Salt Formation

Carboxylic acids are weak acids and their carboxylate anions are strong conjugate bases and are slightly alkaline due to the hydrolysis of carboxylate anions compared to other species, the order of acidity and basicity of corresponding conjugate bases are as follows:

Acidity RCOOH > HOH > ROH > HC ≡ CH > NH₃ > RH

Basicity RCOO– < HO– < RO– < HC ≡ C– < < R–

1. The carboxylic acids react with metals to liberate hydrogen and are soluble in both NaOH and NaHCO₃ solutions.

For example.

2CH₃COOH + 2Na → 2CH₃COO–Na⁺ + H₂

CH₃COOH + NaOH → CH₃COO–Na⁺ + H₂O

CH₃COOH + NaHCO₃ → CH₃COO–Na⁺ + H₂O + CO₂

2. Conversion into functional derivatives

(a) Conversion into acid chlorides

(b) Conversion into esters

Conversion into Esters (Esterification):

Carboxylic acid on reacting with alcohols in presence of dehydrating agent (H₂SO₄ or dry HCl gas) gives esters. The reaction is known as esterification.

This reaction is reversible and the same catalyst, hydrogen ion, that catalyzes the forward reaction, esterification, necessarily catalyzes the reverse reaction hydrolysis.

The equilibrium is particularly unfavorable when phenols (ArOH) are used instead of alcohol; yet if water is removed during the reaction, phenolic esters [RCOOAr] are obtained in high yield.

The presence of a bulky group near the site of reaction, whether in alcohol or acid, slows down esterification (as well as its reverse, hydrolysis).

Reactivity CH₃OH > 1° > 2° > 3°

In esterification HCOOH>CH₃COOH>RCH₂COOH > R₂CHCOOH > R₃CCOOH

The Mechanism of the Esterification Reaction:

The step in the mechanism for the formation of an ester from an acid and an alcohol are the reverse of the steps for the acid-catalyzed hydrolysis of an ester, the reaction can go in either direction depending on the conditions used. A carboxylic acid does not react with alcohol unless a strong acid is used as a catalyst, protonation makes the carbonyl group more electrophilic and enables it to react with the alcohol, which is a weak nucleophile.

3. Reduction

4. Substitution in alkyl or aryl group

Halogenation of Aliphatic Acids (Hell-Volhard-Zelinsky Reaction)

In the presence of phosphorus, aliphatic carboxylic acids react smoothly with chlorine or bromine to yield a compound in which α-hydrogen has been replaced by halogen.

The function of the phosphorus is ultimately to convert a little of the acid into acid halide so that it is the acid halide, not the acid itself, that undergoes this reaction.

P + X₂ → PX₃

R – CH₂ – COOH + PX₃  → RCH₂ – COX

The halogen of these halogenated acids undergoes nucleophilic displacement and elimination the same as it does in the simple alkyl halides. Halogenation is therefore the first step in the conversion of a carboxylic acid into many important substituted carboxylic acids.