We’ve seen that,when Br2 reacts with and alkene,the cyclic bromonium ion intermediate reacts with the only nucleophile present,Br- ion.If the reaction is carried out in the presence of and additional nucleophile,however,the intermediate bromonium ion can be intercepted by the added nucleophile and diverted to to a different product.In the presence of water,forexample,water competes with Br- ion as nucleophile and reacts with the bromoniumion intermediate to yield a bromohydrin.The net effect is addition of H-OBr to the alkene.The reaction takes place by the pathway shown in Figure 7.3(p.238)