What are hydroxy compounds in the context of A-Level Organic Chemistry?

In A-Level Organic Chemistry, hydroxy compounds refer to organic molecules that contain one or more hydroxyl groups (-OH) attached to a carbon atom. These compounds are significant because they include alcohols and phenols, which are fundamental in various chemical reactions and industrial applications.

How do you classify hydroxy compounds in A-Level Chemistry?

Hydroxy compounds are classified into two main categories: alcohols and phenols. Alcohols have the hydroxyl group attached to a saturated carbon atom, while phenols have the hydroxyl group attached directly to an aromatic ring. This classification is crucial for understanding their chemical properties and reactions in A-Level Chemistry.

What are the common reactions involving hydroxy compounds studied in Cambridge International A Levels?

In Cambridge International A Levels, common reactions involving hydroxy compounds include oxidation, dehydration, and esterification. Alcohols can be oxidized to form aldehydes, ketones, or carboxylic acids. Dehydration of alcohols leads to the formation of alkenes, and esterification involves the reaction of alcohols with carboxylic acids to produce esters.

Why is the study of hydroxy compounds important in A-Level Chemistry?

The study of hydroxy compounds is important in A-Level Chemistry because they are prevalent in many biological and industrial processes. Understanding their properties and reactions helps students grasp key concepts in organic chemistry, such as functional group transformations and reaction mechanisms, which are essential for further studies and practical applications.

What are the physical properties of hydroxy compounds that are emphasized in A-Level Chemistry?

In A-Level Chemistry, the physical properties of hydroxy compounds that are emphasized include boiling points, solubility, and hydrogen bonding. Alcohols generally have higher boiling points than hydrocarbons of similar molecular weight due to hydrogen bonding. They are also typically soluble in water, especially those with shorter carbon chains, because of their ability to form hydrogen bonds with water molecules.

Why is "Not specifying distillation vs reflux in oxidation." a common mistake in Hydroxy compounds?

Why it happens: Knowing the products but not the conditions. How to avoid it: Distil → aldehyde; reflux → carboxylic acid. Source: 9701 Examiner Reports 2022-2024

Why is "Saying tertiary alcohols are oxidised." a common mistake in Hydroxy compounds?

Why it happens: Applying the rule for 1°/2°. How to avoid it: Tertiary alcohols are not oxidised (dichromate stays orange).

Why is "Saying alcohols are more acidic than water." a common mistake in Hydroxy compounds?

Why it happens: Misjudging the inductive effect. How to avoid it: Alkyl +I destabilises RO⁻ → alcohols are weaker acids than water.

Why is "Saying all alcohols give the iodoform test." a common mistake in Hydroxy compounds?

Why it happens: Forgetting the CH₃CH(OH)– requirement. How to avoid it: Only CH₃CH(OH)– (or CH₃CO–) groups give a positive test.

Hydroxy compounds(A-Level Organic Chemistry)Cambridge International A Level Chemistry 9701

Hydroxy compounds

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Hydroxy Compounds (Alcohols, A Level) — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

Classifying alcohols, their oxidation products and conditions, esterification (from acids and from acyl chlorides), the tri-iodomethane test, and alcohol acidity.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

32.1 — Classify alcohols and describe their oxidation, esterification and dehydration reactions.
32.1 — Describe the reaction of alcohols with acyl chlorides and the tri-iodomethane test.
32.1 — Explain the acidity of alcohols relative to water.

Classification and oxidation

1° → aldehyde/acid, 2° → ketone, 3° → no reaction; distil vs reflux controls the 1° product.

Alcohols are classified by the number of carbon groups on the carbon bearing the –OH: primary (1°), secondary (2°) and tertiary (3°).

Using acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄, orange → green):

Primary → aldehyde if you distil the product off as it forms; → carboxylic acid if you reflux.
Secondary → ketone (no further oxidation).
Tertiary → no reaction (the dichromate stays orange).

The distil vs reflux choice is a frequent mark: distil to stop at the aldehyde, reflux to push on to the acid.

1° → aldehyde (distil) → acid (reflux).
2° → ketone; 3° → no reaction.
K₂Cr₂O₇/H₂SO₄ goes orange → green when it works.

See the full worked example for hydroxy compounds →

Esterification and dehydration

Two ester routes (acid equilibrium vs fast acyl chloride); dehydration gives an alkene.

Esterification can be done two ways:

With a carboxylic acid (conc H₂SO₄ catalyst) — a slow, reversible equilibrium. $\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \rightleftharpoons \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$
With an acyl chloride — fast and irreversible, releasing HCl. $\text{CH}_3\text{COCl} + \text{C}_2\text{H}_5\text{OH} \rightarrow \text{CH}_3\text{COOC}_2\text{H}_5 + \text{HCl}$

Dehydration (elimination of water) uses hot concentrated H₂SO₄ (or passing the vapour over Al₂O₃) to give an alkene: $\text{C}_2\text{H}_5\text{OH} \rightarrow \text{CH}_2\text{=CH}_2 + \text{H}_2\text{O}$

Alcohols also react with Na (→ alkoxide + H₂), confirming the O–H is weakly acidic.

