Ester

Definition

Esters are organic compounds formed by the reaction of an alcohol or phenol and an acid such as a carboxylic acid. The condensation reaction releases a water molecule. The general formula of esters is: esters can also be formed with thiols (thioesters), with other organic acids, and with inorganic acids such as phosphoric acid (e.g., in nucleic acids), sulfuric acid, or nitric acid. A typical example is ethyl acetate, which is formed from ethanol and acetic acid: or methyl salicylate, which can be synthesized with methanol and salicylic acid. The sulfuric acid serves as a catalyst:

Nomenclature

For the nomenclature of esters, the alcohol part is called the residue (e.g., ethyl-) and the carboxylic acid is named as the corresponding salt (e.g., -acetate). The ester of ethanol and acetic acid is thus called ethyl acetate. There are alternative names such as “ethyl acetate“. Because the carboxylic acid is called a salt, esters can be confused with salts. The danger of confusion exists, for example, with glucocorticoids such as hydrocortisone acetate. Esters in ring structures are called lactones:

Representative

Examples of ester agents:

  • Acetylcholine
  • Acetylsalicylic acid
  • Enalapril
  • Many glucocorticoids
  • Heroin
  • Methyl salicylate
  • Nitrates such as nitroglycerin, isosorbide dinitrate
  • Vitamin C
  • Warfarin

Properties

Esters are generally stable in water. Unlike carboxylic acids, they do not react acidically and they are less polar compared to alcohols because the hydroxyl group is missing. For this reason, the corresponding boiling point is lower. Esters often have a striking, pleasant and fruity odor and are therefore used as flavoring agents. They occur naturally and are produced synthetically (“fruit esters”). Typical natural substances with ester bonds are the triglycerides (fats, fatty oils) and the waxes.

Reactions

Synthesis (esterification): in the simplest ester synthesis, the alcohol and the carboxylic acid are heated in a water bath. An acid such as sulfuric acid serves as the catalyst. A hot plate rather than a Bunsen burner is recommended for the water bath because the substances used are usually flammable. Esters are also synthesized with acid chlorides and acid anhydrides, among others (e.g. acetic anhydride). During transesterification, the alcohol group is exchanged. Synthesis of acetylsalicylic acid (aspirin): Ester hydrolysis: Esters can be hydrolyzed (cleaved) with strong bases such as sodium hydroxide and potassium hydroxide, for example. In this process, the hydroxide acts as a nucleophile. The hydrolysis of triglycerides is referred to as saponification. Salts of fatty acids are formed, which are called soaps. Ester hydrolysis is also possible with acids.

In pharmacy

Esters are components of numerous active pharmaceutical ingredients. These include ester prodrugs, with which higher absorption and bioavailability can be achieved. Many excipients used in pharmaceuticals are also esters, such as flavoring agents, in fats, fatty oils, their derivatives, and waxes.