HOBr H As with the addition of halogen, the addition of hypohalous acid(HOX) to alkenes is a stepwise process which also involves the"halonium"ion intermediate. As described previously for the addition of halogen (X2), the halonium ion intermediate is highly electrophilic and reacts rapidly with the best nucleophile in the system. In the case of HOX addition, the most powerful nucleophile in the system is hydroxide anion Again, because of the steric bulk of the halonium ion, attack by hydroxide must come from the backside (relative to the large halogen atom)to form the trans product. If the alkene is unsymmetrical, hydroxide can potentially attack at either the carbon. The distribution of positive charge, however, will not necessarily be the same on both carbons, and attack will typically occur on the carbon which is best able to stabilize the partial positive charge. In general, this will also be the carbon which would form the most stable carbocation, and, as a general rule, hydroxide anion will attach to the carbon of the alkene which would form the most stable carbocation. another way to view the selectivi Rule, the carbon which would get the halogen in the addition of Hx, will get the hydroxyl group in the ffectively block attack from the top forcing the addition of halogen and hydroxide to be anti (trans: on the opposite side), relative to each other HIH20 heat H The acid-catalyzed addition of water to alkenes is another example of a stepwise process which generally involves a solvent-equilibrated carbocation intermediate. The formation of this intermediate is initiated through a simple acid-base equilibrium in which the acid donates a proton to the alkene T-system, which is functioning as a Lewis base. The protonated T-system to form an sp sigma bond adjacent to an spa carbocation center. If the alkene is unsymmetrical, two carbocations are possible and the addition will proceed to form the most stable carbocation. As before, tertiary centers will be favored over secondary,As with the addition of halogen, the addition of hypohalous acid (HOX) to alkenes is a stepwise process which also involves the "halonium" ion intermediate. As described previously for the addition of halogen (X2), the halonium ion intermediate is highly electrophilic and reacts rapidly with the best nucleophile in the system. In the case of HOX addition, the most powerful nucleophile in the system is hydroxide anion. Again, because of the steric bulk of the halonium ion, attack by hydroxide must come from the backside (relative to the large halogen atom) to form the trans product. If the alkene is unsymmetrical, hydroxide can potentially attack at either the carbon. The distribution of positive charge, however, will not necessarily be the same on both carbons, and attack will typically occur on the carbon which is best able to stabilize the partial positive charge. In general, this will also be the carbon which would form the most stable carbocation, and, as a general rule, hydroxide anion will attach to the carbon of the alkene which would form the most stable carbocation. Another way to view the selectivity is using Markovnikov's Rule; the carbon which would get the halogen in the addition of HX, will get the hydroxyl group in the addition of HOX. As before, the large halogen on the intermediate halonium ion effectively blocks attack from the top, forcing the addition of halogen and hydroxide to be anti (trans; on the opposite side), relative to each other. Return to Top The acid-catalyzed addition of water to alkenes is another example of a stepwise process which generally involves a solvent-equilibrated carbocation intermediate. The formation of this intermediate is initiated through a simple acid-base equilibrium in which the acid donates a proton to the alkene -system, which is functioning as a Lewis base. The protonated -system to form an sp3 sigma bond adjacent to an sp2 carbocation center. If the alkene is unsymmetrical, two carbocations are possible and the addition will proceed to form the most stable carbocation. As before, tertiary centers will be favored over secondary