Classify alcohols by the carbon bearing OH
Classify each OH-bearing carbon as primary, secondary or tertiary, including molecules with several OH groups.
Section 16 · Hydroxy compounds
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?
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.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →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.
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
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
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
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.
After trying it yourself, choose your next review. This is your self-assessment.
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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.