Hydration of Alkenes: Acid-Catalyzed vs Oxymercuration-Demercuration

Alkene to Alcohol

When converting an alkene to an alcohol, the process typically involves adding water across the double bond of the alkene

When converting an alkene to an alcohol, the process typically involves adding water across the double bond of the alkene. This reaction is known as hydration and is commonly achieved using either acid-catalyzed hydration or oxymercuration-demercuration.

In acid-catalyzed hydration, the alkene reacts with water in the presence of an acid catalyst, such as sulfuric acid (H2SO4) or phosphoric acid (H3PO4). The acid serves to protonate the alkene, generating a carbocation intermediate. Water then attacks the carbocation, leading to the formation of a protonated alcohol. The protonated alcohol is subsequently deprotonated to yield the desired alcohol product. Here is the general reaction:

Alkene + H2O + Acid catalyst → Alcohol

For example, let’s consider the addition of water to propene (a 3-carbon alkene) using sulfuric acid as the catalyst:

CH3CH=CH2 + H2O + H2SO4 → CH3CH2CH2OH

In oxymercuration-demercuration, the alkene reacts with a mercuric salt, such as mercuric acetate (Hg(OAc)2). This results in the formation of a mercurinium ion intermediate, which is then attacked by water. Afterward, a reducing agent, such as sodium borohydride (NaBH4), is added to remove the mercury atom and regenerate the alkene. The net result is the addition of water across the double bond, leading to the formation of an alcohol. Here is the general reaction:

Alkene + Hg(OAc)2 + H2O → Mercurinium ion intermediate
Mercurinium ion + H2O → Alcohol
Alcohol + NaBH4 → Regeneration of alkene

For example, let’s consider the addition of water to propene using mercuric acetate and sodium borohydride:

CH3CH=CH2 + Hg(OAc)2 + H2O → CH3CH(OAc)CH2OH
CH3CH(OAc)CH2OH + NaBH4 → CH3CH2CH2OH

Both acid-catalyzed hydration and oxymercuration-demercuration are widely used methods for converting alkenes to alcohols. These reactions are valuable in organic synthesis as they enable the introduction of hydroxyl groups into organic molecules, allowing for the creation of new functional groups and the generation of more complex structures.

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