Carboxylic Acids (Triple only)

Introduction: What Are Carboxylic Acids?

Carboxylic acids are a family of organic compounds that all contain the carboxyl group, written as –COOH.

This functional group is what gives the molecules their acidity and their characteristic reactions.

Breaking down the –COOH group

• The carbon atom forms:
  
  • A double bond with an oxygen atom.
  • A single bond with an –OH group.

The carboxyl functional group consists of a carbon atom bonded to one oxygen by a double bond and to an –OH group by a single bond. This combination is responsible for the acidic behaviour of carboxylic acids.

Names of carboxylic acids end in “-anoic acid” and begin with the standard chain prefixes you already know (meth-, eth-, prop-, but-).

Name	Formula	Structure (displayed formula)
Methanoic acid	HCOOH
Ethanoic acid	CH₃COOH
Propanoic acid	C₂H₅COOH
Butanoic acid	C₃H₇COOH

A comparison of the displayed structures of the four simplest carboxylic acids, showing how the –COOH group attaches to progressively longer carbon chains.

Key idea:

When writing or recognising formulas for carboxylic acids, the –COOH group must remain visible.
For example, ethanoic acid must be written as CH₃COOH, not simply rearranged into an empirical formula.

How to spot one quickly

Look for:

• A –COOH functional group
  or
• A name ending in “-anoic acid”

Carboxylic acids share a set of characteristic behaviours due to the –COOH group.

Reaction with Carbonates

They react with metal carbonates to produce:

• Carbon dioxide
• Water
• A salt (called a carboxylate) ending in “-anoate”

Example Reaction:

Ethanoic acid + sodium carbonate → carbon dioxide + water + sodium ethanoate

Ethanoic acid reacting with sodium carbonate to generate carbon dioxide gas, liquid water, and the salt sodium ethanoate.

Solubility

Carboxylic acids dissolve in water, and when they do, some molecules release H⁺ ions, making the solution acidic.

Weak Acids

Carboxylic acids are weak acids because they only partly ionise in water.
Therefore, their solutions have higher pH values than solutions of strong acids at the same concentration.

Weak acids occur naturally in foods-here, citric acid contributes to the sour taste of citrus fruits and provides a mildly acidic environment.

Esters contain the functional group –COO–.

They form when a carboxylic acid reacts with an alcohol, usually in the presence of a strong acid catalyst such as concentrated sulfuric acid.

General Reaction

alcohol + carboxylic acid ⇌ ester + water

Reaction showing how an alcohol and a carboxylic acid combine under acidic conditions to produce an ester and water.

Example: Making Ethyl Ethanoate
Ethanol + ethanoic acid —(acid catalyst)→ ethyl ethanoate + water

Formation of ethyl ethanoate from ethanol and ethanoic acid. The –OH group of the acid and a hydrogen atom from the alcohol are removed to form water, while the remaining fragments join to create the ester.

General structure: 

Alcohol part + Carboxylic acid part

Rule:

1. First part (ending in –yl) → comes from the alcohol
2. Second part (ending in –anoate) → comes from the carboxylic acid

This is the opposite of how the molecules are joined, so the name must be read carefully.

Step-by-Step Naming Guide

STEP 1 - Identify the alcohol used to make the ester

Remove the –anol ending and replace it with –yl.