Carboxylation

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In chemistry, carboxylation is a reaction to introduce a carboxy group into an organic compound , with carboxylic acids being formed.

Reaction mechanism

The carboxylation runs from the reaction mechanism forth mostly as a nucleophilic addition to a carbon - hetero atom (CX) - multiple bond , in this case at a carbon oxygen - double bond ( see: carbonyl group ). With a different approach, however - from the side of the second reactant - the reaction can also be assigned to the electrophilic substitutions ( see: Examples ).

Reactant

Carboxylation reagent

The carboxylation can be carried out not only with carbon dioxide as the reactant , but also with metal complexes such as nickel tetracarbonyl or phosgene (here with subsequent hydrolysis ). What all these reactants have in common is the presence of a “positive” ( electron-deficient ) carbon atom that the nucleophile attacks.

Nucleophile

As nucleophile serve electron-rich compounds such as anions (eg. B. Acetylenide and phenates ) or compounds with a charge imbalance ( dipoles , such. As alkylmagnesium halides ( Grignard compounds ) or polarized aromatics ).

Examples

Carboxylation of acetylenides

Acetylenides react with carbon dioxide in the form of dry ice to form alkynic acids :

The addition of the sodium acetylenide anion to the carbonyl carbon creates the sodium salt of an alkynoic acid .

Carboxylation of phenates

The preparation of 2-hydroxybenzoic acids is done by the carboxylation of phenates with carbon dioxide ( Kolbe-Schmitt reaction ):

Mechanism of the Kolbe-Schmitt reaction: Sodium phenolate reacts in a base- catalyzed reaction with carbon dioxide to form the sodium salt of salicylic acid (2-hydroxybenzoic acid)

Viewed from the phenol side, there is an electrophilic substitution of a hydrogen for the carboxy group on the aromatic ring.

Carboxylation of alkyl magnesium halides

Carboxylic acids are formed by the carboxylation of alkyl magnesium halides ( Grignard compounds ) at room temperature :

Reaction of tert-butyl magnesium bromide with carbon dioxide to form pivalic acid .

Carboxylation by phosgene

In the presence of a Lewis acid as a catalyst , aromatics can be carboxylated with the help of phosgene :

Reaction of aromatics with phosgene to form aromatic carboxylic acids. Aluminum chloride (AlCl 3 ) serves as the catalyst .

This reaction is based on the mechanism of electrophilic substitution . The resulting carboxylic acid chloride then has to be hydrolyzed to form the carboxylic acid and hydrogen chloride .

Biochemical carboxylation

The transfer of carboxy groups plays an important role in biochemical systems and is mediated by enzymes, so-called carboxylases ( carboxy transferases ):

Carboxylation of enol pyruvate with biotin as a prosthetic group

See also

Individual evidence

  1. ↑ a b c Entry on carboxylation. In: Römpp Online . Georg Thieme Verlag, accessed on June 13, 2014.
  2. ↑ Beyer-Walter, Textbook of Organic Chemistry, 23rd Edition, S. Hirzel Verlag 1998 ISBN 3-7776-0808-4 .