15.18:
Dehydration of Aldols to Enones: Acid-Catalyzed Aldol Condensation
The β-hydroxy ketone dehydrates in acidic conditions via an E1 elimination reaction to form an enone.
The E1 elimination proceeds through two distinct steps: ionization and deprotonation.
In the first step, the hydroxyl group in the presence of an acid functions as a Bronsted–Lowry base and accepts a proton to yield a hydrated hydroxyl group. Then, a neutral water molecule departs to give a tertiary carbocation intermediate.
Finally, the chloride ion abstracts the hydrogen atom from the α carbon to form an enone as the product.
15.18:
Dehydration of Aldols to Enones: Acid-Catalyzed Aldol Condensation
As shown in Figure 1, under acidic conditions, the β-hydroxy ketone undergoes dehydration via an E1 elimination reaction to form an enone.
Figure 1. The dehydration reaction of a β-hydroxy ketone.
Figure 2 depicts the sequential processes involved in the mechanism of the reaction. Here, the acid protonates the hydroxyl group in the β-hydroxy ketone to form a hydrated hydroxyl group, which then departs to form a tertiary carbocation intermediate. Subsequently, the loss of the hydrogen atom from the α carbon yields an enone as the final product.
Figure 2. The mechanism of the dehydration reaction of a β-hydroxy ketone.