2. Carboxylic Acids

Carboxylic Acids

Carboxylic acids are a family of organic compounds containing the carboxyl functional group, –COOH.

Some simple examples are:

Methanoic acid: HCOOH
Ethanoic acid: CH₃COOH
Propanoic acid: CH₃CH₂COOH
Butanoic acid: CH₃CH₂CH₂COOH

The –COOH group gives carboxylic acids their characteristic acidic properties and strongly influences their boiling points, solubility and chemical reactions.

Carboxylic acids occur naturally in many foods and biological systems and are also important industrial chemicals.

The Carboxyl Functional Group

The characteristic functional group is:

–COOH

This is called the carboxyl group.

It contains:

• one carbon atom
• two oxygen atoms
• one hydrogen atom

A simplified representation is:

R–COOH

where R represents the rest of the carbon-containing molecule.

For example:

CH₃–COOH

is ethanoic acid.

The:

–COOH

part identifies the compound as a carboxylic acid.

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Looking More Closely at –COOH

The carboxyl group contains both:

C=O

and:

O–H

The carbon atom is double-bonded to one oxygen and single-bonded to another oxygen that is bonded to hydrogen.

A structural representation is:

R–C(=O)–OH

The combination of these atoms behaves as a single functional group with characteristic chemical properties.

It is important not to confuse the –OH within –COOH with the –OH group of an alcohol. The surrounding structure changes the chemistry considerably.

Carboxylic Acids Form a Homologous Series

Carboxylic acids form a homologous series.

Members:

• contain the same –COOH functional group
• have similar chemical properties
• follow a general structural pattern
• show gradual changes in physical properties
• differ from neighbouring members by CH₂

The first few members are:

Notice that each successive member adds:

CH₂

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Naming Carboxylic Acids

Simple carboxylic acids are named from the corresponding alkane.

The ending:

-ane

is replaced with:

-anoic acid

For example:

methane → methanoic acid

ethane → ethanoic acid

propane → propanoic acid

butane → butanoic acid

pentane → pentanoic acid

The ending:

-oic acid

indicates the presence of a carboxylic acid group.

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Counting Carbon Atoms

When naming a carboxylic acid, the carbon atom inside the:

–COOH

group is included when counting the carbon atoms.

For example:

CH₃COOH

contains two carbon atoms.

Therefore:

ethanoic acid

Not methanoic acid.

Similarly:

CH₃CH₂COOH

contains three carbon atoms.

Therefore:

propanoic acid

This is a common source of mistakes.

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Worked Example: HCOOH

Count the carbon atoms.

There is:

1 carbon

One carbon gives the prefix:

meth-

The molecule contains –COOH.

Therefore:

methanoic acid

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Worked Example: CH₃COOH

Count all carbon atoms, including the carbon in –COOH.

There are:

2 carbon atoms

Prefix:

eth-

Therefore:

ethanoic acid

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Worked Example: CH₃CH₂COOH

Number of carbon atoms:

3

Prefix:

prop-

Therefore:

propanoic acid

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Worked Example: CH₃CH₂CH₂COOH

Number of carbon atoms:

4

Prefix:

but-

Therefore:

butanoic acid

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Common Names

Some carboxylic acids also have older common names that are still widely used.

For example:

methanoic acid = formic acid

ethanoic acid = acetic acid

You may encounter both names.

In systematic chemistry naming, we normally use:

methanoic acid

and:

ethanoic acid

But acetic acid is extremely common in everyday and industrial contexts.

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Acidic Properties

Carboxylic acids behave as acids because they can donate H⁺ ions when dissolved in water.

A simplified representation is:

RCOOH ⇌ H⁺ + RCOO⁻

For ethanoic acid:

CH₃COOH ⇌ H⁺ + CH₃COO⁻

The formation of H⁺ in aqueous solution gives carboxylic acids their acidic properties.

However, most simple carboxylic acids are weak acids.

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Carboxylic Acids Are Weak Acids

A weak acid only partially ionizes in water.

This means that when ethanoic acid is dissolved in water, only some of the acid molecules form ions.

We can represent this using a reversible arrow:

CH₃COOH ⇌ H⁺ + CH₃COO⁻

Most of the ethanoic acid remains as un-ionized CH₃COOH molecules.

This is different from a strong acid such as hydrochloric acid, which ionizes much more extensively in water.

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Weak Does Not Mean Dilute

This distinction is extremely important.

Weak describes the extent to which an acid ionizes.

Dilute describes the concentration of acid in a solution.

