Ester

In chemistry, esters are organic compounds in which an organic group (symbolized by R' in this article) replaces a hydrogen atom (or more than one) in a hydroxyl group. An oxygen acid is an acid whose molecule has an -OH group from which the hydrogen (H) can dissociate as an H+ ion.

The most common esters are the carboxylate esters, where the acid in question is a carboxylic acid. For example, if the acid is acetic acid, the ester is called an acetate. Stable esters such as carbamates, RO(CO)NHR', and dialkyl carbonates, RO(CO)OR, can be formed from unstable carbamic acid or carbonic acid, respectively. Esters may also be formed with inorganic acids; for example, dimethyl sulfate is an ester, and sometimes called "sulfuric acid, dimethyl ester".

Esters are named similarly to salts; although they don't really have cations and anions, the terminology follows the same pattern: a more electropositive part followed by a more electronegative part.

An ester can be thought of as a product of a condensation reaction of an acid (usually an organic acid) and an alcohol (or phenol compound), although there are other ways to form esters. Condensation is a type of chemical reaction in which two molecules are joined together and eliminate a small molecule, in this case two-OH groups are joined eliminating a water molecule. A condensation reaction to form an ester is called esterification. Esterification can be catalysed by the presence of H+ ions. Sulfuric acid is often used as a catalyst for this reaction. The name ester is derived from the German Essig-Äther, an old name for acetic acid ethyl ester (ethyl acetate).

　
有機酸與醇類可進行脫水反應產生酯類，此反應稱為酯化 ( esterification )。


酯類是芳香氣味的液體，有許多水果香或植物體上的酯香，都可用來製備人工芳香劑

和香水，酯也是漆料和合成塑膠的極佳溶劑。

(1)酯的生成:酸中的OH轉變成OR所得之衍生物稱為酯

(2)無機酸的酯:濃硝酸與甘油(丙三醇)生成的酯稱為硝化甘油,可作為炸藥的原料

(3)酯的性質:

　A.低分子量的羧酸與醇所形成的酯,具有水果香味且揮發性大,常用來作香料及人造調味品,並且是優良溶劑.

　　[例]乙酸戊酯俗稱香蕉油,為食品香料及溶劑

　B.難溶於水,比重小於1,無氫鍵,沸點熔點較同分子量酸低.

　C.酯與水緩慢反應分解成酸和醇,這種反應稱為加水分解(即水解)