Reactions of alkenes

H2O-addition to alkenes

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 (${\text{p}\mathit{K}}_{\text{a}}$= -9) and HCl (${\text{p}\mathit{K}}_{\text{a}}$= -7) have compared to water (${\text{p}\mathit{K}}_{\text{a}}$= 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.

Video: Organische Chemie - Reaktionen Alkene - Sekundarstufe I (May 2022).