esterification

Esterification is the general name for a chemical reaction in which two reactants (typically an alcohol and an acid) form an ester as the reaction product. Esters are common in organic chemistry and biological materials, and often have a characteristic pleasant, fruity odor. This leads to their extensive use in the fragrance and flavor industry. Ester bonds are also found in many polymers.

Esterification is a reversible reaction. Hydrolysis—literally "water splitting"—involves adding water and a catalyst (commonly NaOH) to an ester to get the sodium salt of the carboxylic acid and alcohol. As a result of this reversibility, many esterification reactions are equilibrium reactions and therefore need to be driven to completion according to Le Chatelier's principle. Esterifications are among the simplest and most often performed organic transformations.

The most common esterification processes involve nucleophilic acyl substitution where the carbonyl compound is used as an electrophile and is attacked by a nucleophilic alcohol. However, other processes are possible; esterification by alkylation reverses the roles of "classic" carbonyl chemistry: a carboxylate anion is used as a nucleophile that displaces a halide ion in an SN2 reaction.

Fischer esterification

In the Fischer esterification, esters may be prepared by refluxing an acid (usually, but not always a carboxylic acid) and a primary or secondary alcohol in the presence of a catalyst (commonly concentrated sulfuric acid). Water is a byproduct, and it can be removed to force the equilibrium in the desired direction. In addition to being the catalyst for the reaction, the sulfuric acid also removes water to help shift the equilibrium towards forming more of the ester product. Alternatively, where the ester has a suitable boiling point, it may be distilled off, once again causing the equilibrium to favor the product.

For example, esterification of acetic acid in excess ethanol (possibly as the solvent) in the presence of concentrated sulfuric acid as a catalyst results in an ester (ethyl acetate):

Reaction with acyl halides

The reaction of a carboxylic acid halogenide (which is also called acyl halide) with an alcohol/phenol. This reaction is usually very rapid due to the high reactivity of the acyl halide (it is often performed at low temperatures), but for the same reason it tends to be difficult to control, often resulting in a mixture of low purity products and a high percentage of by-products.

H₃C-COCl + HO-CH₂-CH₃ → H₃C-COO-CH₂-CH₃ + H-Cl

Reaction with acid anhydrides

The reaction of a carboxylic acid anhydride with an alcohol. This method is favored for the synthesis of phenyl esters (for example, it is used in the synthesis of aspirin). The anhydride may be generated in situ, and catalysts are usually added (often stoichiometric quantities of amines such as pyridine or triethylamine, which also serve to neutralize the acid formed). This method is very inefficient with respect to the acid (essentially 2 moles are required for each mole of alcohol), so is mainly used either for low molecular weight acids or for very expensive alcohols.

H₃C-CO-O-CO-CH₃ + HO-CH₂-CH₃ → H₃C-COO-CH₂-CH₃ + H₃C-COOH

Alkylation of carboxylate salts

The reaction of an alkali carboxylate and an alkyl halide. This is not a reversible reaction and therefore can run to completion naturally. In the case that an alkyl chloride is used, an iodide salt may be added to catalyze the reaction (Finkelstein reaction). The carboxylate salt may be generated in situ or prior to the reaction. In difficult cases, the silver carboxylate may be used, since the silver ion coordinates to the halide aiding its departure and improving the reaction rate. This reaction can suffer from anion availability problems and therefore can benefit from the addition of phase transfer catalysts or highly polar aprotic solvents such as DMF. As an example, the reaction of sodium acetate with ethyl bromide is shown.

Other methods

Other methods include the Mitsunobu reaction and the Steglich esterification.

See also

• Transesterification, a reaction to transform one ester into a different ester.
• Hydrolysis, the cleavage of an ester with water back to a carboxylic acid and alcohol.
• acetylation
• acylation