11.4 Reactions and Acidity of Carboxylic Acids | Carbonyl Compounds | Chemistry 12

1. General Types of Carboxylic Acid Reactions

Carboxylic acids undergo a variety of reactions, which can be broadly categorized into three types based on the part of the molecule involved:

Reactions involving the hydrogen atom of the -OH group: These typically lead to salt formation and highlight the acidic properties of carboxylic acids.
Reactions involving the replacement of the -OH group: These reactions lead to the formation of carboxylic acid derivatives.
Reactions involving the carboxyl group as a whole: These include reduction and decarboxylation reactions.

2. Reactions Involving the H-atom of the Carboxyl Group (Acidic Properties)

Carboxylic acids are generally weaker acids compared to mineral acids. When dissolved in water, they partially dissociate to produce a hydrogen ion ( $H^{+}$ ) and a carboxylate ion:

$R C O O H + H_{2} O ⇌ R C O O^{-} + H_{3} O^{+}$

$R C O O H ⇌ R C O O^{-} + H^{+}$

The acidic nature arises from the donation of a proton ( $H^{+}$ ) and the subsequent stabilization of the conjugate base (carboxylate ion) through resonance.

Oxidation Reduction Concepts→

3. Reactions Involving the -OH Group (Preparation of Carboxylic Acid Derivatives)

In these reactions, the -OH group of the carboxylic acid is replaced by another group, leading to the formation of various carboxylic acid derivatives such as acyl halides, esters, and amides. This process often involves nucleophilic addition to the carbonyl carbon, followed by the displacement of the -OH group.

3.1. Preparation of Acyl Chlorides

Acyl chlorides are important intermediates in organic synthesis. They can be prepared by treating carboxylic acids with various chlorinating agents.

Reaction with Thionyl Chloride ( $S O C l_{2}$ )

Acyl chlorides are efficiently prepared by reacting a carboxylic acid with thionyl chloride ( $S O C l_{2}$ ) in the presence of a base like pyridine. This reaction is favored because the byproducts ( $S O_{2}$ and $H C l$ ) are gases and escape, driving the reaction to completion.

Reaction with Phosphorus Halides

Carboxylic acids react with phosphorus halides to form acyl chlorides.

With Phosphorus(III) Chloride ( $P C l_{3}$ )

Phosphorus(III) chloride reacts with carboxylic acids at room temperature to produce acyl chloride and phosphorus acid.

$3 C H_{3} C O O H + P C l_{3} \to 3 C H_{3} C O C l + H_{3} P O_{3}$ ethanoic acid ethanoyl chloride

With Phosphorus(V) Chloride ( $P C l_{5}$ )

Phosphorus(V) chloride reacts with carboxylic acids in a cold state to produce acyl chloride, phosphorus oxychloride ( $P O C l_{3}$ ), and hydrogen chloride gas ( $H C l$ ).

$C H_{3} C O O H + P C l_{5} \to C H_{3} C O C l + P O C l_{3} + H C l$ ethanoic acid ethanoyl chloride

4. Oxidation Reactions of Carboxylic Acids

While most carboxylic acids are resistant to oxidation at the carbonyl carbon, some specific carboxylic acids, particularly those with an $α$ -hydrogen or specific structures, can undergo further oxidation. Methanoic acid (formic acid) and ethanedioic acid (oxalic acid) are notable examples.

4.1. Oxidation of Methanoic Acid ( $H C O O H$ )

Methanoic acid contains both a carboxyl group and an aldehyde-like hydrogen atom, making it susceptible to oxidation.

(i) With Tollen's Reagent

Methanoic acid is oxidized to carbon dioxide and water by Tollen's reagent (ammoniacal silver nitrate), producing a characteristic silver mirror.

$H C O O H + 2 O H^{-} + 2 A g (N H_{3})_{2}^{+} \to C O_{2} + 2 H_{2} O + 2 A g ↓ + 4 N H_{3}$

(ii) With Fehling's Solution

Methanoic acid is also oxidized to carbon dioxide and water by Fehling's solution (a copper(II) complex), resulting in the formation of red precipitates of copper(I) oxide ( $C u_{2} O$ ).

