Editing Ester (section)

== Reactions ==
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]].<ref name=wade>Wade 2010, pp. 1005–1009.</ref>

===Hydrolysis and saponification===
{{Main|Ester hydrolysis}}
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''K''<sub>a</sub> 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.

[[File:Fischer esterification-hydrolysis equilibrium.svg|center|The acid-catalyzed hydrolysis of an ester and Fischer esterification correspond to two directions of an equilibrium process.]]

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 acid]]s is an industrially important process, used in the production of soap.<ref name=wade />

Esterification is a reversible reaction. Esters undergo [[hydrolysis]] under acidic and basic conditions. Under acidic conditions, the reaction is the reverse reaction of the [[Fischer esterification]]. Under basic conditions, [[hydroxide]] acts as a nucleophile, while an alkoxide is the leaving group. This reaction, [[saponification]], is the basis of soap making.
:[[Image:Ester hydrolysis.svg|750px|Ester saponification (basic hydrolysis)]]

The alkoxide group may also be displaced by stronger nucleophiles such as [[ammonia]] or primary or secondary [[amine]]s to give [[amide]]s (ammonolysis reaction):
:{{chem2|RCO2R' + NH2R{{''}} → RCONHR{{''}} + R'OH}}
This reaction is not usually reversible. Hydrazines and hydroxylamine can be used in place of amines. Esters can be converted to [[isocyanate]]s through intermediate [[hydroxamic acid]]s in the [[Lossen rearrangement]].

Sources of carbon nucleophiles, e.g., [[Grignard reagent]]s and organolithium compounds, add readily to the carbonyl.

=== Reduction ===
Compared to ketones and aldehydes, esters are [[Carbonyl reduction#Trends in carbonyl reactivity|relatively resistant to reduction]]. The introduction of catalytic hydrogenation in the early part of the 20th century was a breakthrough; esters of fatty acids are hydrogenated to [[fatty alcohol]]s.
:{{chem2|RCO2R' + 2 H2 → RCH2OH + R'OH}}
A typical catalyst is [[copper chromite]]. Prior to the development of [[catalytic hydrogenation]], esters were reduced on a large scale using the [[Bouveault–Blanc reduction]]. This method, which is largely obsolete, uses sodium in the presence of proton sources.

Especially for fine chemical syntheses, [[lithium aluminium hydride]] is used to reduce esters to two primary alcohols. The related reagent [[sodium borohydride]] is slow in this reaction. [[DIBAH]] reduces esters to aldehydes.<ref>{{cite web | author=W. Reusch | title=Carboxyl Derivative Reactivity | url=http://www.cem.msu.edu/~reusch/VirtualText/crbacid2.htm#react2 | work=Virtual Textbook of Organic Chemistry | url-status=dead | archive-url=http://arquivo.pt/wayback/20160516073829/http://www.cem.msu.edu/~reusch/VirtualText/crbacid2.htm#react2 | archive-date=2016-05-16}}</ref>

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.<ref>{{cite journal|last1=Yato|first1=Michihisa|last2=Homma|first2=Koichi|last3=Ishida|first3=Akihiko|title=Reduction of carboxylic esters to ethers with triethyl silane in the combined use of titanium tetrachloride and trimethylsilyl trifluoromethanesulfonate|journal=Tetrahedron|date=June 2001|volume=57|issue=25|pages=5353–5359|doi=10.1016/S0040-4020(01)00420-3}}</ref><ref>{{cite journal|last1=Sakai|first1=Norio|last2=Moriya|first2=Toshimitsu|last3=Konakahara|first3=Takeo|title=An Efficient One-Pot Synthesis of Unsymmetrical Ethers: A Directly Reductive Deoxygenation of Esters Using an InBr3/Et3SiH Catalytic System|journal=The Journal of Organic Chemistry|date=July 2007|volume=72|issue=15|pages=5920–5922|doi=10.1021/jo070814z|pmid=17602594}}</ref>

=== 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 [[enolate]]s. 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<sup>−</sup>) and producing β-keto ester '''4'''.

[[File:Claisen condensation - general mechanism.svg|center|The Claisen condensation involves the reaction of an ester enolate and an ester to form a beta-keto ester.]]

Crossed Claisen condensations, in which the enolate and nucleophile are different esters, are also possible. An [[Intramolecular reaction|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.<ref>Carey 2006, pp. 919–924.</ref> 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.<ref>Kürti and Czakó 2005, p. 30.</ref> 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.<ref>{{March6th|page=1453}}</ref> 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 [[adduct]]s.

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 [[alkoxide]]s. 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 ===
{{refimprove section|date = September 2024}}
* Esters can be directly converted to [[nitriles]].<ref>{{Cite journal | doi=10.1016/S0040-4039(01)86746-0| title=A direct conversion of esters to nitriles| journal=Tetrahedron Letters| volume=20| issue=51| pages=4907| year=1979| last1=Wood | first1=J. L. | last2=Khatri | first2=N. A. | last3=Weinreb | first3=S. M.}}</ref>{{primary source inline|date = September 2024}}
* 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 [[Alpha-hydroxy ketone|α-hydroxyketones]] in the [[acyloin condensation]].

=== Protecting groups ===
As a class, esters serve as [[protecting group]]s for [[carboxylic acid]]s. Protecting a carboxylic acid is useful in peptide synthesis, to prevent self-reactions of the bifunctional [[amino acid]]s. 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.