Chloronium ion

Chlorohydrination. The mechanism for the formation of propylene chlorohydrin is generally beheved to be through the chloronium ion intermediate (109,112). 

The chloronium ion intermediate can react with water to produce the desired propylene chlorohydrin, with chloride ion to produce 1,2-dichloropropane, or with propylene chlorohydrin to produce isomers of dichloro-dipropyl ether. 

Chloronium ions, tetramethylene-reactions, 1, 566 Chloronium salts cyclic... 

Both proton loss and rearrangement reflect the greater positive charge at carbon in a chloronium ion than in a bromonium ion because of the weaker bridging by chlorine. 

Chloronium ion (Section 6.16) A halonium ion in which the halogen is chlorine (see halonium ion). 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

MeBr is a strong poison only with Et2 All coinitiator. Since Et2 All forms the least nucleophilic counterion, Et2AHXe, it is expected to produce a relatively free carbenium ion, facilitating bromonium ion formation by interaction with MeBr solvent. With more nucleophilic counteranions, like Me3 AlXe or Et2 AlXf (X = Cl, Br), bromonium ion formation is more difficult and poisoning is modest. Evidently, the less stable bromonium ions form only with weakly nucleophilic counterions. MeCl is the weakest poison or may be inert, since chloronium ions are highly unstable. 

Twenty years later, by studying the chlorination of cis- and trans-di-t-butylethylenes, Fahey (1966) showed that halogen bridging is a general rule. It is expected that, owing to steric repulsions between the two branched groups, the cis-alkene would prefer to react via an open / -chlorocarbocation [14] where free rotation can occur rather than via a chloronium ion [15]. 

As an alternative to radical chlorination, use has been made of carbon tetrachloride and hexachloroethane in the presence of a quaternary ammonium salt, as source of the chloronium ion for reaction with activated alkylbenzenes [38], Benzyl chlorides need the additional activation of a nitro group for their conversion into the corresponding nitrobenzotrichlorides, whereas benzal chlorides do not need the extra activation for a similar conversion. The same synthetic protocol, using hexachloroethane, has been used for the conversion of allylic sulphones into the 1,1-dichloro derivatives [39],... 

Solvolysis of butadiene monoxide (10.102) in saline solution is a rather unexpected reaction that further documents this compound s reactivity [168]. In aqueous NaCl solution at physiological pH and temperature, butadiene monoxide disappeared rapidly to form 1,2-dichloro-3,4-epoxybutane (10.111, Fig. 10.25). There was a linear dependence of the rate of reaction on the Cl concentration (in the range investigated (34-135 mM)). The reaction pathway was described as slow solvolytic formation of the bu-tenylchloronium ion, followed by Cl attack to yield Cl2 and butadiene. Cl2 is then rapidly trapped by a second molecule of butadiene monoxide to form a different chloronium ion that also reacts with Cl to yield the final, stable dichloro product (10.111). The formation of 1,2-dichloro-3,4-epoxy-butane under physiological conditions is believed to be toxicologically significant. 

Anodic regioselective acetamidosulfeny-lation of alkenes is similarly achieved by oxidation of diphenyldisulfide in acetonitrile [81]. Cyclic enamines, which are intermediates in the oxidation of cyclic N-methoxycarbonyl amines, react in aqueous acetonitrile that contains chloride ions to a-hydroxy- 8-chloro compounds via intermediate chloronium ions [82]. Enolethers undergo a regioselective azidomethoxyla-tion to yield acetals of a-azido carbonyl compounds upon electrolysis in methanol containing sodium azide [83]. The reaction proceeds possibly via addition of an anodicaUy generated azide radical. 

Dehydration of prednisolone acetate (175b) yields the corresponding 9,11 olefin. As a variation on the chemistry we have seen previously, this olefin is allowed to react with chlorine in the presence of lithium chloride. If this addition is assumed to proceed by the customary mechanism, the first intermediate should be the 9a,lla-chloronium ion. Axial attack by chloride anion from the lip position will lead to the observed stereochemistry of the product dichlorisone (240).°°... 

The mechanism in the first step involves an attack of the electrophilic chlorine on the double bond of propylene to form a chloronium ion, which is attacked by a hydroxide ion to complete the first reaction. The dilute... 

Attack by Cl, 111 I, 19 and RS + 2il is similar to that by Br there is a spectrum of mechanisms between cyclic intermediates and open cations. As might be expected from our discussion in Chapter 10 (p. 312), iodonium ions compete with open carbocations more effectively than bromonium ions, while chloronium ions compete less effectively. There is... 

An important application of alkylation with di- and polyhalides is cyclialkylative ring formation. Of different a,co-dihaloalkanes studied, 1,4-dichlorobutane was shown to react at exceptional rate and with least rearrangement.143 This was rationalized in terms of participation by the cyclic chloronium ion 29 formed with anchimeric assistance by the second chlorine atom in the molecule ... 

Ethylene was reacted with chlorine water, or with a mixture of hydrated lime and chlorine. In the latter case the Ca(OCl)2 formed decomposes to yield HOC1. The aqueous opening of the intermediate chloronium ion leads to the formation of the product. Ethylene chlorohydrin then was cyclized to ethylene oxide by addition of calcium hydroxide. 

An interesting observation of the chlorination of allenes is that besides the addition products, monochlorinated compounds are also formed.252-254 Thus allene gave propargyl chloride, and tetramethylallenes gave 3-chloro-2,4-dimethyl-1,3-pentadiene. Since the reactions were carried out in the presence of oxygen, that is, under ionic conditions, proton loss from the intermediate chloronium ion or 2-chlorocarbocation explains these results. 

Cyclic aliphatic halonium ions (I, Br, Cl) have been observed directly in superacid solution by NMR spectroscopy (B-75MI11900). Cyclic halonium ions with ring size three, five and six are formed from open chain dihalides by reaction with strong Lewis acids such as SbFs. Although numerous iodonium, bromonium and chloronium ions are known, no fluoronium ion has been directly observed. NMR spectra of a solution of 2,3-difluoro-2,3-dimethyl-butane (12) in SbF5-S02 at — 90 °C provide evidence for a rapid interconversion of the two open-chain, substituted /3-fluoroethyl cations (67JA4744). The open-chain cation is about 48.2 kJ mol-1 more stable than the closed fluoronium ion (74JA2665). 

C-l, to give initially a chloronium ion, as shown. Favored attack by chloride ion at the mors highly reactive center (C-l) would result in a net inversion of configuration at C-2 of 27, t( yield 29. In the reaction of a-D-lyxopyranosyl chloride 2,3,4-tri(chlorosulfate) 28 wit aluminum chloride, a 1,2-cis product 30 was preponderant, whereas the proposed mechanisrr should yield a 1,2-trans product this result is, presumably, due to the anomerization of the thermodynamically less-stable p anomer of 30 to the (more stable) 1,2-cij product 30. 

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