21.6 Chemistry of Esters

Ester Reactions

Esters undergo nucleophilic acyl substitution because the carbonyl carbon is electrophilic and the alkoxide ($OR'$) group can serve as a leaving group. Different nucleophiles produce different products: hydroxide gives hydrolysis, hydride gives reduction, and carbon nucleophiles like Grignard reagents give new C–C bonds. Mastering these pathways is central to this unit.

Mechanism of Ester Hydrolysis

Base-promoted hydrolysis (saponification)

This reaction is irreversible because the carboxylate product is stabilized by resonance and deprotonation.

1. The hydroxide ion ($OH^-$) attacks the electrophilic carbonyl carbon of the ester (e.g., ethyl acetate).
2. A tetrahedral intermediate forms, with a negative charge on the former carbonyl oxygen.
3. The tetrahedral intermediate collapses: the alkoxide leaving group ($CH_3CH_2O^-$) departs, regenerating the $C=O$ and producing a carboxylic acid.
4. A fast, thermodynamically favorable proton transfer occurs: the carboxylic acid donates a proton to the alkoxide, yielding a carboxylate anion (acetate) and an alcohol (ethanol).

Because step 4 is strongly exergonic, the equilibrium lies far to the product side. That's why base-promoted hydrolysis is effectively irreversible, unlike the acid-catalyzed version.

When this reaction is applied to fats or oils (triacylglycerols), it's called saponification and produces glycerol plus fatty acid salts (soap).

Acid-catalyzed hydrolysis

This is the reverse of Fischer esterification, and every step is reversible. Excess water is used to drive the equilibrium toward hydrolysis products.

1. A strong acid (e.g., $HCl$ or $H_2SO_4$) protonates the carbonyl oxygen, activating the ester toward nucleophilic attack.
2. Water acts as the nucleophile, attacking the now more electrophilic carbonyl carbon to form a tetrahedral intermediate.
3. Proton transfers within the tetrahedral intermediate convert the $OR'$ group into a better leaving group (a neutral alcohol rather than an alkoxide).
4. The alcohol (e.g., ethanol) leaves, generating a protonated carboxylic acid.
5. Deprotonation gives the neutral carboxylic acid product (e.g., acetic acid) and regenerates the acid catalyst.

Reduction of Esters

Full reduction to primary alcohols with $LiAlH_4$

$LiAlH_4$ is a powerful, non-selective reducing agent. It delivers $H^-$ twice, converting an ester all the way to a primary alcohol.

1. A hydride ($H^-$) from $LiAlH_4$ attacks the electrophilic carbonyl carbon of the ester (e.g., methyl benzoate).
2. The tetrahedral intermediate collapses, expelling the alkoxide leaving group ($CH_3O^-$) and forming an aldehyde (benzaldehyde).
3. The aldehyde is more reactive than the starting ester, so a second $H^-$ from $LiAlH_4$ immediately reduces it via nucleophilic addition to give an aluminum alkoxide.
4. Aqueous workup (dilute acid or $NaOH$) protonates the alkoxide, yielding the primary alcohol product (benzyl alcohol).

The net result: an ester ($RCOOR'$) becomes a primary alcohol ($RCH_2OH$) plus the alcohol $R'OH$.

Partial reduction to aldehydes with DIBAL-H

DIBAL-H (diisobutylaluminum hydride) is bulkier than $LiAlH_4$, which allows you to stop the reduction at the aldehyde stage.

1. One equivalent of DIBAL-H delivers a hydride to the carbonyl carbon of the ester (e.g., ethyl benzoate).
2. The resulting tetrahedral intermediate is stabilized as an aluminum alkoxide at low temperature and does not collapse to expel the leaving group the way it would with $LiAlH_4$.
3. Aqueous workup hydrolyzes the aluminum alkoxide, releasing the aldehyde product (benzaldehyde).

Two conditions are critical: the reaction must be run at $-78\,°C$ (dry ice/acetone bath) and with exactly 1 equivalent of DIBAL-H. Warming up or adding excess reagent will push the reduction further to the alcohol.

Reactions with Grignard Reagents

Grignard reagents ($RMgX$) are strong carbon nucleophiles. When they react with esters, two equivalents of the Grignard add, producing a tertiary alcohol (with two identical R groups from the Grignard).

1. The first equivalent of the Grignard reagent (e.g., $CH_3MgBr$) attacks the carbonyl carbon of the ester (e.g., ethyl benzoate).
2. The tetrahedral intermediate collapses, expelling the alkoxide leaving group ($CH_3CH_2O^-$) and forming a ketone intermediate.
3. The ketone is more electrophilic than the starting ester, so a second equivalent of $CH_3MgBr$ attacks immediately, forming a new magnesium alkoxide.
4. Aqueous workup protonates the alkoxide, yielding a tertiary alcohol product.

Ester Formation and Exchange

Fischer esterification combines a carboxylic acid and an alcohol under acid catalysis to form an ester plus water. The mechanism is the exact reverse of acid-catalyzed hydrolysis. Because the reaction is reversible, you drive it forward using excess alcohol or by removing water (Dean-Stark trap).

$RCOOH + R'OH \xrightleftharpoons{H^+} RCOOR' + H_2O$

Transesterification swaps the alkoxy group of an existing ester with a different alcohol. It also requires acid or base catalysis and follows the same nucleophilic acyl substitution mechanism. Excess of the new alcohol drives the equilibrium. This reaction is industrially important: biodiesel production converts triglycerides into fatty acid methyl esters (FAMEs) via transesterification with methanol.