ester(redirected from Acetyloxy)
Also found in: Dictionary, Thesaurus, Medical.
ester, any one of a group of organic compounds with general formula RCO2R′ (where R and R′ are alkyl groups or aryl groups) that are formed by the reaction between an alcohol and an acid. For example, when ethanol and acetic acid react, ethyl acetate (an ester) and water are formed; the reaction is called esterification. Ethyl acetate is used as a solvent. Methyl acetate, formed by the reaction between methanol and acetic acid, is a sweet-smelling liquid used in making perfumes, extracts, and lacquers. Esters react with water (hydrolysis) under basic conditions to form an alcohol and an acid. When heated with a hydroxide certain esters decompose to yield soap and glycerin; the process is called saponification. Common fats and oils are mixtures of various esters, such as stearin, palmitin, and linolein, formed from the alcohol glycerol and fatty acids. Naturally occurring esters of organic acids in fruits and flowers give them their distinctive odors. Esters perform important functions in the animal body; e.g., the ester acetylcholine is a chemical transmitter of nerve stimuli.
any of a number of organic compounds, such as C2H5OCOCH3 or C5H11ONO, that are derived from acids by replacing the hydroxyl group (OH) with an alcohol, enol, or phenol radical (OR).
Esters are structural analogues of the salts of oxygen acids, but where a metal atom would be attached to the salt, a hydrocarbon group (R) is attached to the ester. The nomenclature for the salts and esters is similar; for example, NaOCOCH3 is called sodium acetate, and C2H5OCOCH3 is known as ethyl acetate. Like salts, esters form products of incomplete and complete replacement with dibasic and multibasic acids; thus, there are acid esters, such as monomethylsulfate (HOSO2OCH3), and complete (neutral) esters, such as dimethylsulfate (CH3OSO2OCH3).
Esters differ greatly from salts in their properties, however. Typical organic compounds, esters are usually volatile liquids. In some instances, they have a fruity or flowery fragrance. Virtually insoluble in water, they are readily soluble in organic solvents. Esters are hydrolyzed by the action of water to form the corresponding alcohol and acid, in accordance with the general formula
RCOOR′ + H2O ⇆ RCOOH + HOR′
The reaction is catalyzed by acids and even more so by bases. When alkaline catalysts are used, salts of acids are formed instead of free acids; the reaction is irreversible. Of the other reactions in which esters display acylating properties, the most common are transesterification, alcoholysis, and double exchange reactions. Some esters—for example, those formed by the lower aliphatic alcohols and such acids as sulfuric, trifluoroacetic, phosphoric, and phthalic acids—also have alkylating properties (seeDIMETHYL SULFATE and ).
Esters are usually obtained by esterification (see). They may also be formed by the acylation of alcohols by various acid derivatives (including acid halides and anhydrides), by the action of acid salts on alkyl halides, as in the reaction
C2H5I + AgONO → C2H5ONO + AgI
or by the action of acids on olefins, as in the reaction
HOSO2OH + CH2=CH2 → HOSO2OCH2CH3
Esters of glycerin and the higher carboxylic acids are the principal components of fats, and esters of the higher monohydric aliphatic alcohols and carboxylic acids are the main components of waxes. Other esters—mainly those of the terpene alcohols—are components of essential oils. Esters are used in technology as plastic monomers (acrylates and vinyl acetates), plasticizers for plastics (dioctyl and dibutyl phthalates), detergents (alkyl sulfates), solvents (amyl, butyl, and ethyl acetates), extraction agents and pesticides (esters of phosphoric acid), explosives (esters of nitric acid and polyhydric alcohols, such as nitroglycerin), pharmaceuticals (validol and aspirin), and fragrances (benzyl acetate and terpenyl acetate). Many esters of high molecular weight (including polyethylene terephthalate and cellulose acetates) are used in industry as plastics, lacquers, and synthetic fibers (seeALKYD RESINS and POLYESTER FIBER).
REFERENCESNesmeianov, A. N., and N. A. Nesmeianov. Nachala organicheskoi khimii, books 1–2. Moscow, 1969–70.
Chichibabin, A. E. Osnovnye nachala organicheskoi khimii, 7th ed., vol. 1. Moscow, 1963.