696 CHAPTER 17 Carbonyl Compounds I
the leaving groups in the tetrahedral intermediate formed in transesterification have
approximately the same basicity. Consequently, an excess of the reactant alcohol must
be used to produce more of the desired product.PROBLEM 21Write the mechanism for the acid-catalyzed transesterification reaction of methyl acetate
with ethanol.17.12 Hydroxide-Ion-Promoted Ester Hydrolysis
The rate of hydrolysis of an ester can be increased by carrying out the reaction in a
basic solution. Like an acid, hydroxide ion increases the rates of both slow steps of
the reaction.
Hydroxide ion increases the rate of formation of the tetrahedral intermediate be-
cause is a better nucleophile than so more readily attacks the car-
bonyl carbon. Hydroxide ion increases the rate of collapse of the tetrahedral
intermediate because a smaller fraction of the negatively charged tetrahedral inter-
mediate becomes protonated in a basic solution. A negatively charged oxygen can
more readily expel the very basic leaving group because the oxygen does not
develop a partial positive charge in the transition state.Notice that when is expelled, the final products are not the carboxylic acid and
methoxide ion because if only one species is protonated, it will be the more basic one.
The final products are the carboxylate ion and methanol because is more basic
than Since carboxylate ions are negatively charged, they are not attacked
by nucleophiles.Therefore, the hydroxide-ion-promoted hydrolysis of an ester, unlike the acid-catalyzed
hydrolysis of an ester, is nota reversible reaction.+
O−Nu
CH 3COCH 3 COO-.CH 3 O-CH 3 O-OOH+ −−OCH 3CH 3HO− H 2 Omechanism for hydroxide-ion-promoted hydrolysis of an esterC OCH 3OHOHCH 3 C OCH 3HO
OH+ CH 3 O−O−+ CH 3 OHthe more basic the solution,
the lower its concentrationCH 3COCH 3COCH 3CO(RO-)HO- H 2 O, HO-+ +HClmethyl acetate propyl alcohol propyl acetate methyl alcoholCH 3 CH 2 CH 2 OH CH 3 OH
CH 3 OCH 3 OCH 2 CH 2 CH 3COCH 3COHydroxide ion is a better
nucleophile than water.