Chapter 13: Carboxylic Acids – Chemistry 2nd Year Notes

This chapter covers Carboxylic Acids from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). Carboxylic acids contain the carboxyl group (-COOH), a combination of a carbonyl and a hydroxyl group on the same carbon. They are the most acidic of the common organic compounds and are found everywhere, from vinegar to citrus fruits.

Here you will learn why carboxylic acids are acidic, how they are prepared, and their reactions, many of which give important derivatives such as esters, acid chlorides and amides.

Learning Objectives

• Identify the carboxyl group and name simple carboxylic acids.
• Explain why carboxylic acids are acidic using resonance.
• Describe methods of preparing carboxylic acids.
• Describe the reactions of carboxylic acids and their derivatives.
• Explain esterification and its reverse, hydrolysis.
• State the important uses of carboxylic acids.

Key Concepts

The Carboxyl Group and Acidity

The carboxyl group is the functional group of carboxylic acids. These compounds are acidic because, when a proton is lost, the resulting carboxylate ion is stabilised by resonance: the negative charge is shared equally between the two oxygen atoms, and the two carbon to oxygen bonds become identical. This spreading of charge makes the ion stable and the loss of the proton favourable, so carboxylic acids are far more acidic than alcohols or phenols, though still weak acids. The order of acidity is carboxylic acid greater than phenol greater than water greater than alcohol.

Preparation

Carboxylic acids are prepared by the oxidation of primary alcohols or aldehydes, by the hydrolysis of nitriles, and by treating a Grignard reagent with carbon dioxide followed by hydrolysis, which adds one carbon to the chain.

Reactions and Derivatives

As acids they react with metals, bases and carbonates to form salts; the reaction with sodium carbonate releases carbon dioxide, a useful test. The remaining reactions replace the hydroxyl of the carboxyl group with another group to give a derivative: with an alcohol and an acid catalyst they undergo esterification to form an ester and water; with phosphorus pentachloride or thionyl chloride they form acyl chlorides; with ammonia, through the ammonium salt on heating, they form amides. Strong reducing agents such as lithium aluminium hydride reduce them to primary alcohols, and heating their sodium salts with soda-lime causes decarboxylation, removing carbon dioxide to give an alkane.

Uses

Ethanoic acid (vinegar) is a food preservative and a raw material; methanoic acid is used in leather and rubber processing; benzoic acid and its salts are food preservatives; and long-chain fatty acids make soaps. Esters of carboxylic acids are used as flavourings and perfumes.

Important Definitions

Formulas & Rules

Item	Fact
Functional group	carboxyl (carbonyl plus hydroxyl)
Acidity	carboxylate ion is resonance-stabilised
Acidity order	carboxylic acid greater than phenol greater than water greater than alcohol
Test	with sodium carbonate gives carbon dioxide effervescence
Derivatives	ester (alcohol), acyl chloride (PCl5), amide (ammonia)

Diagrams & Illustrations

Acidity of carboxylic acids: a diagram showing R-COOH losing a proton to form the carboxylate ion, whose negative charge is spread over both oxygen atoms by resonance, giving stability and acidity.

Reactions of carboxylic acids: a map showing salt formation, esterification, formation of acyl chlorides and amides, reduction and decarboxylation.

Solved Examples & Numericals

Example 1: Why acidic

Why is ethanoic acid more acidic than ethanol? When ethanoic acid loses a proton, the carboxylate ion is stabilised by resonance; ethanol alkoxide has no such stabilisation, so ethanoic acid is much more acidic.

Example 2: Distinguishing test

How can you distinguish a carboxylic acid from a phenol? Add sodium carbonate; a carboxylic acid gives brisk effervescence of carbon dioxide, while a phenol does not react.

Example 3: Esterification

Write the product when ethanoic acid reacts with ethanol. With a little concentrated sulphuric acid they form the ester ethyl ethanoate and water.

Short Questions & Answers

Long Questions & Answers

Q1: Explain why carboxylic acids are acidic.

Carboxylic acids are acidic because they readily give up the hydrogen of their hydroxyl group as a proton, and the ion left behind is unusually stable. When a carboxylic acid loses a proton it forms a carboxylate ion, in which the negative charge is not fixed on one oxygen but is delocalised by resonance so that both oxygen atoms share it equally, and the two carbon to oxygen bonds become identical. This spreading of the charge lowers the energy of the ion and makes it stable, so the acid loses its proton easily. In contrast, an alcohol losing a proton gives an alkoxide with its charge fixed on one oxygen and no resonance, which is why alcohols are essentially neutral. Phenols lie in between. This gives the order carboxylic acid greater than phenol greater than water greater than alcohol. Even so, carboxylic acids are still weak acids and only partly ionise.

Q2: Describe the important reactions of carboxylic acids.

Carboxylic acids react both as acids and by forming derivatives. As acids they turn blue litmus red and react with reactive metals, bases and carbonates to form salts; the reaction with sodium carbonate releases carbon dioxide, used as a test. The other reactions replace the hydroxyl of the carboxyl group. With an alcohol and an acid catalyst they undergo esterification to form a sweet-smelling ester and water, a reversible reaction whose reverse is hydrolysis. With phosphorus pentachloride or thionyl chloride they form reactive acyl chlorides. With ammonia, through the ammonium salt on heating, they form amides. Strong reducing agents reduce them to primary alcohols. Finally, heating their sodium salts with soda-lime causes decarboxylation, removing carbon dioxide to leave an alkane with one fewer carbon.

Q3: What is esterification? Discuss the reaction and its importance.

Esterification is the reaction in which a carboxylic acid combines with an alcohol, in the presence of a small amount of a strong acid catalyst such as concentrated sulphuric acid, to form an ester and water; for example ethanoic acid and ethanol give ethyl ethanoate. The reaction is reversible, so it never goes fully to completion; the yield can be improved by using an excess of one reactant or by removing the water or ester. The reverse reaction, in which an ester is split by water back into an acid and an alcohol, is called hydrolysis, and when carried out with alkali it is called saponification, the basis of soap making. Esterification is important because esters have pleasant fruity smells and are used as artificial flavourings and perfumes, as solvents, and as the basis of fats, oils and many polymers.

MCQs with Answers

Quick Revision Summary

• Carboxyl group; acidic because the carboxylate ion is resonance-stabilised.
• Acidity: carboxylic acid greater than phenol greater than water greater than alcohol.
• Preparation: oxidise primary alcohol or aldehyde; hydrolyse nitrile; Grignard plus carbon dioxide.
• Reactions: salts (carbon dioxide with sodium carbonate), esterification, acyl chloride, amide, reduction, decarboxylation.
• Esterification is reversible; the reverse is hydrolysis (with alkali it is saponification).
• Uses: vinegar, preservatives, soaps, flavours and perfumes.

Exam Tips

• Explain acidity with the resonance of the carboxylate ion; a guaranteed question.
• Sodium carbonate effervescence distinguishes acids from phenols and alcohols.
• Learn the derivative reactions as replacing the hydroxyl: ester, acyl chloride, amide.
• Esterification is reversible; know both directions and the catalyst.
• Grignard plus carbon dioxide adds one carbon to give an acid.