11.2 Reactions of Carboxylic Acids

Introduction

In the previous section, we learned that carboxylic acids contain the carboxyl group (-COOH). This group shows the chemistry of both the carbonyl (C=O) and the hydroxyl (-OH) groups.

Because of this, carboxylic acids undergo several important types of reactions. In most reactions, the carboxyl group is retained, but the reactivity of these molecules is due to the presence of the carbonyl group.

This section explains the main reactions of carboxylic acids.

Definition

Reactions of Carboxylic Acids: The chemical reactions that carboxylic acids undergo due to the presence of the carboxyl group (-COOH).

In simple words: Carboxylic acids react in different ways because of the -COOH group.

Main Idea

The carboxyl group shows the chemistry of both the carbonyl (C=O) and the hydroxyl (-OH) groups. In most reactions, the carboxyl group is retained.

Carboxylic acids undergo the following types of reactions:

• The reaction in which the hydrogen atom of the carboxyl group (-C-O-H) is involved (salt formation).
• The reaction in which the -OH group is replaced by another group (-OH).
• The reactions involving the carboxyl group as a whole (-COOH).

This section covers the first two types of reactions.

Core Concept

Key point: Carboxylic acids react with bases to form salts and with alcohols to form esters.

Detailed Explanation

1. Reaction Involving H-Atom of the Carboxyl Group (Acidic Properties of Carboxylic Acid)

Carboxylic acids are weaker acids than mineral acids. They produce H⁺ ion when dissolved in water.

RCOOH ⇌ RCOO^- + H⁺

Example:

CH₃COOH ⇌ CH₃COO^- + H⁺

Key point: Carboxylic acids are weaker acids than mineral acids and produce H⁺ ions in water.

(a) Reactions with Bases

Carboxylic acids react with strong bases (NaOH, KOH) to form salts.

General equation:

RCOOH + NaOH → RCOONa + H₂O

Example:

CH₃COOH_(aq) + NaOH_(aq) → CH₃COONa_(aq) + H₂O_(l)

Names of the reactants and products:

Key point: Carboxylic acids react with strong bases to form salts and water.

(b) Reaction with Carbonates and Bicarbonates

Carboxylic acids decompose carbonate and bicarbonates, evolving carbon dioxide gas with effervescence.

General equations:

RCOOH + Na₂CO₃ → 2RCOONa + CO₂ + H₂O

RCOOH + NaHCO₃ → RCOONa + CO₂ + H₂O

Examples:

CH₃COOH_(aq) + Na₂CO_3(aq) → 2CH₃COONa_(aq) + CO_2(g) + H₂O_(l)

CH₃COOH_(aq) + NaHCO_3(aq) → CH₃COONa_(aq) + CO_2(g) + H₂O_(l)

Key point: Carboxylic acids react with carbonates and bicarbonates, producing carbon dioxide gas with effervescence.

(c) Reactions with Metals

Carboxylic acids react with active metals such as Na, K, Ca, Mg, etc., to form their salts with the evolution of hydrogen gas.

General equation:

2RCOOH + 2Na → 2RCOONa + H₂

Example:

2CH₃COOH_(aq) + 2Na_(s) → 2CH₃COONa_(aq) + H_2(g)

Names of the reactants and products:

Key point: Carboxylic acids react with active metals to form salts and hydrogen gas.

2. Reaction Involving the OH Group of Carboxylic Acid

Reaction with Alcohol (Preparation of Esters)

When a carboxylic acid is heated with an alcohol in the presence of concentrated sulphuric acid as a catalyst, an ester is formed.

This reaction is called the esterification reaction.

General equation:

RCOOH + R'OH Conc. H₂SO₄→ RCOOR' + H₂O

Example:

CH₃COOH + CH₃OH Conc. H₂SO₄→ CH₃COOCH₃ + H₂O

Names of the reactants and products:

Key point: Carboxylic acids react with alcohols to form esters and water. This reaction is called esterification.

Note from the source: Esters are usually significant for their fruity smells and act as flavouring agents. Such a reaction in which two molecules combine with the elimination of a small molecule like water is called a condensation reaction.

Key point: Esterification is a condensation reaction.

Summary Table

Intermediate Level

Reactions of carboxylic acids:

1. Reactions involving the H-atom:

• With bases: RCOOH + NaOH → RCOONa + H₂O
• With carbonates: RCOOH + Na₂CO₃ → 2RCOONa + CO₂ + H₂O
• With bicarbonates: RCOOH + NaHCO₃ → RCOONa + CO₂ + H₂O
• With metals: 2RCOOH + 2Na → 2RCOONa + H₂

2. Reactions involving the OH group:

- With alcohols: RCOOH + R'OH → RCOOR' + H₂O (esterification)

Key point: Carboxylic acids react with bases to form salts, with carbonates and bicarbonates to form salts and CO₂, with metals to form salts and H₂, and with alcohols to form esters.

