Esters

ESTERS THEORY

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Esters come from condensing acids with alcohols and are named as salts of the acid from which they come. IUPAC nomenclature changes the -oic ending of the acid to -oate , ending with the name of the alkyl group attached to the oxygen.

esters nomenclature 1

[1] Methyl methanoate

[2] Methyl ethanoate

[3] Ethyl propanoate

[4] Methyl butanoate

Read more: Lactone hydrolysis

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Esters react with alcohols in acidic media, replacing their alkoxy group with the corresponding alcohol, as can be seen in the following reaction. This reaction is called transesterification.

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Esters transesterify in the presence of alkoxides, depending on the reaction:

Methyl ethanoate [1] reacts with sodium ethoxide to form ethyl ethanoate [2] and sodium methoxide [3]

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Two equivalents of magnesium organometallic add to the ester, to form tertiary alcohols in which two substituents are the same. The mechanism consists of the nucleophilic addition of the organometallic to the carbonyl of the ester. After the first addition, a ketone is formed which is attacked again by a second equivalent of magnesium to form the final alcohol.

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The esters are easily reduced with lithium aluminum hydride to form primary alcohols.

The reductant supplies hydride ions to the carbonyl carbon, transforming it into alcohol.

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Esters are reduced to aldehydes with diisobutylaluminum hydride (DIBAL) at -78°C. A single equivalent must be used to stop the reaction in the aldehyde, avoiding its reduction to alcohol.

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The esters have acidic hydrogens with pKa=25 in their position, which can be subtracted using bases. The conjugate base is an enolate ester, a highly nucleophilic species that attacks a varied number of electrophiles.