Carbocation Stability and the Occurrence of Molecular Rearrangements

A base removes a hydrogen from the p carbon as the double bond forms and the protonated hydroxyl group departs. (The base may be another molecule of the alcohol or the conjugate base of the acid.) 

With an understanding of carbocation stability and its effect on transition states, we can now proceed to explain the rearrangements of carbon skeletons that occur in some alcohol dehydrations. 

Consider again the rearrangement that occurs when 3,3-dimethyl-2-butanol is dehydrated  

The first step of this dehydration is the formation of the protonated alcohol in the usual way  

In the second step the protonated alcohol loses water and a secondary carbocation forms  

---