive chlorine to ammonia or water. 

Nitryl chloride reacts with zinc, mercury, and bismuth to form mixtures of the metal chlorides and nitrates, but does not react with copper, magnesium, iron, aluminum, or tin at 243 K. In the reaction with ammonia, nitryl chloride acts as a source of positive chlorine ... 

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