Solved Example

Example: Complete the Following Reactions and Write the Names of Reactants and Products

1. CH₃COOH(aq) + CH₃OH(aq) → ?

Answer:

CH₃COOH + CH₃OH Conc. H₂SO₄→ CH₃COOCH₃ + H₂O

Reactants: Ethanoic acid + Methanol Products: Methyl ethanoate + Water

2. CH₃CH₂COOH(aq) + Na(s) → ?

Answer:

2CH₃CH₂COOH + 2Na → 2CH₃CH₂COONa + H₂

Reactants: Propanoic acid + Sodium Products: Sodium propanoate + Hydrogen gas

3. HCOOH(aq) + NaHCO₃(aq) → ?

Answer:

HCOOH + NaHCO₃ → HCOONa + CO₂ + H₂O

Reactants: Methanoic acid + Sodium bicarbonate Products: Sodium methanoate + Carbon dioxide + Water

Concept Assessment Exercise 11.1

Complete the following reactions and write the names of reactants and products.

1. CH₃COOH(aq) + CH₃OH(aq) → ?

Answer:

CH₃COOH + CH₃OH Conc. H₂SO₄→ CH₃COOCH₃ + H₂O

Reactants: Ethanoic acid + Methanol Products: Methyl ethanoate + Water

2. CH₃CH₂COOH(aq) + Na(s) → ?

Answer:

2CH₃CH₂COOH + 2Na → 2CH₃CH₂COONa + H₂

Reactants: Propanoic acid + Sodium Products: Sodium propanoate + Hydrogen gas

3. HCOOH(aq) + NaHCO₃(aq) → ?

Answer:

HCOOH + NaHCO₃ → HCOONa + CO₂ + H₂O

Reactants: Methanoic acid + Sodium bicarbonate Products: Sodium methanoate + Carbon dioxide + Water

Activity 11.1: Studying and Analyzing the Reactions Between Vinegar (Acetic Acid) and Two Different Substances: Sodium Metal and Sodium Carbonate

Materials Needed:

• Vinegar (acetic acid, CH₃COOH)
• Small pieces of sodium metal (Na) (Caution: Sodium metal is highly reactive and should be handled with extreme care under the supervision of a qualified instructor)
• Sodium carbonate (Na₂CO₃)
• Two small beakers or glass containers
• Droppers or pipettes
• Safety goggles and gloves

Safety Precautions:

• Ensure all students wear safety goggles, gloves, and lab aprons.
• Explain the safety hazards associated with handling sodium metal and performing reactions that release gases.
• Conduct the experiment in a fume hood or a well-ventilated area.
• Sodium metal should be handled only with tweezers or tongs and kept away from water.

Introduction to Chemical Reactions:

Briefly introduce the concept of chemical reactions, reactants, and products. Discuss the properties of vinegar (acetic acid), sodium metal, and sodium carbonate. Explain that vinegar is an acid that can react with metals and carbonates to produce different products.

Reaction 1: Vinegar with Sodium Metal

Procedure:

• Place a small piece of sodium metal in a dry beaker.
• Using a dropper, carefully add a small amount of vinegar to the beaker with sodium metal.

Observation:

• Reaction: The sodium metal reacts vigorously with vinegar, producing bubbles and fizzing.
• Gas Released: Hydrogen gas (H₂) is released, and sodium acetate (CH₃COONa) is formed in the solution.

Chemical Equation:

2Na_(s) + 2CH₃COOH_(aq) → 2CH₃COONa_(aq) + H_2(g)

Discussion Points:

• Why is hydrogen gas released?
• Discuss the safety concerns of handling sodium metal and why the reaction is exothermic (releases heat).

Reaction 2: Vinegar with Sodium Carbonate

Procedure:

• In a separate beaker, add a small amount (about a teaspoon) of sodium carbonate.
• Slowly add vinegar using a dropper until the bubbling reaction occurs.

Observation:

• Reaction: Vigorous bubbling and fizzing are observed as carbon dioxide gas (CO₂) is released.
• Gas Released: Carbon dioxide (CO₂) is released, forming water (H₂O) and sodium acetate (CH₃COONa).

Chemical Equation:

Na₂CO_3(s) + 2CH₃COOH_(aq) → 2CH₃COONa_(aq) + CO_2(g) + H₂O_(l)

Discussion Points:

• Why does carbon dioxide gas form?
• How can we identify the gas released?

Concept Assessment Exercise 11.2

1. Describe the chemical reaction for the oxidation of ethanol to ethanoic acid using acidified potassium manganate(VII).

Answer: Ethanol is oxidized to ethanoic acid using acidified potassium manganate(VII) (KMnO_frac4H₂SO₄). The reaction proceeds in two steps:

Step 1: Ethanol is oxidized to ethanal.

CH₃CH₂OH + [O] → CH₃CHO + H₂O

Step 2: Ethanal is further oxidized to ethanoic acid.

CH₃CHO + [O] → CH₃COOH

Overall reaction:

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

2. Explain the role of bacteria in the production of vinegar from ethanol.

Answer: Ethanol can be converted into vinegar (ethanoic acid) by the action of microorganisms called acetobacter in an aerobic environment. The reaction is catalysed by enzymes produced by acetobacter bacteria.

