Last updated: 12 October 2026
Organic chemistry becomes much easier when you stop seeing every compound as a completely new substance. Instead, look for the small part of the molecule that controls its characteristic reactions — the functional group.
In this chapter, we will follow three closely connected families. Alcohols contain –OH, carboxylic acids contain –COOH, and alcohols can react with carboxylic acids to form esters. This means the topic is not really three separate lists of facts. It is one reaction story in which one organic family leads naturally to the next.
These compounds are also familiar outside the laboratory. Ethanol is used in fuels, solvents and hand sanitisers, ethanoic acid is responsible for the acidic character of vinegar, and many small esters have distinctive smells used in perfumes and flavourings.
Try these five questions before reading the chapter. They are designed to reveal whether you can already recognise some of the important functional groups and reaction patterns.
- 5 short questions
- Instant feedback after each answer
- Explanations for every option
Recognising the Three Organic Families
A functional group is an atom or group of atoms that gives an organic compound its characteristic chemical reactions. Finding the functional group is therefore usually the best first step when you are given an unfamiliar organic formula.
| Family | Functional group | Name ending | Example |
|---|---|---|---|
| Alcohol | –OH | –ol | Ethanol, CH₃CH₂OH |
| Carboxylic acid | –COOH | –oic acid | Ethanoic acid, CH₃COOH |
| Ester | –COO– | –yl –oate | Ethyl ethanoate, CH₃COOCH₂CH₃ |
The functional group is the clue
Compare CH₃CH₂OH and CH₃COOH. Both compounds contain carbon, hydrogen and oxygen, but they behave differently because their oxygen atoms are arranged in different functional groups.
The first contains –OH, so it is an alcohol. The second contains –COOH, so it is a carboxylic acid.
Do not confuse –OH with OH⁻
The –OH group in an alcohol is covalently bonded as part of a molecule. It is not a separate hydroxide ion, OH⁻.
Ethanol, CH₃CH₂OH, is therefore a covalent molecular substance — not an ionic compound containing OH⁻ ions.
Alcohols and the –OH Functional Group
Alcohols form a homologous series containing the hydroxyl functional group, –OH. Their names end in –ol.
As the carbon chain becomes longer, the familiar prefixes meth–, eth–, prop– and but– tell us how many carbon atoms are present.
The First Four Alcohols
| Number of carbon atoms | Name | Condensed structural formula |
|---|---|---|
| 1 | Methanol | CH₃OH |
| 2 | Ethanol | CH₃CH₂OH |
| 3 | Propanol | CH₃CH₂CH₂OH |
| 4 | Butanol | CH₃CH₂CH₂CH₂OH |
Notice that the –OH group is shown explicitly. Writing ethanol as C₂H₆O gives its molecular formula, but CH₃CH₂OH is more useful when identifying the homologous series because the –OH functional group can be seen immediately.
Two Ways to Produce Ethanol
Ethanol can be produced from two very different starting materials. One method begins with glucose and uses yeast. The other begins with ethene and uses steam.
These two methods are worth comparing because the same chemical product can be made by different routes, each with its own advantages and disadvantages.
Fermentation of Glucose
During fermentation, enzymes in yeast convert glucose into ethanol and carbon dioxide.
C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g)
The important conditions are:
- glucose solution;
- yeast;
- a temperature of about 37 °C;
- an absence of oxygen.
Why is the temperature kept moderate?
Fermentation depends on enzymes in yeast. At low temperatures, enzyme-controlled reactions are slow. If the temperature becomes too high, the enzymes become denatured and stop working properly.
A temperature of about 37 °C allows fermentation to proceed reasonably quickly without damaging the enzymes.
Producing Ethanol from Ethene and Steam
Ethanol can also be manufactured by reacting ethene with steam. Water is added across the carbon–carbon double bond of ethene.
C₂H₄(g) + H₂O(g) → C₂H₅OH(g)
The process uses:
- ethene;
- steam;
- a phosphoric acid catalyst;
- a temperature of about 300 °C;
- a pressure of about 60 atm.
This is an addition reaction because the atoms from the reactants combine to form a single organic product.
Fermentation compared with the reaction of ethene and steam
| Feature | Fermentation | Ethene + steam |
|---|---|---|
| Starting material | Glucose | Ethene |
| Source | Can come from renewable plant material | Usually obtained from crude oil |
| Temperature | About 37 °C | About 300 °C |
| Pressure | Does not require high pressure | About 60 atm |
| Rate | Relatively slow | Relatively fast |
Chemical Reactions of Ethanol
Once we know that ethanol is an alcohol, we can begin to predict the reactions it undergoes. Two particularly important reactions are combustion and oxidation.
Complete Combustion of Ethanol
Ethanol burns in a sufficient supply of oxygen to form carbon dioxide and water. Energy is released, which is why ethanol can be used as a fuel.
C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)
Oxidation of Ethanol to Ethanoic Acid
Ethanol can also undergo a more controlled oxidation. When ethanol is heated with an oxidising agent such as acidified potassium manganate(VII), it is oxidised to ethanoic acid.
