Esters are versatile compounds that undergo various reactions. They can be hydrolyzed, reduced, or transformed into other functional groups. Understanding these reactions is crucial for synthesizing complex molecules and creating useful products.
Ester reactions involve nucleophilic attacks on the carbonyl carbon. Different reagents lead to different outcomes, from hydrolysis to reduction. These transformations are essential in organic synthesis and have wide-ranging applications in industry and everyday life.
Ester Reactions
Mechanism of ester hydrolysis
- Base-promoted ester hydrolysis
- Hydroxide ion ($OH^-$) acts as a nucleophile attacks the electrophilic carbonyl carbon of the ester (ethyl acetate)
- Tetrahedral intermediate is formed contains a negative charge on the oxygen and a positive charge on the carbonyl carbon
- Alkoxide (ethoxide) acts as a leaving group leaves, forming a carboxylic acid (acetic acid)
- Proton transfer from water to the alkoxide yields an alcohol (ethanol) and a carboxylate anion
- Carboxylate anion is protonated by water to form the carboxylic acid product (acetic acid)
- This process is also known as saponification when applied to the hydrolysis of fats or oils
- Acid-catalyzed ester hydrolysis
- Protonation of the carbonyl oxygen by a strong acid (HCl) activates the ester makes it more susceptible to nucleophilic attack
- Water acts as a nucleophile attacks the electrophilic carbonyl carbon forms a tetrahedral intermediate
- Tetrahedral intermediate is formed contains a positive charge on the carbonyl oxygen and a negative charge on the hydroxyl group
- Alcohol (ethanol) acts as a leaving group leaves, forming a protonated carboxylic acid (acetic acid)
- Deprotonation of the protonated carboxylic acid by water yields the neutral carboxylic acid product (acetic acid)
- This reaction is an example of Fischer esterification in reverse
Reduction of esters
- Reduction to primary alcohols using lithium aluminum hydride ($LiAlH_4$)
- Hydride ($H^-$) from $LiAlH_4$ acts as a nucleophile attacks the electrophilic carbonyl carbon of the ester (methyl benzoate)
- Tetrahedral intermediate is formed contains a negative charge on the carbonyl oxygen and a positive charge on the aluminum
- Alkoxide (methoxide) acts as a leaving group leaves, forming an aldehyde (benzaldehyde)
- Another hydride from $LiAlH_4$ reduces the aldehyde to a primary alcohol (benzyl alcohol) via nucleophilic addition
- Aqueous workup protonates the alkoxide to yield the primary alcohol product (benzyl alcohol)
- Partial reduction to aldehydes using diisobutylaluminum hydride (DIBAL-H)
- Bulky hydride from DIBAL-H selectively reduces the ester (ethyl benzoate) to an aldehyde (benzaldehyde) stops reduction at this stage
- Low temperatures (-78 ℃) and careful control of stoichiometry (1 equivalent of DIBAL-H) prevent over-reduction to the alcohol
- Aqueous workup protonates the aluminum alkoxide to yield the aldehyde product (benzaldehyde)
Ester reactions vs reducing agents
- Reaction with Grignard reagents ($RMgX$)
- Nucleophilic addition of the organometallic reagent (methylmagnesium bromide) to the carbonyl carbon of the ester (ethyl acetate)
- Tetrahedral intermediate is formed contains a negative charge on the carbonyl oxygen and a positive charge on the magnesium
- Alkoxide (ethoxide) acts as a leaving group leaves, forming a ketone (acetone)
- Aqueous workup protonates the magnesium alkoxide to yield a tertiary alcohol (2-propanol)
- Overall, Grignard reagents convert esters to tertiary alcohols via nucleophilic addition
- Reaction with lithium aluminum hydride ($LiAlH_4$)
- Hydride ($H^-$) from $LiAlH_4$ reduces the ester (ethyl acetate) to an aldehyde (acetaldehyde) via nucleophilic addition
- Another hydride from $LiAlH_4$ reduces the aldehyde to a primary alcohol (ethanol)
- Aqueous workup protonates the aluminum alkoxide to yield the primary alcohol product (ethanol)
- Overall, $LiAlH_4$ converts esters to primary alcohols via sequential reduction steps
- Esterification: The process of forming an ester from a carboxylic acid and an alcohol
- Transesterification: The exchange of the alkoxy group of an ester with another alcohol
- Both reactions involve nucleophilic acyl substitution at the carbonyl group