Hydration through Mercury Compounds Oxymercuration

The critical difference between addition of halogen (X2) and the many HX additions we saw in Chapter 9 is the presence of the intermediate halonium (bromo-nium or chloronium) ion. This intermediate is demanded by the observed preference for anti addition. However, occasionally the cyclic halonium ion is less stable than the corresponding open ion and the open ion will be favored. One way to stabilize the open ion is through resonance. If the open ion is the intermediate, the products of both cis and trans addition will be observed. When a protic nucleophilic solvent is used in the reaction, the solvent as well as the halide will add to the intermediate. In such cases halohydrins or halogenated ethers are formed along with dihalides. 

For small-scale hydrations, it is convenient to use mercury salts, such as mercuric acetate, Hg(OAc)2, to carry out the addition of H—OH across a double bond. This process is called oxymercuration. In this two-step process, an alkene is first treated with mercuric acetate, then the initial alkylmercury compound formed is reduced with sodium borohydride (Na BH4). 

PROBLEM 10.8 The pATj of acetic acid is 4.8. The corresponding two-carbon alcohol, ethyl alcohol (CH3CH2OH), is a much weaker acid, as its pA j of 15.9 demonstrates. Explain why acetic acid is much more acidic than ethyl alcohol. 

The first step in the oxymercuration reaction is the formation of a cyclic mer-curinium ion (Fig. 10.22). Open carbocations are unlikely intermediates, as they would surely undergo carbocationic rearrangements, and these are not seen even in systems normally prone to rearrangement. 

FIGURE 10.22 Oxymercuration begins by attack of the alkene on the Lewis acid mercuric acetate to form a cyclic mercurinium ion. The nucleophilic solvent, water, then adds in Sn2 fashion to give the first product, a mercury-containing alcohol, where R is a simple alkyl group. 

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