Ester: acid + alcohol (H₂SO₄, equilibrium) OR acyl chloride (fast, one-way).
Dehydration (hot conc H₂SO₄ / Al₂O₃) → alkene.
React with Na → alkoxide + H₂.

See the full worked example for hydroxy compounds →

Tri-iodomethane test and acidity

A CH₃CH(OH)– group gives CHI₃; alcohols are weaker acids than water.

Tri-iodomethane (iodoform) test. Warming an alcohol with alkaline aqueous iodine gives a pale-yellow precipitate of CHI₃ only if it contains a CH₃CH(OH)– group (or a CH₃CO– group). So ethanol and propan-2-ol give a positive test; propan-1-ol does not.

Acidity. Alcohols react with sodium, showing the O–H is weakly acidic. However, an alcohol is a weaker acid than water: the electron-donating (+I) alkyl group pushes electron density onto the alkoxide ion (RO⁻), destabilising it and making dissociation less favourable.

Tri-iodomethane: CH₃CH(OH)– → pale-yellow CHI₃.
Ethanol & propan-2-ol positive; propan-1-ol negative.
Alcohols are weaker acids than water (alkyl +I destabilises RO⁻).

See the full worked example for hydroxy compounds →

How it’s examined

Paper 4 favours oxidation (with the distil/reflux distinction), esterification (acid vs acyl chloride), and the tri-iodomethane test. Examiner reports flag missing conditions, saying tertiary alcohols oxidise, and over-applying the iodoform test.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 32.1; 9701 Examiner Reports 2022-2024; 9701/41 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-20.

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Step-by-step worked examples — Hydroxy compounds

Step-by-step solutions to past-paper-style questions on hydroxy compounds, written exactly the way a tutor would explain them at the board.

1Classifying alcohols (2 marks)
Getting started• classification
Question
Classify ethanol, propan-2-ol and 2-methylpropan-2-ol as primary, secondary or tertiary.
Step-by-step solution
1. Step 1
  Count the alkyl groups on the carbon bearing –OH.
2. Step 2
  Ethanol 1°; propan-2-ol 2°; 2-methylpropan-2-ol 3°.
Answer
Ethanol 1°; propan-2-ol 2°; 2-methylpropan-2-ol 3°.
Examiner tip
Classification is by the number of carbons attached to the C–OH carbon.
2Oxidation products and conditions (3 marks)
Getting started• oxidation
Question
State the product and conditions when (a) a primary alcohol is oxidised to an aldehyde, (b) to a carboxylic acid, and (c) a tertiary alcohol is treated with K₂Cr₂O₇/H₂SO₄.
Step-by-step solution
1. Step 1
  (a) Aldehyde — K₂Cr₂O₇/H₂SO₄, distil off.
2. Step 2
  (b) Carboxylic acid — K₂Cr₂O₇/H₂SO₄, reflux.
3. Step 3
  (c) Tertiary → no reaction (dichromate stays orange).
Answer
(a) aldehyde (distil); (b) carboxylic acid (reflux); (c) no reaction.
Examiner tip
Distil → aldehyde; reflux → acid; tertiary alcohols are not oxidised.
3Esterification with acid vs acyl chloride (3 marks)
Building confidence• esterification
Question
Give the equation for ethanol forming an ester with (a) ethanoic acid and (b) ethanoyl chloride.
Step-by-step solution
1. Step 1
  (a) With the acid (conc H₂SO₄, reversible):
  $\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \rightleftharpoons \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$
2. Step 2
  (b) With the acyl chloride (fast, irreversible):
  $\text{CH}_3\text{COCl} + \text{C}_2\text{H}_5\text{OH} \rightarrow \text{CH}_3\text{COOC}_2\text{H}_5 + \text{HCl}$
Answer
Acid route is a slow equilibrium (needs H₂SO₄); the acyl chloride route is fast and one-way (gives HCl).
Examiner tip
Acyl chlorides esterify alcohols rapidly and irreversibly (no catalyst), releasing HCl fumes.
4Dehydration to an alkene (2 marks)
Building confidence• dehydration
Question
Give the reagents/conditions and product when ethanol is dehydrated.
Step-by-step solution
1. Step 1
  Hot concentrated H₂SO₄ (or Al₂O₃ catalyst) — elimination of water.
  $\text{C}_2\text{H}_5\text{OH} \rightarrow \text{CH}_2\text{=CH}_2 + \text{H}_2\text{O}$
Answer
Hot conc H₂SO₄ (or Al₂O₃) → ethene + H₂O.
5Tri-iodomethane (iodoform) test (4 marks)
Stretch• iodoform
Question
Which of ethanol, propan-1-ol and propan-2-ol give a positive tri-iodomethane test? Give the observation and explain.
Step-by-step solution
1. Step 1
  A positive test needs a CH₃CH(OH)– group (with alkaline aqueous iodine).
2. Step 2
  Ethanol (CH₃CH₂OH) and propan-2-ol (CH₃CH(OH)CH₃) have it → positive (pale yellow precipitate of CHI₃).
3. Step 3
  Propan-1-ol (CH₃CH₂CH₂OH) does not → negative.
Answer
Ethanol and propan-2-ol give a pale-yellow CHI₃ precipitate; propan-1-ol does not.
Examiner tip
The test detects a CH₃CH(OH)– (or CH₃CO–) group — a yellow precipitate of CHI₃ forms.
6Acidity of alcohols (3 marks)
Stretch• acidity
Question
Explain why ethanol is a weaker acid than water but reacts with sodium.
Step-by-step solution
1. Step 1
  Ethanol reacts with Na (2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂), showing the O–H is weakly acidic.
2. Step 2
  But the ethyl group is electron-donating (+I), pushing electron density onto the ethoxide ion.
3. Step 3
  This destabilises the ethoxide, so dissociation is less favourable → ethanol is a weaker acid than water.
Answer
Ethanol's O–H is weakly acidic (reacts with Na); the alkyl +I effect destabilises the ethoxide, so it is weaker than water.

