Addition of aqueous acid to alkenes to give alcohols
Description: Addition of aqueous acid to alkenes leads to the formation of alcohols.
Notes: Note that the alcohol is formed at the most substituted carbon of the alkene (Markovnikoff selectivity!) Since this reaction proceeds through a carbocation, rearrangements can occur in some cases.
Aqueous acid is often written as “H3O(+)” . An alternative way to depict aqueous acid is H2O/H2SO4. There’s no essential difference for our purposes.
Notes: Note that the last example is a rearrangement!
Mechanism: Protonation of the alkene by acid (Step 1, arrows A and B) leads to the more substituted (i.e. stable) carbocation, which is then attacked by water (Step 2, arrow C). Deprotonation of the oxygen then gives the neutral alcohol (Step 3, arrows D and E). Note that the acid is regenerated in step 3, so it acts as a catalyst.
Notes: It’s probably more reasonable to show water as the base in step 3, but HSO4(-) was used here for simplicity’s sake. Note that the acid is catalytic.
In this example a secondary carbocation is formed: if C-3 were a tertiary or quaternary carbon, a rearrangement would have occurred.