Ester : Preparation, Properties and Reaction

Ester

Esters are compounds which are formed when the hydroxylic hydrogen atom in oxygen acids is replaced by an alkyl group; the acid may be organic or inorganic. The most important esters are derived from the carboxylic acids. The general formula of the carboxylic esters is C_nH_2nO₂, which is the same as that of the carboxylic acids, and they are named as the alkyl salts of the acid, e.g.,

CH₃COOC₂H₅                                    ethyl acetate

(CH₃)₂CHCOOCH(CH₃)₂                 isopropyl isobutyrate

Carboxylic esters are formed by the action of the acid on an alcohol:

Acid  +  Alcohol   ⇌   ester  +  water

The reaction is reversible, the forward reaction being known as esterification, and the backward reaction as hydrolysis.

General method of preparation of the carboxylic esters

1.  The usual method is esterification. The reaction is always slow, but is speeded up by the presence of small amounts of inorganic acids as catalysts, e.g., when the acid is refluxed with the alcohol in the presence of 5-10 per cent concentrated sulphuric acid:

R¹COOH   +   R²OH    (H₂SO₄) ⟶     R¹COOR²   +   H₂O

Alternatively, hydrogen chloride is passed into the mixture of alcohol and acid until there is a 3 per cent increase in weight, and the mixture is refluxed (the yields are very good). This is known as the Fischer-Speier method (1895), and is more satisfactory for secondary and tertiary alcohols than the sulphuric acid method, which tends to dehydrate the alcohol to alkene.

Esterification without the use of catalysts, and starting with one mole of acid and one mole of alcohol, gives rise to about 2/3 mole of ester. The yield of ester may be increased by using excess of acid or alcohol, the cheaper usually being the one in excess. Increased yields may also be affected by dehydrating agents, e.g. concentrated sulphuric acid behaves both as a catalyst and a dehydrating agent. The same effect may be obtained by removing the water or ester from the reaction mixture by distillation, which is particularly useful for high-boiling acids and alcohols. On the other hand, the water may be removed from the reaction mixture by the addition of benzeny or carbon tetrachloride, each of which forms a binary mixture with water (and may form a ternary mixture with water and the alcohol), the azeotropic mixtures boiling at a lower temperature than any of the components.

2.  Acid chlorides or anhydrides react rapidly with alcohols to form esters:

R¹COCl   +R²OH    ⟶    R₁CO₂R²   +   HCl

(R¹CO)₂O   +R²OH    ⟶    R¹CO₂R²   +   R¹CO₂H

The reaction with tertiary alcohols is very slow, and is often accompanied by the reactions (alkene or alkyl halide formation), but by using the appropriate conditions, esters are produced. Esters of tertiary alcohols may also be conveniently prepared by means of a Grignard reagent.

3 Esters may be prepared by refluxing the silver salt of an acid with an alkyl halide in ethanolic solution:

R¹CO₂Ag   +   R²Br   ⟶    R¹CO₂R²   +   AgBr

This method is useful where direct esterification is difficult.

4. Methyl esters are very conveniently prepared by treating an acid with an ethereal solution of diazomethane:

RCO₂H   +   CH₂N₂    ⟶    RCO₂CH₃   +   N₂

General properties of the ester

The carboxylic esters are pleasant-smelling liquids or solids. The boiling points of the straight-chain isomers are higher than those of the branched-chain isomers. The boiling points of the methyl and ethyl esters are lower than those of the corresponding acid, and this is due to the fact that the esters are not associated since they cannot form intermolecular hydrogen bonds. The esters of low molecular weight are fairly soluble in water-hydrogen bonding between ester and water is possible- and the solubility decreases as the series is ascended; all esters are soluble in most organic solvents.

General reaction of esters

1.  Esters are hydrolysed by acids or alkalis:

R¹CO₂H   +   R²OH    (H₂O⁺)⟵    R¹CO₂R²    (OH^-) ⟶    R¹XO₂^-   +   R²OH

When hydrolysis is carried out with alkali, the salt of the acid is obtained, and since the alkali salts of the higher acids are soaps, alkaline hydrolysis is known as saponification (derived from Latin word mean soap); saponification is far more rapid than acid hydrolysis.

2.  Esters are converted into alcohols by the Bouveault-Blanc reduction, catalytic hydrogenation and by metallic hydrides.

3.  Esters react with ammonia to form amides. This reaction is an example of ammonolysis.

 R¹COOR²   +   NH₃    ⟶    R¹CONH₂   +R²OH

With hydrazine, esters from acid hydrazides :

R¹COOR²   +   H₂NNH₂    ⟶     R¹CONHNH₂   +   R²OH

Esters react with phosphorus pentachloride or thionyl chloride to form acyl chlorides e.g.,

R¹CO₂R²   +   PCl₅     ⟶     R¹COCl   +   POCl₃   +   R ²Cl