Esters are a class of functional groups and chemical compounds. Esters consist of an organic or inorganic acid in which the -OH group of the acid is replaced by an -OR group. Cyclic esters are usually called lactones. Some acids that are commonly esterified are carboxylic acids, phosphoric acid, nitric acid, and sulfuric acid. Volatile esters, particularly carboxylate esters, often have a pleasant smell and are found in essential oils, perfumes, and pheromones, and give many fruits their characteristic scent. Methyl acetate and ethyl acetate are important solvents; phosphoesters form the backbone of DNA molecules; fats and lipids are the esters of fatty acids; and polyesters are important synthetic fabrics and plastics. Esters can be synthesized in a condensation reaction between an acid and an alcohol in a reaction known as esterification.

Nomenclature

An ester is named according to the two parts that make it up: the part from the alcohol and the part from the acid (in that order), for example ethyl ethanoate (see image below).

For esters derived from the simplest carboxylic acids, the traditional name for the acid constituent is generally retained; for example, formate, acetate, propionate, butyrate. For esters from more complex carboxylic acids, the systematic name for the acid is used, followed by the suffix -oate. For example, methyl formate is the ester of methanol and methanoic acid (formic acid). It could also be called methyl methanoate.

Physical properties

Esters participate in hydrogen bonds as hydrogen-bond acceptors, but unlike their parent alcohols cannot act as hydrogen-bond donors. This ability to participate in hydrogen bonding makes them more water-soluble than their parent hydrocarbons; however, the limitations on their hydrogen bonding also make them more hydrophobic than either their parent alcohols or their parent acids. Their lack of hydrogen-bond-donating ability means that ester molecules cannot hydrogen-bond to each other, which, in general, makes esters more volatile than a carboxylic acid of similar molecular weight. This property makes them very useful in organic analytical chemistry: Unknown organic acids with low volatility can often be esterified into a volatile ester, which can then be analyzed using mass spectrometry, gas chromatography, or gas liquid chromatography. Many esters have distinctive odors and are used as artificial fragrances and flavorings.

Reactions

• Saponification (basic hydrolysis)
• Hydrolysis - the breakdown of an ester by water. This process can be catalyzed by both acids and bases. The base-catalyzed process is called saponification. The hydrolysis yields an alcohol and a carboxylic acid or its carboxylate salt.
• Reaction with primary or secondary amines to form amides.
• Phenyl esters react to form hydroxyarylketones via the Fries rearrangement.
• Esters are converted to isocyanates through intermediate hydroxamic acids in the Lossen rearrangement.
• Di-ester enolates such as diethyl malonate react as nucleophiles with alkyl halides in the malonic ester synthesis.
• Certain esters are functionalized with an α-hydroxyl group via the Chan rearrangement.
• Esters with β-hydrogen atoms can be converted to alkenes via pyrolysis.

Synthesis

Methods of preparing esters include:

• Transesterification.
• Dieckmann condensation or Claisen condensation.
• Favorskii rearrangement of α-haloketones in the presence of base.
• Pinner reaction of a nitrile with an alcohol.
• Nucleophilic displacement of alkyl halides with carboxylic acid salts.
• Nucleophilic displacement of acyl halides with alcohols.
• Baeyer-Villiger oxidation of ketones with peroxides.

Tags: carbonyl, carboxylic acid, chemistry, ester, Functional Groups, Functional Groups, organic chemistry

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