CH₃CH₂OH + O₂ → CH₃COOH + H₂O

3. Write structural formulas for the following compounds:

(a) Ethanoic acid

Answer: CH₃ - COOH

(b) Propanoic acid

Answer: CH₃CH₂ - COOH

Important Points

• - Carboxylic acids contain the -COOH group.
• - The carboxyl group shows the chemistry of both the carbonyl (C=O) and hydroxyl (-OH) groups.
• - Types of reactions:
• 1. Reactions involving the H-atom (salt formation).
• 2. Reactions involving the OH group (ester formation).
• 3. Reactions involving the carboxyl group as a whole.
• - Reactions involving the H-atom:
• - With bases: RCOOH + NaOH → RCOONa + H₂O
• - With carbonates: RCOOH + Na₂CO₃ → 2RCOONa + CO₂ + H₂O
• - With bicarbonates: RCOOH + NaHCO₃ → RCOONa + CO₂ + H₂O
• - With metals: 2RCOOH + 2Na → 2RCOONa + H₂
• - Reactions involving the OH group:
• - With alcohols: RCOOH + R'OH → RCOOR' + H₂O (esterification)
• - Esterification is a condensation reaction.
• - Esters have fruity smells and act as flavouring agents.

Remember

• - Carboxylic acids contain the -COOH group.
• - Reactions involving the H-atom:
• - With bases → salt + water
• - With carbonates → salt + CO₂ + water
• - With bicarbonates → salt + CO₂ + water
• - With metals → salt + hydrogen
• - Reactions involving the OH group:
• - With alcohols → ester + water (esterification)
• - Esterification is a condensation reaction.
• - Esters have fruity smells.
• - Examples:
• - CH₃COOH + NaOH → CH₃COONa + H₂O
• - CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O
• - 2CH₃COOH + 2Na → 2CH₃COONa + H₂
• - CH₃COOH + CH₃OH → CH₃COOCH₃ + H₂O

Common Mistakes

Quick Revision

Reactions of Carboxylic Acids:

The carboxyl group shows the chemistry of both the carbonyl (C=O) and the hydroxyl (-OH) groups. In most reactions, the carboxyl group is retained. However, the reactivity of these molecules is due to the presence of the carbonyl group. Carboxylic acids undergo the following types of reactions.

1. Reaction involving H-atom of the carboxyl group (Acidic properties of Carboxylic acid):

Carboxylic acids are weaker acids than mineral acids. They produce H⁺ ion when dissolved in water.

• RCOOH ⇌ RCOO^- + H⁺

• (a) Reactions with bases:
• Carboxylic acids react with strong bases (NaOH, KOH) to form salts.

• CH₃COOH_(aq) + NaOH_(aq) → CH₃COONa_(aq) + H₂O_(l)

• (b) Reaction with carbonates and bicarbonates:
• Carboxylic acids decompose carbonate and bicarbonates evolving carbon dioxide gas with effervescence.

• CH₃COOH_(aq) + Na₂CO_3(aq) → 2CH₃COONa_(aq) + CO_2(g) + H₂O_(l)

• CH₃COOH_(aq) + NaHCO_3(aq) → CH₃COONa_(aq) + CO_2(g) + H₂O_(l)

• (c) Reactions with metals:
• Carboxylic acids react with active metals such as Na, K, Ca, Mg etc to form their salts with the evolution of hydrogen gas.

• 2CH₃COOH_(aq) + 2Na_(s) → 2CH₃COONa_(aq) + H_2(g)

2. Reaction involving the OH group of Carboxylic acid:

Reaction with Alcohol (Preparation of Esters): When a carboxylic acid is heated with an alcohol in the presence of conc. sulphuric acid as catalyst and esters is formed and the reaction is named "Esterification reaction". Esters are usually significant for their fruity smells and act as flavouring agents. Such a reaction in which two molecules combine with the elimination of a small molecule like water is called a condensation reaction.

• CH₃COOH + CH₃OH Conc. H₂SO₄→ CH₃COOCH₃ + H₂O

• Concept Assessment Exercise 11.1:
• Complete the following reactions and write the names of reactants and products.

• CH₃COOH(aq) + CH₃OH(aq)
• CH₃CH₂COOH(aq) + Na(s)
• HCOOH(aq) + NaHCO₃(aq)

Activity 11.1: Studying and analyzing the reactions between vinegar (acetic acid) and two different substances: sodium metal and sodium carbonate.

60-Second Revision

• Carboxylic acids contain the -COOH group.
• Reactions involving the H-atom:
• With bases → salt + water
• With carbonates → salt + CO₂ + water
• With bicarbonates → salt + CO₂ + water
• With metals → salt + hydrogen
• Reactions involving the OH group:
• With alcohols → ester + water (esterification)
• Esterification is a condensation reaction.
• Esters have fruity smells.
• Examples:
• CH₃COOH + NaOH → CH₃COONa + H₂O
• CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O
• 2CH₃COOH + 2Na → 2CH₃COONa + H₂
• CH₃COOH + CH₃OH → CH₃COOCH₃ + H₂O
• Catalyst for esterification: concentrated H₂SO₄.
• ⚠️ Warning: esterification requires an acid catalyst (concentrated H₂SO₄) — without it, the reaction is very slow.