# Reactions of alkenes

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The acids HBr and HCl add easily to alkenes. H2O can also be viewed as an HX reagent, since water can also split off a proton. So water should add to alkenes in the same way as e.g. HBr and should lead to hydration2However, O and alkenes only react with one another in the presence of an acid as a catalyst. The acids HBr ($pKa$= -9) and HCl ($pKa$= -7) have compared to water ($pKa$= 15.7) a much higher proton concentration. The presence of an acid during hydration opens up a new (completely different) reaction path, which obviously has a much lower activation energy than that for direct hydration.

The addition of H takes place under acid catalysis2Oan alkenes in 3 steps.

Protonation is the slowest and thus the rate-limiting step in the reaction.

Acid catalysis of the ${\text{H}}_{2}\text{O}$-Addition
Only in the presence of an acid as a catalyst does water add to alkenes to form an alcohol.

However, directly acid-catalyzed hydration has two major disadvantages:

• rearrangements can occur
• the regiochemistry cannot be controlled.

Therefore other methods such as oxymercuration and hydroboration are also used, especially for reactions on a laboratory scale.