A Level · Cambridge (CIE) · 9701

Classify alcohols by the carbon bearing OH

Classify each OH-bearing carbon as primary, secondary or tertiary, including molecules with several OH groups.

Hydroxy compounds · Cambridge 9701 AS Chemistry · 16.1

AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State reagents and conditions, follow the stated reaction pathway, and distinguish each alcohol class and oxidation product. Written practice is self-checked

Before you begin

Use functional-group formulae and reaction types from Topic 13, alkene reactions from Topic 14, and halogenoalkane substitution from Topic 15.

Work on paper. Try each question before revealing help, and explain why your method works.

Quick readiness check

Which atom's carbon neighbours determine alcohol class?

What you will learn

Locate each carbon bonded to OH
Count its carbon neighbours
Classify molecules containing multiple OH groups

Count neighbours at each C–OH carbon

A primary alcohol has its OH-bearing carbon attached to one other carbon; a secondary alcohol has two carbon neighbours; a tertiary alcohol has three. Count around the carbon bonded directly to O.

Do not classify by the total number of carbons in the molecule or by counting hydrogens attached to oxygen. The classification is local to each C–OH site.

A molecule can contain different alcohol sites

For an alcohol with more than one OH group, classify each OH-bearing carbon separately. A molecule may have two primary alcohol groups, two secondary groups, or a mixture of classes.

For example, HOCH₂CH₂OH contains two primary alcohol sites. Butane-2,3-diol contains two secondary sites. A polyol is not automatically tertiary because it has several OH groups.

Pause and explain

How many carbon neighbours does a secondary alcohol carbon have?

Put the idea to work

Worked example

Classify the OH groups in HOCH₂CH(OH)CH₃.

Show the worked solution

The terminal CH₂OH carbon has one carbon neighbour.
The middle CH(OH) carbon has two carbon neighbours.
Classify each OH site separately.

Answer The terminal OH is primary; the middle OH is secondary.

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From guided practice to a new situation

Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.

1 · Guided

Classify the OH group in 2-methylpropan-2-ol.

Give me a hint

Inspect the carbon bonded to oxygen.

Compare my reasoning

The C–OH carbon is attached to three methyl carbons.
That gives three carbon neighbours.
Use the tertiary definition.

Tertiary alcohol.

Look for these in your work

Correct carbon selected
Three neighbours counted
Class named

2 · Independent

Classify both OH groups in butane-1,3-diol.

Give me a hint

Inspect the carbon carrying each OH separately.

Compare my reasoning

The OH at C1 is on a carbon with one carbon neighbour.
The OH at C3 is on a carbon with two carbon neighbours.
The two sites can have different classes.

C1 OH is primary; C3 OH is secondary.

Look for these in your work

Both sites checked
Neighbour counts correct
Both classes stated

3 · Transfer

A student says ethane-1,2-diol is a secondary alcohol because it has two OH groups. Correct the reasoning.

Give me a hint

Number of OH groups is not the definition of secondary.

Compare my reasoning

Each OH is attached to a terminal CH₂ carbon.
Each OH-bearing carbon has one carbon neighbour.
Both alcohol sites are primary.

Both OH groups are primary; classify each C–OH carbon by its carbon neighbours.

Look for these in your work

Site-by-site method used
Neighbour count correct
Misconception corrected

Common mistakes

Classifying by the total number of OH groups
Counting oxygen as a carbon neighbour
Giving a molecule with multiple OH groups just one class without checking each site

Recall without your notes

What can you explain now?

Which atom's carbon neighbours determine alcohol class?

Compare with the explanation

The carbon directly bonded to OH

Classification depends on that C–OH carbon.

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Make it stick

How do you classify a polyol with more than one OH group?

If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.

Keep learning Chemistry

Check the official syllabus 2028–2030

Course scope and assessment

Four hundred and forty-two original lessons map the published learning outcomes across AS Chemistry Topics 1–22 and A Level Topics 23–37, with guided, independent and transfer practice and explained answers. Every numbered subject-content topic and listed outcome tag now has a mapped lesson sequence. Topic 32 maps all 12 hydroxy-compound outcome tags in 11 lessons; Topic 33 maps all 15 carboxylic-acid and derivative outcome tags in 16 lessons; Topic 34 maps all 22 nitrogen-compound outcome tags in 20 lessons; Topic 35 maps all 12 polymerisation outcome tags in 16 lessons; Topic 36 maps all four organic-synthesis outcome tags in 14 lessons; Topic 37 maps all 22 chromatography and NMR outcome tags in 18 lessons. Cambridge's 2028–2030 Version 1 syllabus states that there are no significant changes which affect teaching from the preceding edition; the routes are aligned to that edition. This is 37 of the 37 numbered subject-content topics. Full cumulative exam assessment, Paper 3 laboratory competence, Paper 5 investigation practice and human subject-teacher review remain outstanding. Written practice is self-checked and does not certify exam readiness.