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
The symbol [O] represents oxygen supplied by the oxidising agent.
When acidified potassium manganate(VII) is used, its purple colour is decolourised.
Combustion is not the same as controlled oxidation
In complete combustion, ethanol is broken down to carbon dioxide and water.
In the oxidation reaction studied here, ethanol is converted into another organic compound: ethanoic acid.
Carboxylic Acids and the –COOH Functional Group
Carboxylic acids contain the carboxyl functional group, –COOH. Their names end in –oic acid.
The First Four Carboxylic Acids
| Total number of carbon atoms | Name | Condensed structural formula |
|---|---|---|
| 1 | Methanoic acid | HCOOH |
| 2 | Ethanoic acid | CH₃COOH |
| 3 | Propanoic acid | CH₃CH₂COOH |
| 4 | Butanoic acid | CH₃CH₂CH₂COOH |
Remember to count the carbon in –COOH
The carbon atom inside the carboxyl group is part of the carbon chain.
For example, CH₃COOH contains two carbon atoms altogether: one in CH₃ and one in COOH. It is therefore ethanoic acid.
Why Are Carboxylic Acids Weak Acids?
Carboxylic acids are weak acids because they only partially ionise in water.
Using ethanoic acid as an example:
CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)
Only some ethanoic acid molecules form ions. Many remain as un-ionised CH₃COOH molecules.
Weak does not mean dilute
Acid strength describes how completely an acid ionises in water. A weak acid ionises only partially.
Concentration describes how much acid is present in a given volume of solution. A weak acid can therefore still be concentrated.
Typical Reactions of Carboxylic Acids
Although carboxylic acids are weak acids, they still undergo the characteristic reactions of acids.
| Reactant | General products | Useful observation |
|---|---|---|
| Reactive metal | Salt + hydrogen | Effervescence |
| Base or alkali | Salt + water | Neutralisation occurs |
| Carbonate | Salt + water + carbon dioxide | Effervescence |
For example, ethanoic acid reacts with sodium hydroxide to form sodium ethanoate and water:
CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l)
Ethanoic acid also reacts with sodium carbonate:
2CH₃COOH(aq) + Na₂CO₃(aq) → 2CH₃COONa(aq) + H₂O(l) + CO₂(g)
Notice the salt name ethanoate. The ending of the carboxylic acid changes from –oic acid to –oate when its salt is named.
Esters: Joining an Alcohol and a Carboxylic Acid
We can now connect the first two families. When an alcohol reacts with a carboxylic acid, a new organic family is formed: the esters.
Esterification
The reaction between an alcohol and a carboxylic acid is called esterification.
alcohol + carboxylic acid → ester + water
The alcohol and carboxylic acid are heated together with concentrated sulfuric acid, which acts as a catalyst.
For example, ethanol reacts with ethanoic acid to form ethyl ethanoate:
CH₃CH₂OH + CH₃COOH → CH₃COOCH₂CH₃ + H₂O
Esters contain the characteristic –COO– linkage.
Water really is a product
A common mistake is to write only the ester as the product. During esterification, atoms from the alcohol and carboxylic acid are rearranged and a molecule of H₂O is also formed.
How to Name an Ester
An ester name contains two words. Each word tells you where part of the ester came from.
- The first word comes from the alcohol. Change the alcohol name to an –yl ending.
- The second word comes from the carboxylic acid. Change –oic acid to –oate.
| Alcohol | Carboxylic acid | Ester formed |
|---|---|---|
| Methanol | Ethanoic acid | Methyl ethanoate |
| Ethanol | Ethanoic acid | Ethyl ethanoate |
| Ethanol | Propanoic acid | Ethyl propanoate |
| Propanol | Methanoic acid | Propyl methanoate |
Reading an Ester Structural Formula
To name an ester from its structural formula, first locate the –COO– linkage. This separates the alcohol-derived part from the carboxylic-acid-derived part.
Consider CH₃COOCH₂CH₃.
First locate the –COO– linkage.
- The CH₂CH₃ group attached to the single-bond oxygen came from ethanol, so the first word is ethyl.
- The CH₃COO– portion containing the C=O came from ethanoic acid, so the second word is ethanoate.
The ester is therefore ethyl ethanoate.
A useful way to split an ester
Find the –COO– linkage first. The carbon group attached to the single-bond oxygen gives the –yl part of the name.
The part containing the carbon of the C=O group gives the –oate part.
Properties and Uses of Esters
Many small esters are volatile liquids with distinctive, often fruity smells. This helps explain their use in:
- perfumes and fragrances;
- food flavourings;
- solvents.
However, not every ester has the same smell, and smell should not be treated as a reliable identification test in the laboratory.
Connecting the Three Families
The most useful way to revise this chapter is to connect the reactions rather than memorise each one separately.
Follow the organic reaction pathway
ethene + steam → ethanol
Ethene is converted into ethanol by an addition reaction.
ethanol + 2[O] → ethanoic acid + water
Ethanol is converted into ethanoic acid by oxidation.
ethanol + ethanoic acid → ethyl ethanoate + water
Finally, the alcohol and carboxylic acid react together in esterification.
