Acidity of a-Hydrogens pKa for a-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. => Chapter 22 8Chapter 22 8 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. =>