Therefore:

• a weak acid can be concentrated
• a strong acid can be dilute

These terms describe different properties.

A concentrated carboxylic acid can still be hazardous even though it is classified as a weak acid.

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pH of Carboxylic Acids

Solutions of carboxylic acids generally have:

pH < 7

because they produce H⁺ ions in water.

They can therefore:

• turn blue litmus red
• react with bases
• react with carbonates
• react with some reactive metals

Their exact pH depends on factors including:

• which acid is present
• concentration
• temperature

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Reactions with Bases

Like other acids, carboxylic acids react with bases in neutralization reactions.

General pattern:

carboxylic acid + base → salt + water

For example:

ethanoic acid + sodium hydroxide → sodium ethanoate + water

Symbol equation:

CH₃COOH + NaOH → CH₃COONa + H₂O

The salt produced from ethanoic acid is called an:

ethanoate

Naming Carboxylate Salts

When a carboxylic acid forms a salt, the ending changes.

-oic acid → -oate

Examples:

methanoic acid → methanoate

ethanoic acid → ethanoate

propanoic acid → propanoate

butanoic acid → butanoate

For example:

ethanoic acid + sodium hydroxide → sodium ethanoate + water

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Reactions with Carbonates

Carboxylic acids also react with carbonates.

The general pattern is:

acid + carbonate → salt + water + carbon dioxide

For example:

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

A key observation is:

bubbling or fizzing

because CO₂ gas is produced.

Carbon dioxide can be tested using limewater.

CO₂ causes limewater to become:

cloudy or milky

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Reactions with Metals

Carboxylic acids can react with sufficiently reactive metals.

General pattern:

acid + metal → salt + hydrogen

For example:

ethanoic acid + magnesium → magnesium ethanoate + hydrogen

Hydrogen gas can be identified using a burning splint.

A positive test produces a characteristic:

squeaky pop

Because carboxylic acids are weak acids, their reactions may be slower than comparable reactions involving strong acids of similar concentration.

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Ethanoic Acid

Ethanoic acid is one of the most familiar carboxylic acids.

Formula:

CH₃COOH

Common name:

acetic acid

Ethanoic acid is found in vinegar.

Vinegar is not pure ethanoic acid. It is an aqueous solution containing a relatively small concentration of ethanoic acid along with water and other substances.

The ethanoic acid contributes to vinegar's:

• sour taste
• characteristic smell
• acidic properties

Methanoic Acid

Methanoic acid:

HCOOH

is the simplest carboxylic acid.

Its common name is:

formic acid

Methanoic acid occurs naturally in some organisms and has historically been associated with ants—the common name "formic" comes from the Latin word for ant.

It is also used in industrial chemical processes.

Concentrated methanoic acid is corrosive and must be handled carefully.

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Carboxylic Acids in Foods

Carboxylic acids occur naturally in many foods.

Examples include:

• ethanoic acid in vinegar
• citric acid in citrus fruits
• lactic acid in fermented foods and biological systems
• malic acid in apples and other fruits
• tartaric acid in grapes
• oxalic acid in some plants

These compounds contribute to properties such as:

• sourness
• acidity
• preservation
• flavour

Citric Acid

Citric acid occurs naturally in citrus fruits such as:

• lemons
• limes
• oranges

It contributes to their sour taste.

Citric acid is also widely used in:

• foods
• beverages
• cleaning products
• chemical processes

Unlike simple monocarboxylic acids such as ethanoic acid, citric acid contains more than one –COOH group.

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Lactic Acid

Lactic acid is another important carboxylic acid.

It occurs in:

• fermented dairy products
• fermented foods
• biological metabolic processes

Lactic acid is more structurally complicated than the simple acids at the beginning of the homologous series because it contains additional functional features.

It is also important industrially, including in the manufacture of certain biodegradable polymer materials.

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Fatty Acids

Many fats and oils contain molecules built partly from long-chain fatty acids.

Fatty acids contain a:

–COOH

group attached to a long hydrocarbon chain.

For example, a simplified fatty acid structure might look like:

long hydrocarbon chain–COOH

Fatty acids are important in:

• nutrition
• cell biology
• energy storage
• soap manufacture
• industrial chemistry

This demonstrates that the carboxyl functional group appears in molecules ranging from very small acids to large biological molecules.

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Comparing Carboxylic Acids and Alcohols

Alcohols and carboxylic acids are both oxygen-containing organic compounds, but they contain different functional groups.

Alcohol

Functional group:

–OH

Example:

ethanol: CH₃CH₂OH

Carboxylic Acid

Functional group:

–COOH

Example:

ethanoic acid: CH₃COOH

Their different functional groups give them different chemical properties.

