Acidity of a-Hydrogens the Enolate Anion

The a-hydrogen in a carbonyl compound is more acidic than a normal hydrogen bonded to a carbon atom. Table 9.2 shows the values for a typical aldehyde and ketone as well as for reference compounds. The result of placing a carbonyl group adjacent to methyl protons is truly striking, an increase in their acidity of over 10 (Compare acetaldehyde or acetone with propane.) Indeed, these compounds are almost as acidic as the O—H protons in alcohols. Why is this  

There are two reasons. First, the carbonyl carbon carries a partial positive charge. Bonding electrons are displaced toward the carbonyl carbon and away from the a-hydrogen (shown by the red arrows below), making it easy for a base to remove the a-hydrogen as a proton (that is, without its bonding electrons). 

The anion is called an enolate anion. Its negative charge is distributed between the a-carbon and the carbonyl oxygen atom. 

An enolate anion is formed by removal of the a-hydrogen of a ketone or aldehyde. 

An enolate anion is a resonance hybrid of two contributing structures that differ only in the arrangement of the electrons. 

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