An ester occurs when an alcohol reacts with an acid or a phenol to eliminate water. With carboxylic acids and alcohols, one contains the carboxylic acid esters, which occur naturally in fruits, fats, and oils. They are used as fragrances and flavorings in great variety. A well-known nitric acid ester made from glycerine and nitric acid is explosive nitroglycerine. In gun cotton, cellulose is esterified with nitric acid. Phosphoric acid esters are involved in the structure of DNA. Some representatives were used as insecticides and, unfortunately, as Warfare agents used in the First World War. Simple sulfuric acid esters such as sulfuric acid dimethyl ester (dimethyl sulfate) are highly toxic. Long-chain sulfuric acid esters are known as fatty alcohol sulfates and are used in detergents.
Esters are responsible for numerous fruit flavors.
The German chemist Gmelin coined the term ester in 1850 from the now-outdated term "vinegar ether." Based on the inorganic salts, the esters have the ending -at, ethyl acetate is an example of this. Another possibility is to give the name of the original acid, and then the alcohol involved with the ending -ester, ethyl acetate is another name for ethyl acetate.Â
How Esters React?
The ester reaction is an equilibrium reaction with a slow reaction rate so that the corresponding ester often takes days to form. Strong acids like concentrated sulfuric acid act as a catalyst and accelerate the reaction. This ester reaction summarized here is a complicated reaction mechanism that is initiated by a proton from sulfuric acid and takes place in individual steps. The proton's attack takes place on the double bond's oxygen atom in the acetic acid molecule. Through various changes within the molecules, water is split off, and the catalyst is re-formed.
The reverse reaction, i.e., the cleavage of esters with water, alkalis, or enzymes, is called saponification. In fats, the trihydric alcohol glycerine is esterified with three fatty acid molecules to form a triglyceride. If a triglyceride, which was formed from three molecules of stearic acid, is heated with sodium hydroxide solution, glycerine and the salt of the original fatty acid, soap, are obtained in saponification:
Triglyceride + 3 NaOH glycerine + sodium stearate (soap)Â
Chemical properties
-
Acid hydrolysis: Faced with heat, they decompose, regenerating the alcohol and the corresponding acid. An excess of water is used to tilt the reaction to the right this time. As mentioned, it is the inverse of esterification. -
Hydrolysis in an alkaline medium: In this case, strong hydroxides are used to attack the ester, and in this way, regenerate the alcohol. And the salt of the organic acid is formed.
Applications of Esters in everyday life
Esters also have notable applications in everyday life. Plexiglas is a rigid, transparent plastic made of long chains of esters. Dacron, a fiber that is used for fabrics, is polyester (of many esters).
- Fats and Oils
Esters are derived from glycerol and a carboxylic acid of medium or high molecular weight.
Fats, which are solid esters, and oils, which are liquid, are often called glycerides. A typical example of natural wax is produced by bees, which use it to build the honeycomb.
- Waxes
Esters made from long-chain carboxylic acids and long-chain alcohols are known as waxes. The esters of short-chain acids and alcohols are colorless liquids and are characterized by intense, fruity smells. They are components of many natural and artificial flavorings.
Final Words
Esters are only poorly soluble in water and have a lower density than water. They irritate the airways in high concentrations and have a narcotic effect. Besides their use as flavorings, they are mainly used as solvents.
