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Ester

Esters are a class of compounds widely distributed in nature. The simple ester tend to have pleasant odors.

In many cases although not exclusively so the characteristic flavors and fragrances of flowers and fruits are due to compounds with the ester functional group. An exception is the case of the essential oils.

 

Naming Esters

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Esters are commonly prepared from carboxylic acids and alcohols. Esters names have two words, the first word comes from the alcohol portion, and the second word is derived from the acid portion. The first word is the carbon group named as a substituent. The second word has the ester ending -oate. Some examples are shown below.
  • The alcohol portion is named as 3-methylbutyl. The acid portion is ethanoic acid, which is changed to ethanoate. Therefore the IUPAC name is 3-methylbutyl ethanoate. This is the major odor producing chemical in bananas.
3-Methylbutyl Ethanoate
  • The alcohol portion is simply a methyl group. The acid portion would be 2-hydroxybenzoic acid. The ester name is methyl-2-hydroxybenzoate. This is the major odor producing chemical in oil of wintergreen.
Methyl-2-Hydroxybenzoate
  • This ester is 2-chloropropyl propanoate.
2-Chloropropyl Propanoate
  • This ester is propyl-2-chloropropanoate. It is important to put the substituent in the proper part of the name.
Propyl-2-Chloropropanoate

Ester Hydrolysis

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Esters hydrolyzed by boiling water slowly to carboxylic acids and phenols. The hydrolysis is accelerated in the presence of mineral acid on alkali.

Hydrolysis of Esters

The alkaline hydrolysis is known as saponification. This is because esters with high molecular mass (C12 - C17) give soap on hydrolysis with a base.

Soap on hydrolysis with a base. Soaps are sodium or potassium salts of Carboxylic acids with high molecular mass (C12-C17). The carboxylic acid is obtained by acidification of the salt with mineral acid (H2SO4 or HCl).

Formation of Carboxylic Acid


Ester Synthesis

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Esters are prepared by the acylation of alcohols or phenols.

Esterification

The formation of esters is known as esterification.

Esterification of carboxylic acids with alcohols requires a mineral acid such as concentrated H2SO4 or HCl gas a catalyst.


Fischer Esterification

Fischer Esterification

The esterification of carboxylic acids with alcohols is a kind of nucleophilic acyl substitution. The first step is the protonation of the carbonyl oxygen, which activates the carbonyl group towards nucleophilic addition of the alcohol.

Esterification of Carboxylic Acid
The tetrahedral intermediate, which is formed transfers a proton converting the hydroxyl group into OH2+ group. This species OH2+ is a better leaving group and is eliminated at a neutral water molecule.
Esterification of Carboxylic Acids

The protonated ester finally losses a proton to give the ester.

The above mechanism is supported by the using isotopically labeled methanol (CH3O18H) with acetic acid to give methyl acetate (having labeled oxygen) and water not containing any isotopic oxygen.

Mechanism of esterification of carboxylic acids


Esters of phenols are prepared by reversible acylation of phenols with acyl chloride or anhydrides rather than the reaction with carboxylic acid in which all the steps are reversible.

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Alcoholysis of Ester

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Esters react with alcohols in the presence of an acid catalyst to undergo exchange of alcohol resides i.e., alkoxy parts. The equilibrium mixture consists of the reactants and a new ester and a new alcohol. The reaction involves nucleophilic acyl substitution of the alkoxy group of the ester with the alkoxy group of the alcohol and is known as trans esterification.

Transesterification

Esters react with ammonia and amines to form amides

Amide

The acyl group of the ester is reduced with LiAlH4 (but not with NaBH4) to a primary alcohol.

Catalytic Hydrogenation of Ester

Catalytic hydrogenation of esters to alcohols is not easy unlike that of aldehydes and ketones. The reaction requires high temperature and pressure. The catalyst used is a mixture of oxides known as copper chromite. The alkoxy part of the ester gives the corresponding alcohol as by product.

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