Fundamentals of Organic ChemistryChemistryGeneral Organic Chemistry
The -hydrogens of esters typically have a p K_ a 25 , whereas those of ketones have a p K_ a 20 . This difference in acidity is primarily because:
Options
- AThe electron donating alkoxy group in esters stabilises the enolate.
- BThe inductive effect of the oxygen in the ester destabilises the ester enolate.
- CThe electron donating alkoxy group in esters destabilises the enolate.
- DThere is no resonance stabilisation of the enolates of esters.
Correct answer
C. The electron donating alkoxy group in esters destabilises the enolate.
Step-by-step solution
The acidity of -hydrogens depends on the stability of the conjugate base, which is the enolate ion. In ketones, the negative charge on the -carbon is effectively delocalized onto the electronegative carbonyl oxygen through resonance. In esters, the alkoxy group ( -OR ) is attached directly to the carbonyl carbon. The oxygen atom of the alkoxy group donates electron density to the carbonyl carbon through resonance (+R effect). This cross-conjugation competes with the delocalization of the negative charge from the -c