Esters

Submitted by Uttakarsh Tikku on Sun, 02/01/2009 - 20:41

Esters are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind. This could be an alkyl group like methyl or ethyl, or one containing a benzene ring like phenyl.

An ester is an often fragrant organic or partially organic compound formed by the reaction between an acid (including amino acids) and an alcohol (alkyl, R) or aromatic alcohol (aryl, R') (including a more basic amino acid) with the elimination of water. For examples,

acetic acid + an alcohol <=> acetic ester + water,

CH3COOH + ROH <=> CH3COOR + H2O

"Esterification" (condensation of an alcohol and an acid) is not the only way to synthesize an ester. Esters can be prepared in the laboratory in a number of other ways:

Transesterifications between other esters
Dieckmann condensation or Claisen condensation of esters carrying acidic ?-protons
Favorskii rearrangement of ?-haloketones in presence of base
Nucleophilic displacement of alkyl halides with carboxylic acid salts
Nucleophilic displacement of acyl halides with alcohols
Baeyer-Villiger oxidation of ketones with peroxides
Pinner reaction of nitriles with an alcohol

Esters react in a number of ways:

Esters may undergo hydrolysis - the breakdown of an ester by water. This process can be catalyzed both by acids and bases. The base-catalyzed process is called saponification. The hydrolysis yields an alcohol and a carboxylic acid or its carboxylate salt.
Esters also react if heated with primary or secondary amines, producing amides.
Phenyl esters react to hydroxyarylketones in the Fries rearrangement.
Di-esters such as diethyl malonate react as nucleophile with alkyl halides in the malonic ester synthesis after deprotonation.
Specific esters are functionalized with an ?-hydroxyl group in the Chan rearrangement.
Esters are converted to isocyanates through intermediate hydroxamic acids in the Lossen rearrangement.
Esters with ?-hydrogen atoms can be converted to alkenes in ester pyrolysis.
(source: various blogs)