How do alkenes react with water in the presence of an acid?

Alkenes react with water in the presence of an acid to form alcohols, in a process known as acid-catalysed hydration.

In more detail, the reaction between alkenes and water in the presence of an acid is a type of addition reaction. This is because a molecule of water, H2O, is added across the double bond of the alkene. The acid, often sulphuric acid, acts as a catalyst to speed up the reaction without being used up itself. This reaction is known as acid-catalysed hydration.

The mechanism of this reaction involves three steps. Firstly, the alkene reacts with the acid to form a carbocation, which is a positively charged carbon atom. This is the rate-determining step, meaning it is the slowest and therefore determines the overall rate of the reaction. The double bond of the alkene is broken, and one of the carbon atoms forms a bond with a hydrogen atom from the acid, leaving the other carbon atom positively charged.

In the second step, a water molecule is attracted to the positively charged carbon atom and forms a bond with it. This results in a molecule with an OH group and a hydrogen atom attached to the same carbon atom, known as a protonated alcohol.

In the final step, another water molecule removes the extra hydrogen atom from the protonated alcohol to form the final product, an alcohol. The removed hydrogen atom combines with a hydroxide ion, OH-, from the water to form another water molecule, which can then participate in the reaction again. This is why the acid is not used up in the reaction and acts as a catalyst.

In summary, the reaction of alkenes with water in the presence of an acid is a useful way to synthesise alcohols. It involves the addition of a water molecule across the double bond of the alkene, facilitated by an acid catalyst.