Orthoester

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General structural formula of orthoesters, R 1 = hydrogen atom or organyl radical, R 2 = organyl radical (alkyl radical, aryl radical, alkylaryl radical, etc.)
Structural formula of an ortho ester of acetic acid, R = organyl residue (alkyl residue, aryl residue, alkylaryl residue, etc.)

Orthoesters (more precisely: orthocarboxylic acid esters ) with the general formula R – C (OR ') 3 are triple esters of the orthocarboxylic acids [R – C (OH) 3 ], which do not exist in free form , the addition products of water with carboxylic acids ( hydrates ). Orthocarboxylic acids follow the Erlenmeyer rule ; the equilibria of the addition of water are clearly on the side of the carboxylic acids.

Examples of orthoesters are trimethyl orthoformate HC (OCH 3 ) 3 , triethyl orthoformate HC (OCH 2 CH 3 ) 3 , triethyl orthoacetate CH 3 C (OCH 2 CH 3 ) 3 , and mezerein or daphnetoxin (two orthoesters of benzoic acid ). In the tetrodotoxin , the structural element of a dialkyl orthocarboxylate is present as an anion.

Other carbon-containing orthoesters are orthocarbonic acid esters , quadruple esters of the likewise unstable orthocarbonic acid C (OH) 4 , such as tetramethyl orthocarbonate .

presentation

The preparation of orthoesters can be carried out by reacting nitriles with alcohols under acid catalysis:

RCN + 3 R'OH → R-C (OR ') 3 + NH 3

The orthoformic acid esters are accessible through a Williamson ether synthesis from chloroform and the sodium alcoholates of the corresponding alcohols .

In weakly acidic aqueous solutions, an orthoester becomes a carboxylic acid ester and an alcohol:

R-C (OR ') 3 + H 2 O → R-CO-O-R' + 2 R'OH

Use and properties

Orthoesters are used in organic synthesis as protective groups for carboxylic acids . The reaction of orthoesters with ketones yields ketals and carboxylic acid esters. They are also suitable for removing water, e.g. B. from reaction mixtures.

Inorganic acids can also form orthoesters, such as the orthocarbonic acid esters and the orthosilicic acid esters .

Orthoesters are chemically related to carboxylic acid esters , ethers and acetals .

Individual evidence

  1. ↑ WE Kaufmann, EE Dreger: Ethyl orthoformate In: Organic Syntheses . 5, 1925, p. 55, doi : 10.15227 / orgsyn.005.0055 ; Coll. Vol. 1, 1941, p. 258 ( PDF ).
  2. ↑ Ivan Ernest: Binding, structure and reaction mechanisms in organic chemistry , Springer-Verlag, 1972, ISBN 3-211-81060-9 , p. 193.
  3. ↑ Entry on Orthoester. In: Römpp Online . Georg Thieme Verlag, accessed on June 14, 2014.

Web links

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