Carboxylic Acid Derivatives
Esters are derivatives of carboxylic acids, in which the hydroxyl hydrogen is exchanged for a carbon residue R. One of the key pathways through which esters may be obtained is the acid-catalyzed reaction of a carboxylic acid with an excess of an alcohol. This reaction is known as the Fischer esterification, named after the German chemist Emil Fischer.
The mechanism of the Fischer esterification is comparable to the addition of alcohols ROH to carbonyl compounds. In the first reaction step, the protonation of the carboxyl group's carbonyl oxygen yields a resonance-stabilized intermediate oxonium or carbenium ion.
In the second reaction step, the alcohol nucleophilically attacks the carbonyl carbon.
If an aldehyde or a ketone is applied in place of a carboxylic acid, a hemiacetal is formed in the second step. However, in the case of a carboxylic acid, the product of the second reaction step contains a carbon with three oxygens. This intermediate therefore stabilizes by the elimination of water in the third reaction step. In summary, the Fischer esterification is an addition-elimination reaction.
If the carboxyl group and the alcoholic hydroxy group are members of the same starting product, that is, if the starting product is a hydroxycarboxylic acid with an adequate distance between the carboxyl group and the alcoholic hydroxy group, Fischer esterification may proceed intramolecularly. The product is then a cyclic ester. A cyclic ester is known as a.
According to the systematic nomenclature, lactones are oxacycloalkanones. A five-membered ring lactone thus is an oxacyclopentan-2-one. The position number of the carbonyl carbon is "2", since, according to CIP rules, the ring atom with the highest mass is labeled number "1". If the term "lactone" is used in the classification of a lactone, the (longer) distance between the carbonyl carbon and the ring oxygen is indicated by a Greek symbol. Thus, while a four-membered ring lactone is a β-lactone, a five-membered ring lactone is a γ-lactone and a six-membered ring lactone is a δ-lactone. Since the ring strain would be too strong, α-lactones cannot be formed.
|Acetic acid (systematic: ethanoic acid)||Propionic acid (systematic: propanoic acid)||Butyric acid (systematic: butanoic acid)||Valeric acid (systematic: pentanoic acid)|
|Oxacyclopropan-2-one (α-lactone. Cannot be formed!)||Oxacyclobutan-2-one (β-lactone)||Oxacyclopentan-2-one (γ-lactone)||Oxacyclohexan-2-one (δ-lactone)|