Acid-Catalyzed Dehydration

Dehydration reactions eliminate alcohols, yielding double bonds. 

Recall that for elimination to take place, you need a good leaving group. Alcohols are terrible leaving groups, but in the presence of acid, they can be converted into water, which is an amazing leaving group.

If an alcohol can form a stable carbocation, the E1 mechanism will be favored. If it can’t, then the mechanism will follow an E2 pathway. Let’s start off 1° ROH, which usually follow E2.  

Protonation:

E2 Concerted β-Elimination:

Protonation:

Formation of a Carbocation (Slow Step):

E1 β-Elimination (Fast Step):

Remember how I mentioned that 1° alcohols usually follow E2?

This isn’t the case of 1° alcohols that can rearrange to 3° alcohols. Since the 1,2-rearrangement creates a super stable carbocation, the reaction will follow the E1 pathway.

Problem: Predict the major product of the reaction

6m

Problem: Predict the major product of the reaction

4m