Alcohol vs Carboxylic Acid

The key chemical distinction is:

alcohols are not normally acidic in the same way as carboxylic acids.

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Why Both Can Form Hydrogen Bonds

Alcohols contain:

O–H

Carboxylic acids also contain:

O–H

Both can therefore form hydrogen bonds between molecules.

Carboxylic acids can form particularly strong intermolecular associations.

As a result, many carboxylic acids have relatively high boiling points compared with similar-sized hydrocarbons.

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Solubility in Water

Small carboxylic acids are generally quite soluble in water.

The –COOH group is polar and can interact strongly with water molecules.

Examples such as:

• methanoic acid
• ethanoic acid
• propanoic acid

have significant water solubility.

As the hydrocarbon chain becomes longer, the non-polar part of the molecule becomes more influential.

Therefore:

increasing carbon-chain length generally decreases water solubility.

This is similar to the trend seen in alcohols.

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Boiling Point Trends

Carboxylic acids can form strong intermolecular attractions, including hydrogen bonding.

As a result, they generally have relatively high boiling points.

Within the homologous series:

longer carbon chain → generally higher boiling point

because larger molecules also experience stronger intermolecular attractions overall.

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From Alcohol to Carboxylic Acid

Some alcohols can be converted into carboxylic acids through oxidation.

A very important example is:

ethanol → ethanoic acid

Simplified:

ethanol + oxygen → ethanoic acid + water

This helps explain why wine or other ethanol-containing liquids can eventually develop acidic characteristics under suitable conditions when ethanol is oxidized.

It also provides an important connection between the alcohol and carboxylic acid families.

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Oxidation of Ethanol

Ethanol:

CH₃CH₂OH

can be oxidized to ethanoic acid:

CH₃COOH

We can represent the overall change as:

CH₃CH₂OH + O₂ → CH₃COOH + H₂O

Notice the change in functional group:

–OH → –COOH

The carbon framework remains related, but the molecule becomes a different organic family with very different chemical properties.

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Carboxylic Acids and Esters

Carboxylic acids can react with alcohols to produce compounds called esters.

General reaction:

carboxylic acid + alcohol ⇌ ester + water

For example:

ethanoic acid + ethanol ⇌ ethyl ethanoate + water

Esters often have distinctive smells and are important in:

• fragrances
• flavourings
• solvents
• biological molecules
• manufacturing

This reaction will become particularly useful when connecting the different organic families together.

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Functional Groups Change Properties

Compare:

ethane: CH₃CH₃

ethanol: CH₃CH₂OH

ethanoic acid: CH₃COOH

These molecules are related to small carbon frameworks, but their functional groups cause major differences.

Ethane behaves like a hydrocarbon.

Ethanol behaves like an alcohol.

Ethanoic acid behaves like an acid.

This demonstrates a central principle of organic chemistry:

Functional groups strongly influence chemical behaviour.

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Everyday Uses of Carboxylic Acids

Carboxylic acids and their derivatives have many uses.

They are found in or used for:

• foods
• food preservation
• flavouring
• pharmaceuticals
• cosmetics
• soaps
• polymers
• cleaning products
• chemical manufacturing
• agriculture

Their usefulness comes from the wide range of molecules that can contain one or more carboxyl groups.

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Food Preservation

Acids can help preserve some foods because acidic conditions can inhibit the growth of certain microorganisms.

Vinegar has therefore been used in food preservation, including:

• pickling
• sauces
• preserved vegetables

The ethanoic acid lowers the pH of the environment.

This can make conditions less favourable for some spoilage microorganisms.

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Carboxylic Acids in Soap

Long-chain carboxylic acids, commonly called fatty acids, are important in soap chemistry.

Their salts can have:

• a water-interacting end
• a long hydrocarbon portion that interacts with oils and grease

This allows soap molecules to help disperse oily substances in water.

The chemistry of fatty acids therefore connects organic chemistry with everyday cleaning.

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Safety

Although many carboxylic acids occur naturally in foods, this does not mean that all carboxylic acids are harmless.

Concentrated acids may:

• irritate skin
• damage eyes
• cause chemical burns
• produce irritating vapours

The effect depends on:

• the particular acid
• its concentration
• amount
• exposure route

Laboratory carboxylic acids should therefore be handled using appropriate safety procedures.

