Carboxylic acids and derivatives

Chemistry (9701) · Cambridge International AS Level · 6 min read · Updated: 28 Mar 2026

Welcome to the World of Carboxylic Acids!

In this chapter, we are going to explore a group of organic compounds that you probably interact with every single day. Have you ever tasted the sour zing of vinegar in a salad? That’s ethanoic acid. Have you smelled the sweet scent of a ripe pear or a strawberry? Those are esters, which are derivatives of carboxylic acids.

By the end of these notes, you’ll understand how to make these molecules, how they behave as acids, and how they can be transformed into sweet-smelling esters. Don’t worry if organic chemistry feels like a lot of symbols at first—we will break it down step-by-step!

1. What are Carboxylic Acids?

Carboxylic acids contain the carboxyl functional group: -COOH.
This group is a combination of a carbonyl group ($ C=O $) and a hydroxyl group ($ -OH $) both attached to the same carbon atom.

Prerequisite Check: Remember that in organic chemistry, Carbon always wants to form 4 bonds. In the $ -COOH $ group, the carbon uses two bonds for the $ O $, one for the $ OH $, and one to attach to the rest of the molecule.

How to Make Carboxylic Acids

According to your syllabus, there are three main ways you need to know to produce these acids:

A. Oxidation of Primary Alcohols or Aldehydes
If you take a primary alcohol (like ethanol) and "cook" it with a strong oxidising agent, it turns into a carboxylic acid.
• Reagents: Acidified $ K_2Cr_2O_7 $ (Potassium Dichromate) or acidified $ KMnO_4 $ (Potassium Manganate).
• Conditions: You must use reflux.
Analogy: Think of Reflux like a "closed loop" kitchen. You boil the liquid, the vapors rise, hit a cold tube, turn back into liquid, and fall back into the pot. This ensures the alcohol stays in contact with the oxidising agent long enough to fully oxidise into an acid without escaping as a gas.

B. Hydrolysis of Nitriles
Nitriles are compounds with a $ -C \equiv N $ group. You can break them down using water (hydrolysis).
• Acid Hydrolysis: Heat with dilute acid (like $ HCl $). The nitrile turns directly into a carboxylic acid.
• Alkaline Hydrolysis: Heat with dilute alkali (like $ NaOH $). This first forms a salt (like $ RCOO^-Na^+ $), so you must add a dilute acid afterward (acidification) to get your final carboxylic acid.

C. Hydrolysis of Esters
Esters can be split back into the acid they came from by heating them with dilute acid or alkali. Just like nitriles, if you use an alkali, you must finish with acidification to get the acid.

Quick Review: To get a carboxylic acid, you can either "add oxygen" (oxidation) or "add water to break a bond" (hydrolysis).

2. The Chemical Behavior of Carboxylic Acids

As the name suggests, these molecules are acids. However, they are weak acids. This means they only partially ionise in water.
$ RCOOH \rightleftharpoons RCOO^- + H^+ $

Common Acid Reactions

Carboxylic acids do the typical "acid things" you learned in lower years, but they react more slowly than strong acids like $ HCl $.

1. Reaction with Reactive Metals (e.g., Sodium)
Produces a salt and hydrogen gas ($ H_2 $).
$ 2CH_3COOH + 2Na \to 2CH_3COONa + H_2 $
Observation: You will see bubbles (effervescence) of hydrogen gas.

2. Reaction with Alkalis (e.g., NaOH)
This is a simple neutralisation.
Produces a salt and water ($ H_2O $).
$ CH_3COOH + NaOH \to CH_3COONa + H_2O $

3. Reaction with Carbonates (e.g., $ Na_2CO_3 $)
Produces a salt, water, and carbon dioxide ($ CO_2 $).
Test: This is a great way to identify a carboxylic acid in a lab! If you add a carbonate and see bubbles that turn limewater cloudy, you’ve likely got a carboxylic acid.

Special Reactions: Reduction and Esterification

Reduction to Primary Alcohols
If oxidation turns an alcohol into an acid, reduction does the opposite.
• Reagent: $ LiAlH_4 $ (Lithium Tetrahydridoaluminate) in dry ether.
Common Mistake to Avoid: You cannot use $ NaBH_4 $ for this. $ NaBH_4 $ is too weak to reduce carboxylic acids; it only works for aldehydes and ketones. You need the "heavy lifter" $ LiAlH_4 $.

