Acidity of a-Hydrogens p for a-H of aldehyde or ketone ~20. Much more acidic than alkane or alkene(pK>40)or alkyne (pK =25). Less acidic than water (p=15.7)or alcohol (p 16-19). In the presence of hydroxide or alkoxide ions,only a small amount of enolate ion is present at equilibrium. Acidity of -Hydrogens ◼pKa for -H of aldehyde or ketone ~20. ◼Much more acidic than alkane or alkene (pKa > 40) or alkyne (pKa = 25). ◼Less acidic than water (pKa = 15.7) or alcohol (pKa = 16-19). ◼In the presence of hydroxide or alkoxide ions, only a small amount of enolate ion is present at equilibrium