19.7 Reactivity of Carboxylic Acids | Carbonyl Compounds | Chemistry 11

This section explores the chemical behavior of carboxylic acids, focusing on the unique reactivity imparted by the carboxyl functional group.

The Carboxyl Group: A Hybrid Functional Group

The defining feature of a carboxylic acid is the carboxyl group ( $- C O O H$ ), which is a composite of two functional groups:

A carbonyl group ( $> C = O$ )
A hydroxyl group ( $- O H$ )

The close proximity of these two groups on the same carbon atom creates a unique chemical reactivity that is different from that of simple ketones/aldehydes or alcohols.

$Carboxylic Acid R - C ∣∣ O - O H \equiv Carbonyl Group R - C ∣∣ O - + Hydroxyl Group - O H$

The overall reactivity of carboxylic acids is primarily dictated by the electrophilic nature of the carbonyl carbon and the acidic nature of the hydroxyl proton. In most reactions, the core carboxyl group is retained but modified.

Types of Reactions

The reactivity of carboxylic acids can be categorized based on which part of the functional group is involved in the reaction.

1. Reactions Involving Cleavage of the O-H Bond (Acidity)

Carboxylic acids are acidic because the hydrogen atom of the hydroxyl group can be donated as a proton ( $H^{+}$ ). The resulting carboxylate anion is stabilized by resonance, which delocalizes the negative charge over both oxygen atoms.

Reaction: Deprotonation in the presence of a base.
Equation: $R - COOH + H_{2} O ⇌ R - CO O^{-} + H_{3} O^{+}$

The stability of the carboxylate ion ( $R - CO O^{-}$ ) makes the carboxylic acid significantly more acidic than an alcohol ( $R - OH$ ).

2. Reactions Involving Cleavage of the C-OH Bond (Nucleophilic Acyl Substitution)

The carbonyl carbon is electrophilic and can be attacked by nucleophiles. This often leads to the replacement of the entire $- O H$ group with the incoming nucleophile. This is a characteristic reaction of carboxylic acid derivatives.

Mechanism: Nucleophilic Acyl Substitution.
Key Feature: The $- O H$ group is converted into a better leaving group (like $- O H_{2}^{+}$ in acidic conditions) before being displaced.

Common Nucleophilic Acyl Substitution Reactions:

Reaction Type	Reagent Used	Product Formed	General Reaction
Esterification	Alcohol ( $R^{'} - OH$ ) / Acid Catalyst ( $H^{+}$ )	Ester	$R - COOH + R^{'} - OH ⇌ H^{+} R - COO R^{'} + H_{2} O$
Amide Formation	Ammonia ( $N H_{3}$ ) or Amine ( $R^{'} N H_{2}$ )	Amide	$R - COOH + R^{'} N H_{2} \to R - CONH R^{'} + H_{2} O$
Acyl Halide Formation	Thionyl Chloride ( $SOC l_{2}$ ) or $PC l_{5}$	Acyl Chloride	$R - COOH + SOC l_{2} \to R - COCl + S O_{2} ↑ + HCl ↑$

3. Reactions Involving the Entire Carboxyl Group (Reduction)

The carboxyl group is resistant to reduction but can be reduced to a primary alcohol using a strong reducing agent like Lithium Aluminum Hydride ( $LiAl H_{4}$ ).

Reagent: Lithium Aluminum Hydride ( $LiAl H_{4}$ ), followed by an aqueous workup ( $H_{3} O^{+}$ ).
Reaction: $R - COOH 1. L i A l H_{4}, Ether 2. H_{3} O^{+} R - C H_{2} OH$

Note: Milder reducing agents like $NaB H_{4}$ do not reduce carboxylic acids.

4. Decarboxylation

Carboxylic acids or their salts can undergo decarboxylation, where the carboxyl group is removed as carbon dioxide ( $C O_{2}$ ).

Reaction with Soda-lime: Heating a sodium salt of a carboxylic acid with soda-lime ( $N a O H + C a O$ ) produces an alkane. $R - COONa + NaOH C a O, Δ R - H + N a_{2} C O_{3}$

Worked Example

Problem: Predict the product of the reaction between propanoic acid ( $C H_{3} C H_{2} COOH$ ) and ethanol ( $C H_{3} C H_{2} OH$ ) in the presence of a sulfuric acid catalyst.

Solution:

This reaction is a Fischer Esterification, a type of nucleophilic acyl substitution.

Identify Reactants and Reaction Type:
- Carboxylic Acid: Propanoic acid ( $C H_{3} C H_{2} COOH$ )
- Nucleophile: Ethanol ( $C H_{3} C H_{2} OH$ )
- Catalyst: $H_{2} S O_{4}$ (provides $H^{+}$ )
- Reaction: Esterification
Write the General Equation: The general equation for esterification is: $R - COOH + R^{'} - OH ⇌ H^{+} R - COO R^{'} + H_{2} O$
Substitute the Specific Molecules and Solve:
- $R = C H_{3} C H_{2} -$ (from propanoic acid)
- $R^{'} = C H_{3} C H_{2} -$ (from ethanol)
- The product ( $R - COO R^{'}$ ) will be Ethyl Propanoate.
The balanced chemical equation is: $C H_{3} C H_{2} COOH + C H_{3} C H_{2} OH ⇌ H_{2} S O_{4} C H_{3} C H_{2} COOC H_{2} C H_{3} + H_{2} O$

Reaction Type

Reagent Used

Product Formed

General Reaction

Esterification

Alcohol (

R^{'} - OH

) / Acid Catalyst (

H^{+}

)

Ester

R - COOH + R^{'} - OH ⇌ H^{+} R - COO R^{'} + H_{2} O

Amide Formation

Ammonia (

N H_{3}

) or Amine (

R^{'} N H_{2}

)

Amide

R - COOH + R^{'} N H_{2} \to R - CONH R^{'} + H_{2} O

Acyl Halide Formation

Thionyl Chloride (

SOC l_{2}

) or

PC l_{5}

Acyl Chloride

R - COOH + SOC l_{2} \to R - COCl + S O_{2} ↑ + HCl ↑

Worked Example

Problem: Predict the product of the reaction between propanoic acid (

C H_{3} C H_{2} COOH

) and ethanol (

C H_{3} C H_{2} OH

) in the presence of a sulfuric acid catalyst.

Solution:

This reaction is a Fischer Esterification, a type of nucleophilic acyl substitution.

Identify Reactants and Reaction Type:

Carboxylic Acid: Propanoic acid ( $C H_{3} C H_{2} COOH$ )
Nucleophile: Ethanol ( $C H_{3} C H_{2} OH$ )
Catalyst: $H_{2} S O_{4}$ (provides $H^{+}$ )
Reaction: Esterification

Write the General Equation: The general equation for esterification is: $R - COOH + R^{'} - OH ⇌ H^{+} R - COO R^{'} + H_{2} O$

Substitute the Specific Molecules and Solve:

$R = C H_{3} C H_{2} -$ (from propanoic acid)
$R^{'} = C H_{3} C H_{2} -$ (from ethanol)
The product ( $R - COO R^{'}$ ) will be Ethyl Propanoate.

The balanced chemical equation is: $C H_{3} C H_{2} COOH + C H_{3} C H_{2} OH ⇌ H_{2} S O_{4} C H_{3} C H_{2} COOC H_{2} C H_{3} + H_{2} O$