Adding one hydroxyl group how to add water across a double bond

Adding one hydroxyl group how to add water across a double bond 

In Chapter 17 you saw alkenes being made from alcohols by El elimination—dehydration— under acid catalysis. The question we are going to answer in this sedion is how can you make this elimination run backwards—in other words, how can you hydrate a double bond  

It is possible on occasion simply to use aqueous acid to do this. The reaction works only if protonation of the alkene can give a stable, tertiary cation. The cation is then trapped by the aqueous solvent. 

In general, though, it is difficult to predict whether aqueous acid will hydrate the alkene or dehydrate the alcohol. The method we are about to show you is much more reliable. The key is to use a transition metal to help you out. Alkenes are soft nucleophiles (p. 357) and interact well with soft electrophiles such as transition metal cations. In the margin, for example, is the complex formed between an alkene and mercury(II) cation. The complex should remind you of a bromonium ion, and rightly so because its reactions are similar. Even relatively feeble nucleophiles such as water and alcohols, when used as the solvent, open the mercurinium ion and give alcohols and ethers. In the next scheme, the mercury(ll) is supplied as mercury(ll) acetate, Hg(OAc)2, which we shall represent with two covalent Hg—O bonds. Unsurprisingly, water attacks at the more substituted end of the positively charged mercurinium ion. 

We ve added OH and Hg(II) across the alkene, and the reaction is termed an oxymercura-tion. But a problem remains how to get rid of the metal. The C-Hg bond is very weak and the simplest way to replace Hg with H is by using a reducing agent NaBH4 works fine. 

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ADDING ONE HYDROXYL GROUP HOW TO ADD WATER ACROSS A DOUBLE BOND... 

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