Alkenes, reaction with aqueous chlorine

An aqueous solution of bromine or chlorine reacts with alkenes to form addition products called halohydrins. These compounds have a halogen and a hydroxyl group on adjacent carbon atoms. The reaction of aqueous chlorine with propene is shown below. 

A further support for the identification of the species responsible for the unimolecular conductance increase in terms of a chlorine-containing radical is the fact that in a blank experiment, i.e. one in which chloride is left out, a slow conductance increase is not observed and the overaU conductance yield is only half of that in the presence of chloride. Since in isobutene-saturated aqueous solution the lifetime of SOi is only 90 ns due to its rapid reaction with the alkene (as determined by optical experiments at 450 nm) [46], the non-observability of a unimolecular conductance increase means that the rate constant for heterolysis of the SO adduct to isobutene is 10 s" (cf. Eq. 32) ... 

Halohydrins are easily generated by treating alkenes with aqueous solutions of halogens. Bromine water and chlorine water add across double bonds with Markovnikov orientation (Section 8-11). The following reaction shows cyclopentene reacting with chlorine water to give the chlorohydrin. Treatment of the chlorohydrin with aqueous sodium hydroxide gives the epoxide. 

Another addition reaction involves reaction of alkenes with aqueous solutions of bromine or chlorine rather than the anhydrous conditions noted above. The reaction can proceed by formation of a cation under aqueous... 

Aqueous sodium hypochlorite is another low-priced oxidant. Very efficient oxidative systems were developed which contain a meso-tetraarylporphyrinato-Mn(III) complex salt as the metal catalyst and a QX as the carrier of hypochlorite from the water phase to the organic environment. These reactions are of interest also as cytochrome P-450 models. Early experiments were concerned with epoxidations of alkenes, oxidations of benzyl alcohol and benzyl ether to benzaldehyde, and chlorination of cyclohexane at room temperature or 0°C. A certain difficulty arose from the fact that the porphyrins were not really stable under the reaction conditions. Several research groups published extensively on optimization, factors governing catalytic efficiency, and stability of the catalysts. Most importantly, axial ligands on the Mn porphyrin (e.g., substituted imidazoles, 4-substituted pyridines and their N-oxides), 2 increases rates and selectivities. This can be demonstrated most impressively with pyridine ligands directly tethered to the porphyrin [72]. Secondly, 2,4- and 2,4,6-trihalo- or 3,5-di-tert-butyl-substituted tetraarylporphyrins are more... 

For addition of HOC1 the olefins are stirred with 0.125-0.5 m-HOCl at 10°. The yields of chlorohydrin from 1-alkenes of higher molecular weight are considerably increased by addition of pyridine and sulfuric acid (initial pH 6-6.5)278. Chlorohydrins of higher molecular weight are also, accessible by reaction of alkyl hypochlorites with the olefin in an aqueous medium tert-butyl hypochlorite (a yellow oil, b.p. 77-78°/760 mm) is suitable for this purpose it is obtained by passage of chlorine into tert-butyl alcohol in aqueous sodium hydroxide at 0-20°. 

In the preceding section, diatomic bromine, chlorine, or iodine reacted with an alkene in a nonaqueous solvent such as carbon tetrachloride. Exploring a reaction usually requires experimenting with reaction conditions, including changing the solvent. A reasonable experiment with an alkene that involves chlorine might use an aqueous solution saturated with chlorine gas. 

It is known that dissolving chlorine in water leads to a solution that contains hypochlorous acid (HOCl) and bromine dissolved in water contains hypobromous acid (HOBr). In one experiment, 1-pentene is mixed with chlorine and water (HOCl in aqueous media) and the major product is l-chloro-2-pentanol (48), in 43% isolated yield. ° In the previous section, chlorine reacted with 1-pentene in a nonaqueous solvent such as carbon tetrachloride to give a dichloride. To ascertain why this reaction is different, the first useful observation is that HOCl is in solution rather than Cl-Cl. The polarization of HOCl is HO -Cl, where chlorine is the electrophilic atom. The 7t-bond of an alkene should react with the positive chlorine atom, and cleavage of the Cl-H bond will give hydroxide ion, which is a... 

In the reaction of chlorine dioxide with water, hypochlorous, hydrochloric and chloric acids are formed temporarily, and in alkaline solutions chlorites (C102 ), chlorates (ClOj ) and other products arise. The cation H20C1 formed in aqueous solutions of chlorine, chlorine dioxide and hypochlorites may react with alkenes and other unsaturated compounds. The electrophilic addition of HOCl to alkenes is an established reaction mechanism for a, P-chloroalcohol (chlorohydrin) and a, -dichloro derivative formation (Figure 11.16). The reaction yielding chlorohydrins follows the Markovnikov rule with the hydroxyl group adding to the more substituted carbon. Oxidation of chlorohydrins by hypochlorites... 

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. 

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