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Worked Example: Identify the Functional Group

Compound:

CH₃CH₂COOH

Look for:

–COOH

Therefore:

Functional group:

carboxyl group

Organic family:

carboxylic acid

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Worked Example: Name the Acid

Compound:

CH₃CH₂CH₂COOH

Count all carbon atoms.

There are:

4

Four carbons gives:

but-

Carboxylic acid ending:

-anoic acid

Therefore:

butanoic acid

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Worked Example: Alcohol or Carboxylic Acid?

Compound A:

CH₃CH₂OH

Functional group:

–OH

Therefore:

alcohol

Compound B:

CH₃COOH

Functional group:

–COOH

Therefore:

carboxylic acid

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Worked Example: Reaction with a Carbonate

Suppose ethanoic acid is added to sodium carbonate.

Expected products:

salt + water + carbon dioxide

Therefore:

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

What would you observe?

Fizzing or bubbling

Why?

CO₂ gas is being produced.

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Worked Example: Predicting Acidity

Two liquids contain:

A: ethanol

B: ethanoic acid

Which should show typical acidic behaviour?

B: ethanoic acid

Why?

Ethanoic acid can partially ionize in water and produce H⁺ ions.

Ethanol does not behave as an acid in the same way.

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Common Mistakes

Confusing –OH with –COOH

Alcohol:

–OH

Carboxylic acid:

–COOH

They are different functional groups.

Forgetting the Carbon in –COOH

The carbon in the carboxyl group counts when naming the molecule.

CH₃COOH has two carbons, not one.

Using the –ol Ending

Alcohols end in:

-ol

Carboxylic acids end in:

-oic acid

Calling Ethanoic Acid Ethanol

Ethanol:

CH₃CH₂OH

Ethanoic acid:

CH₃COOH

They are different compounds.

Thinking Weak Means Harmless

A weak acid can still be concentrated and hazardous.

Thinking Weak Means Dilute

Weak refers to ionization.

Dilute refers to concentration.

Saying Weak Acids Do Not Ionize

They do ionize, but only partially.

Forgetting Carbon Dioxide in Carbonate Reactions

Carboxylic acid + carbonate produces:

salt + water + carbon dioxide

Assuming Every Carboxylic Acid Is Found in Food

Many occur naturally in foods, but the family includes a huge range of compounds.

Assuming Natural Means Safe

The safety of a chemical depends on its identity, concentration and exposure—not simply whether it occurs naturally.

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Key Terms

Carboxylic acid — An organic compound containing the –COOH functional group.

Carboxyl group — The –COOH functional group characteristic of carboxylic acids.

Functional group — An atom or group of atoms responsible for characteristic properties and reactions of an organic compound.

Homologous series — A family of organic compounds with the same functional group and similar chemical properties.

Methanoic acid — HCOOH, the simplest carboxylic acid.

Formic acid — Common name for methanoic acid.

Ethanoic acid — CH₃COOH, a common carboxylic acid found in vinegar.

Acetic acid — Common name for ethanoic acid.

Propanoic acid — CH₃CH₂COOH, a three-carbon carboxylic acid.

Butanoic acid — CH₃CH₂CH₂COOH, a four-carbon carboxylic acid.

Weak acid — An acid that only partially ionizes in aqueous solution.

Ionization — Formation of ions from molecules in solution.

Hydrogen ion — H⁺, associated with acidic behaviour in aqueous solutions.

Carboxylate ion — The negative ion formed when a carboxylic acid loses H⁺.

Carboxylate salt — A salt formed from a carboxylic acid.

Neutralization — Reaction between an acid and a base producing a salt and usually water.

Carbonate — A substance containing the carbonate ion, CO₃²⁻.

Hydrogen bonding — A relatively strong intermolecular attraction involving hydrogen attached to a strongly electronegative atom.

Solubility — The ability of a substance to dissolve in a solvent.

Oxidation — A chemical process that can convert certain alcohols into carboxylic acids.

Ester — An organic compound that can be produced by reaction between a carboxylic acid and an alcohol.

Esterification — Reaction between a carboxylic acid and an alcohol to form an ester and water.

Fatty acid — A carboxylic acid containing a relatively long hydrocarbon chain.

Citric acid — A carboxylic acid occurring naturally in citrus fruits.

Lactic acid — A carboxylic acid important in biological and fermentation processes.

Biodegradable — Capable of being broken down biologically under suitable conditions.

Concentrated solution — A solution containing a relatively large amount of solute per volume.

Dilute solution — A solution containing a relatively small amount of solute per volume.

