Classify primary, secondary and tertiary halogenoalkanes
Classify the carbon attached to halogen by counting its carbon neighbours.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the named reagents, solvents, temperatures and pressure conditions; begin curly arrows at a bond or lone pair and distinguish typical mechanisms from absolute rules. Written practice is self-checked
Before you begin
- Use covalent formulas, nucleophiles, electrophiles, homolytic and heterolytic fission, and the alkene and alcohol pathways in Topics 13–14.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What determines whether a halogenoalkane is primary, secondary or tertiary?
What you will learn
- Locate the carbon bonded to X
- Count its carbon neighbours
- Distinguish primary, secondary and tertiary structures
Classification depends on the C–X carbon
Find the carbon directly bonded to the halogen X. Count how many other carbon atoms are directly attached to that same carbon.
A primary halogenoalkane has one carbon neighbour at the C–X carbon, a secondary has two, and a tertiary has three. The halogen identity does not set this classification.
Use connectivity rather than the whole molecule
CH₃CH₂Br is primary because the carbon attached to Br is connected to one other carbon. CH₃CHBrCH₃ is secondary; (CH₃)₃CBr is tertiary.
Do not classify from the total number of carbons or from the position of the halogen in the written formula alone. Trace the bonds at the carbon–halogen site.
Pause and explain
What class is CH₃CHBrCH₃?
Worked example
Classify CH₃CH₂CHBrCH₃ and (CH₃)₃CCl.
Show the worked solution
- For CH₃CH₂CHBrCH₃, inspect the carbon bonded to Br; it has two carbon neighbours.
- That halogenoalkane is secondary.
- The carbon bonded to Cl in (CH₃)₃CCl has three carbon neighbours, so it is tertiary.
Answer CH₃CH₂CHBrCH₃ is secondary; (CH₃)₃CCl is tertiary.
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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 1-bromobutane and 2-bromobutane.
Give me a hint
Count carbon neighbours of the carbon directly attached to Br.
Compare my reasoning
- At C1 of 1-bromobutane, the Br-bearing carbon is attached to one carbon.
- At C2 of 2-bromobutane, that carbon is attached to two carbons.
- Use the primary/secondary definitions.
1-bromobutane is primary; 2-bromobutane is secondary.
Look for these in your work
- Correct atom selected
- Neighbours counted
- Both labels correct
A structure has a C–I carbon bonded to three methyl groups. Classify it and explain why iodine does not affect the label.
Give me a hint
Use the carbon connectivity at the C–I site.
Compare my reasoning
- The carbon carrying I is attached to three carbon atoms.
- That makes the halogenoalkane tertiary.
- The identity of I versus Cl or Br does not change the neighbour count.
It is tertiary because the C–I carbon has three carbon neighbours; classification is independent of halogen identity.
Look for these in your work
- Three neighbours counted
- Tertiary label stated
- Halogen role distinguished
A learner calls a six-carbon primary halogenoalkane tertiary because the molecule contains many carbons. Correct the method.
Give me a hint
Do not count the whole skeleton; inspect only the carbon bonded to X.
Compare my reasoning
- Locate the C–X carbon.
- Count the carbons directly attached to that carbon.
- Use one, two or three neighbours for primary, secondary or tertiary.
Classify from the number of carbon neighbours at the C–X carbon, not from the molecule's total carbon count.
Look for these in your work
- Site located
- Local count used
- Rule applied
Common mistakes
- Counting the halogen as a carbon neighbour
- Classifying by total chain length
- Calling a halogenoalkane primary because X is on an end of the written formula without checking connectivity
What can you explain now?
What determines whether a halogenoalkane is primary, secondary or tertiary?
Compare with the explanation
The number of carbon atoms bonded to the carbon carrying X
Count carbon neighbours at the C–X carbon.
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Make it stick
Which single carbon do you inspect to classify a halogenoalkane, and what do you count?
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