Ester Hydrolysis in Base: Saponification

Ester Hydrolysis in Base:Saponification

O O + RCOR' HO– RCO– Ester Hydrolysis in Aqueous Base • is called saponification • is irreversible, because of strong stabilization of carboxylateion • if carboxylic acid is desired product, saponification is followedby a separate acidification step (simply a pH adjustment) + R'OH

O CH2OCR1 O R2COCH CH2OCR3 O O O O Soap-Making • Basic hydrolysis of the glyceryl triesters (from fats and oils) gives salts of long-chain carboxylic acids. • These salts are soaps. NaOH , H2O, heat R1CONa R2CONa R3CONa

•• •• O O •• •• – – •• •• •• •• RC OR' OH OR' RC OH •• •• •• •• •• •• Which bond is broken when esters arehydrolyzed in base? • nucleophilicacyl substitution. + +

H O C6H5 CH3C C O CH3 H O C6H5 CH3CONa C HO CH3 Stereochemistry gives the same answer • alcohol has same configuration at chirality center as ester; therefore, nucleophilicacyl substitution NaOH, H2O +

Mechanism of Ester Hydrolysisin Base • Involves two stages: • 1) formation of tetrahedral intermediate2) dissociation of tetrahedral intermediate

O + RCOR' H2O OH OR' RC OH First stage: formation of tetrahedral intermediate • water adds to the carbonyl group of the ester • this stage is analogous to the base-catalyzed addition of water to a ketone HO–

Mechanism of formationoftetrahedral intermediate

•• O •• H RC O •• •• – •• OR' •• •• – •• O H •• •• RC O •• •• OR' •• •• Step 1

•• O H H •• – •• O RC O •• •• •• •• H OR' •• •• – •• •• O H O •• H •• •• H RC O •• •• OR' •• •• Step 2

Dissociation oftetrahedral intermediate

•• O H H •• – •• O RC O •• •• •• •• H OR' •• •• •• •• O H O •• •• H RC – •• OR' O H •• •• •• •• Step 3

•• O •• RC – O •• •• •• •• OR' H •• •• HO– O •• RC – •• OR' O H •• •• •• •• Step 4 H2O

Key Features of Mechanism • Nucleophilic addition of hydroxide ion to carbonylgroup in first step • Tetrahedral intermediate formed in first stage • Hydroxide-induced dissociation of tetrahedralintermediate in second stage

Ester Hydrolysis in Base: Saponification