Once you can follow this pathway, many examination questions become pattern-recognition questions: identify the functional group, identify what has changed, and then name the reaction or product.
Common Mistakes to Avoid
| Common mistake | Why it is wrong | Better way to think |
|---|---|---|
| Not counting the carbon atom in –COOH | The carbon in the carboxyl group is part of the carbon chain. | CH₃COOH contains two carbon atoms, so it is ethanoic acid. |
| Treating the –OH in ethanol as OH⁻ | The –OH group is covalently bonded within the ethanol molecule. | Recognise CH₃CH₂OH as a covalent alcohol molecule, not an ionic hydroxide. |
| Writing CO₂ as the product of oxidation of ethanol | CO₂ is formed during complete combustion. Controlled oxidation forms ethanoic acid. | Distinguish combustion from oxidation to a carboxylic acid. |
| Using oxygen during fermentation | Fermentation is an anaerobic process. | Remember: glucose + yeast, about 37 °C, absence of oxygen. |
| Forgetting water during esterification | Esterification produces both an ester and water. | Remember: alcohol + carboxylic acid → ester + water. |
| Reversing the two parts of an ester name | The first word comes from the alcohol and the second from the carboxylic acid. | Alcohol → –yl; carboxylic acid → –oate. |
| Thinking a weak acid cannot react | Weak refers to partial ionisation, not an inability to undergo acid reactions. | Carboxylic acids still react with reactive metals, bases and carbonates. |
Alcohols, Carboxylic Acids and Esters Checklist
Recognising the Families
- Identify –OH as the alcohol functional group.
- Identify –COOH as the carboxylic acid functional group.
- Identify –COO– as the ester linkage.
- Use the correct name endings.
Ethanol
- Describe fermentation of glucose.
- State the conditions needed for fermentation.
- Describe the reaction of ethene with steam.
- Write equations for combustion and oxidation.
Carboxylic Acids
- Name the first four members.
- Count the carbon atom in –COOH.
- Explain why carboxylic acids are weak acids.
- Predict their reactions with metals, bases and carbonates.
Esters
- State the products of esterification.
- Name an ester from its alcohol and carboxylic acid.
- Deduce the parent alcohol and acid from an ester structure.
- Recall common uses of esters.
These ten questions go beyond simple recall. Use structures, observations and reaction patterns to work out each answer.
- 10 application questions
- Structural formula and reaction reasoning
- Detailed feedback for every option
ethene → ethanol → ethanoic acid
X is the first reaction and Y is the second reaction.
Frequently asked questions
Questions Students Often Ask About Alcohols, Carboxylic Acids and Esters
These questions bring together some of the ideas that students commonly mix up when moving between the three organic families.
Why is the –OH group in ethanol not the same as the OH⁻ ion?
In ethanol, the oxygen and hydrogen atoms form part of a covalent molecule. The group is written –OH because it is bonded to the carbon chain. A hydroxide ion, OH⁻, is a separate negatively charged ion. Ethanol therefore does not contain freely moving OH⁻ ions simply because its formula ends in OH.
Why must oxygen be excluded during fermentation?
The production of ethanol by yeast is an anaerobic process, meaning that it takes place without oxygen. The required conditions are glucose solution, yeast, a suitable temperature of about 37 °C and an absence of oxygen.
Why is ethanoic acid weak even though it can react with metals and carbonates?
The word weak refers to partial ionisation in water. It does not mean that ethanoic acid is incapable of reacting. The H⁺ ions produced can still take part in the typical reactions of acids.
How do I know how many carbon atoms are in a carboxylic acid?
Count every carbon atom in the formula, including the carbon atom inside the –COOH functional group. For example, CH₃CH₂COOH contains three carbon atoms altogether and is therefore propanoic acid.
How can I tell which part of an ester name comes from the alcohol?
Find the –COO– linkage. The carbon group attached to the single-bond oxygen came from the alcohol and gives the first, –yl part of the ester name. The section containing the carbonyl carbon gives the –oate part.
Why is water formed during esterification?
The alcohol and carboxylic acid do not simply join together unchanged. Atoms from their functional groups rearrange as the ester linkage forms, and H₂O is produced as a second product.
Is oxidation of ethanol the same as burning ethanol?
No. During complete combustion, ethanol reacts with oxygen to form carbon dioxide and water. During the controlled oxidation studied here, ethanol is converted into ethanoic acid using an oxidising agent such as acidified potassium manganate(VII).
Do all esters smell like fruit?
No. Many small esters have distinctive smells that may be described as fruity, which is why some are useful in flavourings and perfumes. However, different esters have different properties, so smell should not be treated as a defining identification test for every ester.
Alcohols, carboxylic acids and esters become much easier when you follow the functional groups rather than memorising isolated facts. Recognise –OH, –COOH and –COO– first; then follow the reaction pathway from ethene to ethanol, from ethanol to ethanoic acid, and finally from an alcohol and a carboxylic acid to an ester. Once those connections are clear, the naming and reactions begin to fit together as one coherent piece of organic chemistry.