$H C O O H + 4 O H^{-} + 2 C u^{2 +} \to C O_{2} + 3 H_{2} O + C u_{2} O ↓$

4.2. Oxidation of Ethanedioic Acid ( $(C O O H)_{2}$ )

Ethanedioic acid can be oxidized to carbon dioxide.

(iii) With Acidified Potassium Manganate(VII) ( $K M n O_{4}$ )

Ethanedioic acid is oxidized to carbon dioxide by warm potassium manganate(VII) solution acidified with dilute sulfuric acid. In this reaction, the purple color of $K M n O_{4}$ disappears as it is reduced to colorless manganese(II) ions ( $M n^{2 +}$ ).

$5 (C O O H)_{2} + 2 K M n O_{4} + 3 H_{2} S O_{4} \to 10 C O_{2} + 2 M n S O_{4} + K_{2} S O_{4} + 8 H_{2} O$

Oxidizing And Reducing Agents→

5. Comparing Strengths of Carboxylic Acids, Phenols, and Alcohols

The relative strengths of weak acids are quantitatively compared using their $p K_{a}$ values. A smaller $p K_{a}$ value indicates a stronger acid. The acidic strength of a compound is directly related to the stability of its conjugate base formed after donating a proton.

5.1. Acid-Base Equilibria and $p K_{a}$ Values

Let's compare ethanoic acid, phenol, and ethanol:

Compound	Dissociation Reaction	$p K_{a}$ Value
Ethanoic Acid ( $C H_{3} C O O H$ )	$C H_{3} C O O H ⇌ C H_{3} C O O^{-} + H^{+}$	4.76
Phenol ( $C_{6} H_{5} O H$ )	$C_{6} H_{5} O H ⇌ C_{6} H_{5} O^{-} + H^{+}$	10
Ethanol ( $C H_{3} C H_{2} O H$ )	$C H_{3} C H_{2} O H ⇌ C H_{3} C H_{2} O^{-} + H^{+}$	$\approx 16$

5.2. Stability of Conjugate Bases (Resonance)

The differences in $p K_{a}$ values are explained by the stability of the conjugate bases:

Ethanoate Ion ( $C H_{3} C O O^{-}$ ):

The ethanoate ion is highly stabilized due to resonance. The negative charge is delocalized over the two highly electronegative oxygen atoms within the entire carboxylate ( $C O O^{-}$ ) group. This effective charge delocalization makes it a very stable conjugate base.

Figure 11.4: Resonance stabilization of carboxylate ion.

Phenoxide Ion ( $C_{6} H_{5} O^{-}$ ):

The phenoxide ion is also stabilized by resonance. The negative charge is delocalized into the benzene ring. However, this delocalization occurs over less electronegative carbon atoms of the ring. This makes the phenoxide ion less stable than the ethanoate ion. Consequently, phenols are less acidic than carboxylic acids.

Ethoxide Ion ( $C H_{3} C H_{2} O^{-}$ ):

The ethoxide ion has no resonance stabilization. The negative charge is localized solely on the single oxygen atom. Furthermore, the electron-donating alkyl group ( $C H_{3} C H_{2}$ ) destabilizes the negative charge by intensifying it. Therefore, it is highly unstable.

5.3. Order of Acidic Strength

Based on the $p K_{a}$ values and the stability of their conjugate bases, the acidic strength of these compounds follows this order:

$Ethanoic Acid > Phenol > Ethanol$

6. Reduction of Carboxylic Acids

Carboxylic acids can be reduced to primary alcohols using strong reducing agents such as lithium aluminium hydride ( $L i A l H_{4}$ ).

$R C O O H + 4 [H] LiAlH_{4} R C H_{2} O H + H_{2} O$

Compound

Dissociation Reaction

p K_{a}

Value

Ethanoic Acid (

C H_{3} C O O H

)

C H_{3} C O O H ⇌ C H_{3} C O O^{-} + H^{+}

4.76

Phenol (

C_{6} H_{5} O H

)

C_{6} H_{5} O H ⇌ C_{6} H_{5} O^{-} + H^{+}

Ethanol (

C H_{3} C H_{2} O H

)

C H_{3} C H_{2} O H ⇌ C H_{3} C H_{2} O^{-} + H^{+}

\approx 16