Chloronium ions, with alkenes

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. 

Electrophilic addition of Cl., and In tn nlkenes is similar in mechanism to the electrophilic addition of Br0.46 The rate of chlorination in acetic acid is second-order, first-order each in olefin and in chlorine.47 Predominantly anti addition to alkyl-substituted double bonds occurs, indicating that a chloronium ion is formed.48 Further evidence for the chloronium ion is that addition of hypo-chlorous acid to double bonds is not entirely regiospecific. For example, addition to propene gives 91 percent of the Markownikoff product 29, and 9 percent of the anti-Markownikoff product, 30. Phenyl-substituted alkenes give a mixture of syn and anti adducts with Cl2 as they do with Br2.49... 

Halohydrins are /J-halogenated alcohols. They can be obtained in H20-containing solvents from alkenes and reagents, which transfer Hal ions. N-Broniosuccinnuide (transfers Br Figures 3.43 and 3.44 as well as 3.47), chloramine-T (transfers Cl Figure 3.46), and elemental iodine (transfers I Figure 3.47) have this ability. Bromonium and chloronium ions react with H20 via an SN2 mechanism. This furnishes the protonated bromo- or chlorohydrins, which are subsequently deprotonated. 

In the last reaction, the pi electrons of an alkene attack the bromine molecule, expelling bromide ion. A bromonium ion results, containing a three-membered ring with a positive charge on the bromine atom. This bromonium ion is similar in structure to the mercurinium ion discussed in Section 8-5. Similar reactions with other halogens form other halonium ions. The structures of a chloronium ion, a bromonium ion, and an iodonium ion are shown next. 

This is an example of electrophilic addition of Cl to an alkene. The mechanism of this reaction involves the following steps. In the first step, the ethylene reacts with chlorine to form the cyclic ethylene chloronium ion (intermediate) and chloride ion. Note that in this cyclic intermediate, the chlorine has a positive charge. This step is followed by the nucleophilic attack by chloride ion on the chloronium ion. The reaction is enhanced by electron-donating substituents such as alkyl groups on the carbon-carbon double bond, since such groups can further stabilize the formation of the transition state which results in the formation of the chloronium ion. Halogen addition is usually an anti addition process. 

Alkenes readily react with chlorine to give chloronium ions, which prefer to exist in the open form, that is, with a carbocation intermediate initial addition of chlorine may occur at the top or bottom face, which gives enantiomeric intermediate carbocations. Either of these intermediates may be intercepted by water to give a chlorohydrin product, with an ont/ -outcome in which water attacks at the opposite side to chlorine, and as a racemic pair. Deprotonation of the oxonium intermediate generates the chlorohydrin product. 

The analogy with alkene chemistry can be continued in that alkynes react with bromine, chlorine, or iodine, but only one of the two Ji-bonds is used. The reaction is known as dihalogenation of alkynes. When 2-hexyne reacts with chlorine (CI2), the alkyne reacts as a Lewis base and the isolated product is the vinyl dichloride 110 (J5-l,2-dichloro-l-pentene). Formation of this product is explained by an intermediate vinyl-chloronium ion, 109, which is analogous to the halonium ions formed from alkenes in Section 10.4.1. As with alkenes, the chloronium ion reacts with the nucleophihc chloride ion (CL) via anti attack... 

An explanation for the observed stereochemistry of alkene addition came in 1937 with the suggestion that the reaction occurs through an intermediate bromonium ion (R2Br" ), formed hy electrophilic addition of Br" " to the alkene. (Similarly, a chloronium ion contains a positively charged, divalent chlorine, R2C1. ) The bromonium ion is formed in a single step hy interaction of the alkene with Br2 and simultaneous loss of Br (Figure 8.1). 

A second method of synthesizing epoxides is an intramolecular variation of the Williamson ether synthesis. First, a halohydrin forms in the reaction of an alkene with an aqueous solution of a halogen. For example, chlorine gives a cyclic chloronium ion, which then reacts with water as the nucleophile to give the chlorohydrin. 

We may seem to have contradicted ourselves because Equation 10-1 shows a carbocation to be formed in bromine addition, but Equation 10-5 suggests a bromonium ion. Actually, the formulation of intermediates in alkene addition reactions as open ions or as cyclic ions is a controversial matter, even after many years of study. Unfortunately, it is not possible to determine the structure of the intermediate ions by any direct physical method because, under the conditions of the reaction, the ions are so reactive that they form products more rapidly than they can be observed. However, it is possible to generate stable bromonium ions, as well as the corresponding chloronium and iodonium ions. The technique is to use low temperatures in the absence of any strong nucleophiles and to start with a 1,2-dihaloalkane and antimony penta-fluoride in liquid sulfur dioxide ... 

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). 

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