Esterification (Making Esters)
When you react a carboxylic acid with an alcohol, you create an ester and water.
• Reagent: Concentrated $ H_2SO_4 $ (acts as a catalyst and a dehydrating agent).
• Conditions: Heat.
Memory Aid: "Acid + Alcohol $\to$ Ester + Water". It’s a condensation reaction because a small molecule (water) is lost.

Key Takeaway: Carboxylic acids act as weak acids with metals, bases, and carbonates, but they can also be reduced back to alcohols or combined with alcohols to make sweet-smelling esters.

3. Esters: The Fragrant Derivatives

Esters have the functional group -COO-. They are famous for their fruity smells and are used in perfumes and food flavorings.

How to Name an Ester

This is where many students get confused, but there is a simple trick! An ester name has two parts:
1. The first part comes from the Alcohol (ends in -yl).
2. The second part comes from the Carboxylic Acid (ends in -oate).

Example: If you react Ethanol and Propanoic acid, you get Ethyl propanoate.

Hydrolysis of Esters (Breaking them down)

This is the reverse of making them. You add water to split the ester back into an alcohol and a carboxylic acid (or salt).

1. Acid Hydrolysis
• Reagents: Heat with dilute $ HCl $ or dilute $ H_2SO_4 $.
• Result: This is a reversible reaction. You get the Carboxylic Acid and the Alcohol.
$ RCOOR' + H_2O \rightleftharpoons RCOOH + R'OH $

2. Alkaline Hydrolysis (Saponification)
• Reagents: Heat with dilute $ NaOH $.
• Result: This is not reversible (it goes to completion). You get the Sodium Salt of the acid and the Alcohol.
$ RCOOR' + NaOH \to RCOONa + R'OH $

Did you know? This alkaline hydrolysis is the same chemical process used to make soap from fats! That's why it's called saponification.

Summary Table for Quick Review

To make a Carboxylic Acid:
• Oxidise a primary alcohol/aldehyde (Reflux + $ K_2Cr_2O_7 $)
• Hydrolyse a nitrile (Heat + Dilute acid/alkali)
• Hydrolyse an ester (Heat + Dilute acid/alkali)

To identify a Carboxylic Acid:
• Add a carbonate $\to$ see $ CO_2 $ bubbles.

To make an Ester:
• Carboxylic Acid + Alcohol (Heat + Conc. $ H_2SO_4 $)

To reduce a Carboxylic Acid:
• Use $ LiAlH_4 $ $\to$ get a primary alcohol.

Don't worry if this seems tricky at first! The key is to practice drawing the structures. Once you see where the bonds break (usually between the C and the O in the middle of the group), the reactions start to make a lot more sense. You've got this!

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the functional group found in carboxylic acids?

The functional group is the carboxyl group, represented as $ -COOH $.

How is a carboxylic acid synthesized from a primary alcohol?

By heating the primary alcohol under reflux with an oxidising agent like acidified $ K_2Cr_2O_7 $ or $ KMnO_4 $.

What are the two ways to hydrolyse a nitrile into a carboxylic acid?

Nitriles can be hydrolysed by heating with either a dilute acid or by heating with a dilute alkali followed by acidification.

What does it mean for a carboxylic acid to be a "weak acid"?

It means the acid only partially ionises in aqueous solution to form $ H^+ $ ions.

What products are formed when a carboxylic acid reacts with a reactive metal like sodium?

The reaction produces a carboxylate salt and hydrogen gas ($ H_2 $).

Why is the reaction with carbonates a useful test for carboxylic acids?

It produces carbon dioxide gas ($ CO_2 $), which can be identified by turning limewater cloudy.

Which reducing agent is used to turn a carboxylic acid back into a primary alcohol?

Lithium tetrahydridoaluminate ($ LiAlH_4 $) in dry ether.

What reagents and conditions are required for esterification?

A carboxylic acid and an alcohol heated with a concentrated sulfuric acid ($ H_2SO_4 $) catalyst.

How is the name of an ester constructed?

The first part comes from the alcohol (ending in -yl) and the second part from the carboxylic acid (ending in -oate).

What is saponification?

Saponification is the alkaline hydrolysis of an ester using a base like $ NaOH $ to produce an alcohol and a carboxylate salt.

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