Acid-Catalyzed Enolization of an Aldehyde or Ketone in Aqueous Solution

Acid-Catalyzed Enolization of an Aldehyde or Ketone in Aqueous Solution THE OVERALL REACTION  

Step 1 A proton is transferred from the acid catalyst to the carbonyl oxygen. 

Step 2 A water molecule acts as a Br0nsted base to remove a proton from the a-carbon atom of the protonated aldehyde or ketone. 

Both mechanisms, acid-catalyzed and base-catalyzed, consist of two proton-transfer steps proton abstraction from carbon and proton transfer to oxygen. The difference between the two is that the sequence of steps is reversed. Proton abstraction from the a carbon is the first step in the base-catalyzed mechanism it is the second step in the acid-catalyzed one. In each mechanism, proton abstraction from the a carbon is rate-determining. 

With this as background, weTl look at a few reactions of carbonyl compounds that proceed by way of enol intermediates. 

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FIGURE 18 1 Mechanism of acid catalyzed enolization of an aldehyde or ketone in aqueous solution... 

Acid-Catalyzed Enolization of an Aldehyde or Ketone in Aqueous Solution 899 Acid-Catalyzed Bromination of Acetone 901 Cleavage of a Tribromomethyl Ketone 903... 

When an aldehyde or a ketone with one or more a-hydrogens is dissolved in an aqueous solution that is enriched with D2O and contains catalytic amounts of either D or OD, exchange of a-hydrogens occurs af a rate that is proportional to the concentration of the acid or base catalyst. We account for incorporation of deuterium by proposing a rate-determining acid- or base-catalyzed enolization followed by incorporation of deuterium as the enol form converts to the keto form. 

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