Halonium ion, bridged

Halonium ion (Section 6 16) A species that incorporates apos itively charged halogen Bridged halonium 10ns are inter mediates in the addition of halogens to the double bond of an alkene... 

Interpretation of reaction stereochemistry has focused attention on the role played by bridged halonium ions. 

Thus, fluorination of 1,3-dienes proceeds through an allylic ion, while weakly bridged halonium ions are the intermediates in chlorination and bromination of dienes (vide infra). Furthermore, starting from the experimental evidence that 13 is produced under kinetic conditions and not from subsequent rearrangement of the 1,2- and 1,4-adducts, the authors suggested that 13 arose from rearrangement of the allyl cation intermediate, 17. Consistent with an open ion pair intermediate is also the stereoselective formation of the threo isomer from both 1,3-pentadienes, as well as the preference for the addition to the 1,2-bond observed in the reaction of both isomeric pentadienes. This selectivity may indeed... 

When halogenation of alkenes is carried out in aqueous solvent, a vicinal halohydrm is obtained. The reaction is regioselective, and follows the Markovnikov rule. The halide adds to the less substituted carbon atom via a bridged halonium ion intermediate, and the hydroxyl adds to the more substituted carbon atom. The reaction mechanism is similar to the halogenation of alkenes, except that instead of the halide nucleophile, the water attacks as a nucleophile. 

Direct observation of a number of bridged halonium ions (X = Br, Cl, I) is possible [74] when, for example, 2,3-dihalo-2,3-dimethylbutanes are ionised in SbFs/SOi mixtures however, as yet, no analogous fluoronium ions have been observed in solution (Figure 4.27). 

Bridged halonium ions resemble carbocations in that they are short-lived intermediates that react readily with nucleophiles. Carbocations are inherently unstable because only six electrons surround carbon, whereas halonium ions are unstable because they contain a strained three-membered ring with a positively charged halogen atom. 

How does the proposed mechanism invoking a bridged halonium ion intermediate explain the observed trans products of halogenation For example, chlorination of cyclopentene affords both enantiomers of rram-l,2-dichlorocyclopentane, with no cis products. 

The ring-opening of bridged halonium ion intermediates resembles the opening of epoxide rings with nucleophiles discussed in Section 9.15. 

Addition of X forms an unstabie bridged halonium ion in the rate-determining step. 

The mechanism for halohydrin formation is similar to the mechanism for halogenation addition of the electrophile X (from X2) to form a bridged halonium ion, followed by nucleophilic attack by H2O from the back side on the three-membered ring (Mechanism 10.4). Even though X is formed in Step [1] of the mechanism, its concentration is small compared to H2O (often the solvent), so H2O and not X" is the nucleophile. 

Four bonds are broken or formed in this step the electron pair in the Ji bond and a lone pair on a halogen atom are used to form two new C-X bonds in the bridged halonium ion. The X-X bond is also cleaved heterolytically, forming X . This step is ratedetermining. 

Because the bridged halonium ion ring is opened by backside attack of H2O, addition of X and OH occurs in an anti fashion and trans products are formed. 

This result is reminiscent of the opening of epoxide rings with acids HZ (Z = a nucleophile), which we encountered in Section 9.15B. As in the opening of an epoxide ring, nucleophilic attack occurs at the more substituted carbon end of the bridged halonium ion because that carbon is better able to accommodate a partial positive charge in the transition state. 

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