Predict whether substitution is usually SN1 or SN2
Use halogenoalkane class to identify the typical substitution mechanism while allowing secondary cases to vary.
Section 15 · Halogen compounds
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
Which mechanism do primary halogenoalkanes tend to use?
What you will learn
- Relate primary structures to SN2
- Relate tertiary structures to SN1
- Describe secondary structures as condition-dependent
Primary and tertiary compounds favor different routes
Primary halogenoalkanes tend to react by SN2: attack and C–X departure happen together, and the carbon is less crowded.
Tertiary halogenoalkanes tend to react by SN1: alkyl groups stabilize the carbocation formed after C–X ionisation.
Secondary cases can use a mixture
Secondary halogenoalkanes may react by a mixture of SN1 and SN2 depending on their structure and reaction conditions.
Use the syllabus wording tend to, rather than an absolute claim. Structure is a useful first prediction, but nucleophile, solvent and other conditions can affect the observed pathway.
Pause and explain
What is the syllabus tendency for tertiary halogenoalkanes?
Worked example
Predict the usual mechanism tendency for 1-bromopropane, 2-bromopropane and 2-bromo-2-methylpropane.
Show the worked solution
- 1-bromopropane is primary and tends to SN2.
- 2-bromopropane is secondary and may show a mixture depending on structure and conditions.
- 2-bromo-2-methylpropane is tertiary and tends to SN1.
Answer Primary: SN2 tendency; secondary: possible mixture; tertiary: SN1 tendency.
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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-chlorobutane and predict its usual nucleophilic substitution mechanism.
Give me a hint
Count carbon neighbours at the C–Cl carbon.
Compare my reasoning
- The C1 carbon has one carbon neighbour.
- This is a primary halogenoalkane.
- Primary halogenoalkanes tend to react by SN2.
Primary; it tends to react by SN2.
Look for these in your work
- Carbon class correct
- Primary label correct
- Mechanism tendency stated
A tertiary halogenoalkane reacts by substitution. Explain why SN1 is a reasonable first prediction.
Give me a hint
Consider the intermediate required by SN1.
Compare my reasoning
- SN1 begins by forming a carbocation.
- Three alkyl groups stabilize the tertiary carbocation.
- Therefore the SN1 route is commonly favored.
A tertiary carbocation is stabilized by the inductive effect of three alkyl groups, supporting an SN1 tendency.
Look for these in your work
- Intermediate named
- Stabilization explained
- Prediction justified
A classmate says every secondary halogenoalkane is SN1. Correct the statement.
Give me a hint
Use the qualification in the syllabus.
Compare my reasoning
- Secondary compounds can react through SN1 and SN2.
- The mixture depends on structure and conditions.
- State a tendency only when the supplied evidence supports it.
Secondary halogenoalkanes may use a mixture of SN1 and SN2 depending on structure and conditions.
Look for these in your work
- Both pathways included
- Dependence stated
- Absolute claim removed
Common mistakes
- Saying every primary compound must react only by SN2
- Saying secondary halogenoalkanes have one fixed mechanism
- Ignoring carbocation stability when predicting SN1
What can you explain now?
Which mechanism do primary halogenoalkanes tend to use?
Compare with the explanation
SN2
Primary compounds commonly substitute in one concerted step.
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Make it stick
What mechanism tendencies apply to primary, secondary and tertiary halogenoalkanes?
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.