Alkaline hydrolysis of an ester involves initial attack by hydroxide ion on the carbonyl carbon.The presence of substituents on the aromatic ring of ethyl benzoate may be expected to increase/decrease the rate of hydrolysis of this ester.In what order should the five substituents below be arranged to represent the decreasing order of the rates of hydrolysis when these substituents are present in the para position of the aromatic ring in ethyl benzoate? A)-NO₂ -H -Cl -CH₃ -OCH₃ B)-NO₂ -Cl -H -CH₃ -OCH₃ C)-OCH₃ -CH₃ -Cl -H -NO₂ D)-Cl -NO₂ -H -OCH₃ -CH₃ E)-H -Cl -CH₃ -OCH₃ -NO₂

Question

Alkaline hydrolysis of an ester involves initial attack by hydroxide ion on the carbonyl carbon.The presence of substituents on the aromatic ring of ethyl benzoate may be expected to increase/decrease the rate of hydrolysis of this ester.In what order should the five substituents below be arranged to represent the decreasing order of the rates of hydrolysis when these substituents are present in the para position of the aromatic ring in ethyl benzoate? A)-NO₂ > -H > -Cl > -CH₃ > -OCH₃ B)-NO₂ > -Cl > -H > -CH₃ > -OCH₃ C)-OCH₃ > -CH₃ > -Cl > -H > -NO₂ D)-Cl > -NO₂ > -H > -OCH₃ > -CH₃ E)-H > -Cl > -CH₃ > -OCH₃ > -NO₂

Quizplus · Accepted Answer

The presence of an electron-withdrawing group on the aromatic ring will stabilize the transition state, making it easier for the carbonyl carbon to be attacked by hydroxide ion, and thus increasing the rate of hydrolysis. The order of electron-withdrawing ability from strongest to weakest is -NO₂ > -Cl > -CH₃ > -OCH₃ > -H. Therefore, the correct order for decreasing rate of hydrolysis is -NO₂ > -Cl > -H > -CH₃ > -OCH₃, which is equivalent to choice B.

Alkaline Hydrolysis of an Ester Involves Initial Attack by Hydroxide