Esters are less reactive than acid halides and anhydrides. As with more reactive acyl derivatives, they can react with
ammonia and primary and secondary amines to give amides, although this type of reaction is not often used, since acid halides give better yields.
Transesterification Esters can be converted to other esters in a process known as
transesterification. Transesterification can be either acid- or base-catalyzed, and involves the reaction of an ester with an alcohol. Unfortunately, because the leaving group is also an alcohol, the forward and reverse reactions will often occur at similar rates. Using a large excess of the
reactant alcohol or removing the leaving group alcohol (e.g. via
distillation) will drive the forward reaction towards completion, in accordance with
Le Chatelier's principle.
Hydrolysis and saponification Acid-catalyzed hydrolysis of esters is also an equilibrium process – essentially the reverse of the
Fischer esterification reaction. Because an alcohol (which acts as the leaving group) and water (which acts as the nucleophile) have similar p
Ka values, the forward and reverse reactions compete with each other. As in transesterification, using a large excess of reactant (water) or removing one of the products (the alcohol) can promote the forward reaction. Basic hydrolysis of esters, known as
saponification, is not an equilibrium process; a full equivalent of base is consumed in the reaction, which produces one equivalent of alcohol and one equivalent of a carboxylate salt. The saponification of esters of
fatty acids is an industrially important process, used in the production of soap. Direct reduction to give the corresponding
ether is difficult as the intermediate
hemiacetal tends to decompose to give an alcohol and an aldehyde (which is rapidly reduced to give a second alcohol). The reaction can be achieved using
triethylsilane with a variety of Lewis acids.
Claisen condensation and related reactions Esters can undergo a variety of reactions with carbon nucleophiles. They react with an excess of a
Grignard reagent to give tertiary alcohols. Esters also react readily with
enolates. In the
Claisen condensation, an enolate of one ester (
1) will attack the carbonyl group of another ester (
2) to give tetrahedral intermediate
3. The intermediate collapses, forcing out an alkoxide (R'O−) and producing β-keto ester
4. Crossed Claisen condensations, in which the enolate and nucleophile are different esters, are also possible. An
intramolecular Claisen condensation is called a
Dieckmann condensation or Dieckmann cyclization, since it can be used to form rings. Esters can also undergo condensations with ketone and aldehyde enolates to give β-dicarbonyl compounds. A specific example of this is the
Baker–Venkataraman rearrangement, in which an aromatic
ortho-acyloxy ketone undergoes an intramolecular nucleophilic acyl substitution and subsequent rearrangement to form an aromatic β-diketone. The
Chan rearrangement is another example of a rearrangement resulting from an intramolecular nucleophilic acyl substitution reaction.
Other ester reactivities Esters react with nucleophiles at the carbonyl carbon. The carbonyl is weakly electrophilic but is attacked by strong nucleophiles (amines, alkoxides, hydride sources, organolithium compounds, etc.). The C–H bonds adjacent to the carbonyl are weakly acidic but undergo deprotonation with strong bases. This process is the one that usually initiates condensation reactions. The carbonyl oxygen in esters is weakly basic, less so than the carbonyl oxygen in amides due to resonance donation of an electron pair from nitrogen in amides, but forms
adducts. As for
aldehydes, the hydrogen atoms on the carbon adjacent ("α to") the carboxyl group in esters are sufficiently acidic to undergo deprotonation, which in turn leads to a variety of useful reactions. Deprotonation requires relatively strong bases, such as
alkoxides. Deprotonation gives a nucleophilic
enolate, which can further react, e.g., the
Claisen condensation and its intramolecular equivalent, the
Dieckmann condensation. This conversion is exploited in the
malonic ester synthesis, wherein the diester of
malonic acid reacts with an electrophile (e.g.,
alkyl halide), and is subsequently decarboxylated. Another variation is the
Fráter–Seebach alkylation.
Other reactions • Esters can be directly converted to
nitriles. • Methyl esters are often susceptible to decarboxylation in the
Krapcho decarboxylation. • Phenyl esters react to hydroxyarylketones in the
Fries rearrangement. • Specific esters are functionalized with an α-hydroxyl group in the
Chan rearrangement. • Esters with β-hydrogen atoms can be converted to alkenes in
ester pyrolysis. • Pairs of esters are coupled to give
α-hydroxyketones in the
acyloin condensation.
Protecting groups As a class, esters serve as
protecting groups for
carboxylic acids. Protecting a carboxylic acid is useful in peptide synthesis, to prevent self-reactions of the bifunctional
amino acids. Methyl and ethyl esters are commonly available for many amino acids; the
t-butyl ester tends to be more expensive. However,
t-butyl esters are particularly useful because, under strongly acidic conditions, the
t-butyl esters undergo elimination to give the carboxylic acid and
isobutylene, simplifying work-up. == List of ester odorants ==