Chloronium complexes

Synthesis is carried out in two separate steps (Scheme 6.1). In the first reactor, propene reacts with CI2 to produce propene chlorohydrin via intermediate formation of the propene chloronium complex, then quenched by water. In the epoxidation reactor, the dehydrochlorination of propene chlorohydrin occurs using a base (usually calcium hydroxide). 

Possible mechanism of ohlorohydrine formation in solution -is chemical interaction of reacting substances via chloronium complex. 

Bromonium or chloronium complexes, which are shown in the examples above, are good electrophiles and react with benzene by replacing the hydrogen atom with the halogen. 

In Ref. [34], this attachment is discussed in detail along an optimized path obtained for reaction in aqueous solution here, instead, a comparison with the reaction in gas phase will be presented. It is useful to recall that the ring structure has a maximum weight at a geometry close to that of the transition state, while Er3 4 increase monotonically, starting from zero at the T-shaped complex and arriving at a maximum when the chloronium ion intermediate is formed. The wavefunction of the intermediate is very well represented by the resonance of structures I 2, 3 and F4 with the... 

From a mechanistic point of view, two different ionic mechanisms have to be considered (due to the presence of oxygen the radical chain mechanism plays no role in the technical process) first, the uncatalyzed reaction of ethylene and chlorine and second, the metal halide catalyzed reaction. Both routes compete in this process. The uncatalyzed halogenation was studied extensively for the bromina-tion of olefins [14, 15] (Scheme 4). It is commonly accepted that the halogenation of olefins starts with formation of a 1 1 -complex of halogen and alkene followed by formation of a bromonium ion. Subsequent nucleophilic attack of a bromine anion leads to the dibromoalkane. However, when highly hindered olefins (such as tetraneopentylethylene) are used, formation of a 2 1 r-complex, as an intermediate between 1 1 ir-complex and a bromonium ion, is detectable by UV spectroscopy. In the catalyzed reaction the metal halide polarizes the chlorine bond, thus leading to formation of a chloronium or carbonium ion. Subsequent nucleophilic attack of a chloride anion gives the dichloroalkane [12] (Scheme 5). 

In the presence of polar species, here the solvent water, the above complex can evolve towards the heterolytic breaking of the Cl-Cl a-bond and to the formation of a cyclic three-membered chloronium ion intermediate [28]. In the gas phase this process shows a barrier greater than 50 kcal/mol [34] and formation of an ionic intermediate is thns clearly nnfavonrable. A study of the thermodynamic stability of this intermediate in solution [35] and of the effect of the dielectric constant of the solvent on the reaction mechanism [36] have been made recently but so far no full theoretical treatment is available to explain all the details of the process. 

This CAS(6,5) falls into two limiting CAS(4,4) descriptions in the regions of the reactants and of the intermediate the p orbital on the approaching Cl for the T-shape complex and the p orbital on the detached Cl when the chloronium ion is formed are doubly occupied in these two limits. 

Lewis acid catalyst gives a molecular complex with a positive charge on chlorine and a negative charge on iron. Redistribution of electrons in this complex generates a chloronium ion, C1+, a very strong electrophile, as part of an ion pair. 

The o-complexes present in the S Ar mechanism are even more difficult to detect than x-complexes due to their ultrafast deprotonation. This problem can be circumvented by the preparation of o-complexes without aromatic hydrogens from reactions between electrophiles and hexasubsti-tuted benzenes. The first such example was the preparation of the heptamethylbenzenium cation, which was characterized by NMR [47]. Using the same principle, o-complex intermediates of nitration and chlorinations have been prepared and subsequently characterized by NMR and UV-VIS spectroscopy [48]. The o-complexes formed from the interaction of hexamethylbenzene and the cations silyl, methyl, bromonium, and chloronium were crystallized and analyzed by X-ray diffraction [43]. 

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