Carboxylic acids

THEORY OF CARBOXYLIC ACIDS

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IUPAC names carboxylic acids by replacing the ending -ane of the alkane with the same number of carbons with -oic .

nomenclature-carboxylic-acids

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Carboxylic acids are molecules with trigonal planar geometry. They have acidic hydrogen in the hydroxyl group and behave like bases on carbonyl oxygen.

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The most characteristic property of carboxylic acids is the acidity of the hydrogen located on the hydroxyl group. The pKa of this hydrogen ranges from 4 to 5 depending on the length of the carbon chain.

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Carboxylic acids can be prepared using the following methods:

♦ Oxidation of alkylbenzenes: Carboxylic acids can be obtained from alkyl-substituted benzenes by oxidation with potassium permanganate or sodium dichromate.

Oxidation of benzylic positions

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Alkanoyl halides are obtained by reacting carboxylic acids with PBr ₃ . ₂ can also be used .
Thus, ethanoic acid [1] is transformed into ethanoyl bromide [2] by reaction with phosphorus tribromide. Ethanoic acid by reaction with thionyl chloride forms compound [3]

synthesis-alkanoyl-halides

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The anhydrides are obtained by condensation of carboxylic acids with loss of water. The reaction requires strong heating and a long reaction time.

Heating butanedioic acid [1] produces butanedioic anhydride (succinic anhydride) [2] . This type of cyclization requires 5- or 6-membered rings.

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Esters are obtained by reaction of carboxylic acids and alcohols in the presence of mineral acids. The reaction is carried out in excess of alcohol to shift the equilibria to the right. The presence of water is detrimental since it hydrolyzes the ester formed.

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Lactones are cyclic esters that are obtained by intramolecular esterification from molecules containing acid and alcohol groups. This cyclization forms 5- or 6-membered cycles.

Read more: Synthesis of Lactams

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Carboxylic acids react with two equivalents of organolytic followed by aqueous hydrolysis to form ketones.

The reaction requires two equivalents of organolytic, the first deprotonates the acid group, while the second equivalent adds as a nucleophile to the carboxylic group.

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Lithium aluminum hydride reduces carboxylic acids to alcohols.

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The a of carboxylic acids are acidic and can be removed using strong bases such as LDA.

The first equivalent of LDA strips the hydrogen from the hydroxyl group (pKa = 4.7), forming the carboxylate. The second equivalent of LDA deprotonates the a- , forming the acid enolate.

A highly polar solvent (HMPA) is used to stabilize the enolate.

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The Hell-Volhard-Zelinsky reaction makes it possible to halogenate the a-position of carboxylic acids. Phosphorus-catalyzed bromine is used as the reagent. Phosphorus in the presence of bromine generates phosphorus tribromide, which is actually the compound that acts as a catalyst.

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The Hunsdiecker reaction makes it possible to prepare bromoalkanes from carboxylic acids. In this reaction, the acid is treated with silver nitrate in a basic medium, forming the silver carboxylate. In a later stage, the decarboxylation of the carboxylate occurs by treatment with bromine dissolved in carbon tetrachloride.

Read more: Hunsdiecker reaction

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Acid derivatives, alkanoyl halides, anhydrides, esters, nitriles, and amides, show important differences in reactivity toward nucleophiles.

The order of reactivity of carboxylic acids is related to the ability of the L group to transfer charge.

The greater the capacity of the L group to yield lone pairs, a lower reactivity is observed, due to the formation of a stabilizing limit structure.

The greater the weight of the latter structure, the less reactivity the corresponding acid derivative possesses.

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Acid derivatives behave like bases through carbonyl oxygen. The basicity of this oxygen depends on the resonance stabilization of the conjugate acid.

basicity-of-derivatives-of-carboxylic-acids

Let's compare the structures that stabilize the base on the halide and the amide.

basicity-of-derivatives-of-carboxylic-acids-02

basicity-of-derivatives-of-carboxylic-acids-03

As the last structure gains weight, the acid becomes more stable (weak) and therefore the base stronger. Amides are the strongest bases of all acid derivatives.

Acid derivatives feature acidic hydrogens on carbon.

Alkanoyl halides have the most acidic hydrogens in the a , while amides have the least acidic.