How to add water across a double bond

In the last chapter, you saw alkenes being made from alcohols by El elimination—dehydration—under acid catalysis. The question we are going to answer in this section 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 very difficult to predict whether aqueous acid will hydrate the alkene or dehydrate the alcohol. The method we are about to introduce is much more reliable. The key is to use a transition metal to help you out. Alkenes are soft nucleophiles (p. 237) and interact well with soft electrophiles such as transition metal cations. Here, for example, is the complex formed between an alkene and mercury(II) cation. Don t be too concerned about the weird bond growing from the middle of the alkene this is a shorthand way of expressing the rather complex bonding interaction between the alkene and mercury. An alternative, and more useful, representation is the three-mem-bered ring on the right. 

The complex should remind you of a bromonium ion, and rightly so, because its reactions are really rather 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(II) is supplied as mercury(II) acetate, Hg(OAc)2 which we shall represent with two covalent Hg-0 bonds (simply because it helps with the arrows and with electron-accounting to do so). Unsurprisingly, water attacks at the more substituted end of the mercurinium ion. oxymercuration 

WeVe added OH and Hg(II) across the alkene, and the reaction is termed an oxymercuration. 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 to cleave 

---

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

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

Hydration and hydroboration/oxidation give you the ability to add water across an unsymmetrical double bond in either a Markovnikov or an anti-Markovnikov fashion, respectively. Although the mechanistic details of the hydroboration process are complex, the synthetic outcome is decidedly not. Be sure you know how to use these reactions to good effect in making alcohols. 

Over the next two sections, we will learn how to add H and OH across a double bond. The process of adding water across an alkene is called hydration, and it can be done through a Markovnikov addition or through an anti-Markovnikov addition. We just need to carefully choose our reagents. In this section, we will explore the reagents that give a Markovnikov addition of water. Then, in the next section, we will explore how to do an anti-Markovnikov addition of water. 

---