Bridged halonium ion intermediates

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. 

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. 

Each addition of X2 involves a two-step process with a bridged halonium ion intermediate, reminiscent of the addition of X2 to alkenes (Section 10.13). A trans dihalide is formed after addition of one equivalent of X2 because the intermediate halonium ion ring is opened upon backside attack of the nucleophile. Mechanism 11.2 illustrates the addition of two equivalents of CI2 to CHaC CCH, to form CH3CCI2CCI2CH3. 

Addition of Bromine and Chlorine (Section 6.3D) Clj or Brj is used to convert an alkene into a vicinal dihalide. The mechanism involves attack by the alkene tt bond on one atom of Xj to give a bridged halonium ion intermediate (a cation) that is, in turn, attacked by X from the backside to give the vicinal dihalide. Rearrangements do not occur.The reaction displays anti addition stereoselectivity because of the halonium ion intermediate.The reaction is stereospecific because Z alkenes give different products than do Ealkenes. 

Halonium ion (Section 6.16) A species that incorporates apos-itively charged halogen. Bridged halonium ions are intermediates in the addition of halogens to the double bond of an alkene. 

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... 

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. 

Bridged halonium ions are formed as intermediates No rearrangements can occur. 

Addition of bromine and chlorine to an alkene is the addition of two halogens across the carbon-carbon double bond.The mechanism involves a bridged halonium ion as an intermediate and is an anti stereoselective. 

Reaction of the v electrons of the alkene with bromine (an electrophile) gives a bridged bromonium ion intermediate. This intermediate has some of the character of a carbocation (to account for the regioselectivity) and some of the character of a halonium ion (to account for the anti stereoselectivity). The secondary carbocation makes a substantial contribution to the stmcture of the resonance hybrid the primary carbocation is higher in energy and makes little contribution. 

The mechanism involves protonation of the epoxide oxygen to give a cation intermediate (analogous to a bridged halonium ion) followed by backside nucleophilic attack on the carbon more able to accept a positive charge (in unsymmetrical epoxides). 

In Summary Halogens add as electrophiles to alkenes, producing vicinal dihalides. The reaction begins with the formation of a bridged halonium ion. This intermediate is opened stereospecifically by the halide ion displaced in the initial step to give overall anti addition to the double bond. In subsequent sections, we shall see that other stereochemical outcomes are possible, depending on the electrophile. 

---