Key Formulae — Hydroxy compounds

The formulae you need to memorise for hydroxy compounds on the Cambridge International A Level 9701 paper, with every variable defined in plain English and a note on when to use it.

General formula of alcohols
$C_nH_{2n+1}OH$
$n$
number of carbon atoms
When to use
Deducing a saturated alcohol's formula (also written CₙH₂ₙ₊₂O).

Key Definitions and Keywords — Hydroxy compounds

Definitions to memorise and the exact keywords mark schemes credit for hydroxy compounds answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Primary / secondary / tertiary alcohol
Examiner keyword
Classified by the number of alkyl groups on the carbon bearing –OH (one, two or three).
Oxidation of alcohols
Examiner keyword
1° → aldehyde (distil) → carboxylic acid (reflux); 2° → ketone; 3° not oxidised. Uses K₂Cr₂O₇/H₂SO₄ (orange → green).
Esterification
Examiner keyword
Alcohol + carboxylic acid (conc H₂SO₄, reversible) OR alcohol + acyl chloride (fast, irreversible) → ester.
Tri-iodomethane (iodoform) test
Examiner keyword
Alkaline aqueous iodine gives a pale-yellow precipitate (CHI₃) with a CH₃CH(OH)– or CH₃CO– group.
Dehydration
Examiner keyword
Elimination of water from an alcohol (hot conc H₂SO₄ or Al₂O₃) to form an alkene.

Common Mistakes and Misconceptions — Hydroxy compounds

The traps other students keep falling into on hydroxy compounds questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Not specifying distillation vs reflux in oxidation.
9701 Examiner Reports 2022-2024
Why it happens
Knowing the products but not the conditions.
How to avoid it
Distil → aldehyde; reflux → carboxylic acid.
✕Saying tertiary alcohols are oxidised.
Why it happens
Applying the rule for 1°/2°.
How to avoid it
Tertiary alcohols are not oxidised (dichromate stays orange).
✕Saying alcohols are more acidic than water.
Why it happens
Misjudging the inductive effect.
How to avoid it
Alkyl +I destabilises RO⁻ → alcohols are weaker acids than water.
✕Saying all alcohols give the iodoform test.
Why it happens
Forgetting the CH₃CH(OH)– requirement.
How to avoid it
Only CH₃CH(OH)– (or CH₃CO–) groups give a positive test.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Hydroxy compounds — frequently asked questions

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Hydroxy compounds

Short Study Notes in the form of Slides

Short Study Notes — Hydroxy compounds

Detailed Notes

What you’ll learn

How it’s examined

1Classifying alcohols (2 marks)

2Oxidation products and conditions (3 marks)

3Esterification with acid vs acyl chloride (3 marks)

4Dehydration to an alkene (2 marks)

5Tri-iodomethane (iodoform) test (4 marks)

6Acidity of alcohols (3 marks)

General formula of alcohols

Primary / secondary / tertiary alcohol

Oxidation of alcohols

Esterification

Tri-iodomethane (iodoform) test

Dehydration

✕Not specifying distillation vs reflux in oxidation.

✕Saying tertiary alcohols are oxidised.

✕Saying alcohols are more acidic than water.

✕Saying all alcohols give the iodoform test.

Practice questions

Past paper style quiz

Assess your understanding

Hydroxy compounds — frequently asked questions