Hydration of Alkenes
H2O Addition to Alkenes
HBr and HCl easily add to alkenes. Since water also is a molecule of the type HX which can donate a proton, H2O should be able to add to alkenes in the same way as HBr, for example, resulting in the hydration of an alkene. However, for the addition of H2O to alkenes to occur acid catalysts are required. HBr (pKa = -9) and HCl ( = -7) have a much smaller value than water ( = 15.7), i.e. they are much stronger acids than water. The presence of an acid allows for a new and different course of hydration, obviously requiring a much smaller activation energy than the direct hydration without the catalytic effect of the proton.
Acid catalyzed addition of H2O to alkenes proceeds in three reaction steps.
Protonation is the slowest and therefore the rate-determining step of the addition reaction.
- Acid catalyzed addition of H2O
- Only in the presence of an acid catalyst does water add to alkenes resulting in the formation of an alcohol.
However, the acid-catalyzed hydration has two important disadvantages:
- Rearrangements can occur
- Regioselectivity cannot be controlled