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Key Takeaways

• Carboxylic acids contain the –COOH functional group.
• –COOH is called the carboxyl group.
• The carboxyl group contains both C=O and O–H bonds.
• Carboxylic acids form a homologous series.
• Successive members differ by CH₂.
• Simple carboxylic acid names end in -oic acid.
• Methanoic acid contains one carbon.
• Ethanoic acid contains two carbons.
• Propanoic acid contains three carbons.
• Butanoic acid contains four carbons.
• The carbon inside –COOH must be counted when naming the compound.
• Methanoic acid is also called formic acid.
• Ethanoic acid is also called acetic acid.
• Carboxylic acids produce H⁺ ions in aqueous solution.
• Most simple carboxylic acids are weak acids.
• Weak acids ionize only partially.
• Weak does not mean dilute.
• Carboxylic acid solutions generally have pH values below 7.
• Carboxylic acids react with bases to form salts and water.
• Their salts are called carboxylates.
• Carboxylic acids react with carbonates to produce salt, water and carbon dioxide.
• They can react with reactive metals to produce a salt and hydrogen.
• Small carboxylic acids can form hydrogen bonds with water.
• Their water solubility generally decreases as carbon-chain length increases.
• Alcohols contain –OH, while carboxylic acids contain –COOH.
• Alcohols do not normally show the same acidic behaviour as carboxylic acids.
• Ethanol can be oxidized to ethanoic acid.
• Carboxylic acids can react with alcohols to form esters.
• Ethanoic acid occurs in vinegar.
• Citric acid occurs in citrus fruits.
• Lactic acid occurs in biological and fermented systems.
• Fatty acids are long-chain carboxylic acids.
• Carboxylic acids have important applications in foods, medicines, cleaning products, polymers and chemical manufacturing.
• Functional groups are central to understanding organic chemistry because they strongly influence molecular properties and reactions.

The most important structural distinction is:

Alcohol: –OH

Carboxylic acid: –COOH

And the naming pattern is:

alkane → alkanoic acid

For example:

methane → methanoic acid

ethane → ethanoic acid

propane → propanoic acid

butane → butanoic acid

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Check Your Understanding

1. What functional group identifies a carboxylic acid?

2. What is the name of the –COOH group?

3. Identify the two different oxygen-containing bonds present in –COOH.

4. Name HCOOH.

5. Name CH₃COOH.

6. Name CH₃CH₂COOH.

7. Name CH₃CH₂CH₂COOH.

8. Why must the carbon atom in –COOH be included when naming the molecule?

9. What is the common name of methanoic acid?

10. What is the common name of ethanoic acid?

11. What food product commonly contains ethanoic acid?

12. What does it mean to describe a carboxylic acid as a weak acid?

13. Explain why "weak" and "dilute" do not mean the same thing.

14. What happens when a carboxylic acid reacts with a base?

15. Complete:

carboxylic acid + base → ______ + ______

16. Complete:

carboxylic acid + carbonate → ______ + ______ + ______

17. What gas is produced when a carboxylic acid reacts with a carbonate?

18. How could you test this gas?

19. Complete:

carboxylic acid + reactive metal → ______ + ______

20. What gas is produced in this reaction?

21. Explain the difference between the functional groups of alcohols and carboxylic acids.

22. Which is acidic in aqueous solution: ethanol or ethanoic acid?

23. Why are small carboxylic acids soluble in water?

24. Describe how water solubility changes as the carbon chain becomes longer.

25. What carboxylic acid can be produced by oxidizing ethanol?

26. What type of compound can form when a carboxylic acid reacts with an alcohol?

27. Give three examples of carboxylic acids found in foods or biological systems.

28. What is a fatty acid?

29. Explain why a concentrated weak carboxylic acid can still be hazardous.

30. Challenge: Consider the four compounds:

A: CH₃CH₂OH

B: CH₃COOH

C: CH₃CH₂COOH

D: CH₃CH₂CH₂OH

a. Which compounds are alcohols?
b. Which compounds are carboxylic acids?
c. Identify the functional group in A.
d. Identify the functional group in B.
e. Name compound A.
f. Name compound B.
g. Name compound C.
h. Name compound D.
i. Which compound is found in vinegar?
j. Which compounds would show typical acidic behaviour in water?
k. Predict what would happen if sodium carbonate were added to B.
l. Name the gas produced.
m. Describe a test for this gas.
n. Explain why B is described as a weak acid.
o. Explain why B and A have different chemical properties even though both contain oxygen and hydrogen.
p. Describe how A could be chemically converted into B.
q. State the type of compound that could form if A reacted with B.
r. Explain why functional groups are useful for predicting the chemical behaviour of organic compounds.