My lecturer told me that 2° and 3° alcohols prefer to dehydrate through E-1 mechanism. On the other hand, 1° alcohols tend to prefer the E-2 mechanism, which does not involve the formation of a carbocation intermediate, and thus rues out scope for rearrangement.
This is backed by online sources:
Primary alcohols undergo bimolecular elimination (E2 mechanism) while secondary and tertiary alcohols undergo unimolecular elimination (E1 mechanism). Source
Another site tells me:
You will find two versions of the mechanism for the dehydration of primary alcohols on the web and in various textbooks.
One of these is exactly the same as the mechanism for the reaction involving propan-2-ol and other secondary or tertiary alcohols (known technically as an E1 mechanism), but the other is different (known as an E2 mechanism).
The more reliable sources give the E2 mechanism for the dehydration of primary alcohols including ethanol. I am going to treat this as the "correct" version, and have added a note as to why I think is better following the mechanism. Sourced from chemguide.
The note goes on to read:
[...]you would get a primary carbocation formed, CH3CH2+, but this is much less stable than a secondary or tertiary carbocation. That would lead to a very high activation energy for the reaction. The alternative mechanism avoids the formation of the carbocation, and so avoids the high activation energy.
Hence my first question:
What should be the preferred mechanism?
But wait a minute. That wasn't my original question.
My original question is derived from this other post: Benzene from cyclopropane-1,2,3-triyltrimethanol in acidic medium. The comments seem to suggest that this reaction is not known, while another writes that "I strongly doubt it is real".
The top voted answer suggests the formation of a carbocation intermediate leading to ring expansion. And here's where my questions coalesce. The above mentioned compound consists of three $1° OH^-$ groups. This seems to suggest that an E-2 elimination reaction should occur, which rules out the possibility of rearrangement, or for that matter, ring expansion.
- What should be the preferred mechanism for 1°, 2° and 3° alcohols?
- If the mechanism for 1° is E-2, why should carbocation induced ring expansion occur in the dehydration of cyclopropane-1,2,3-triyltrimethanol?