Explain carbocation stability and inductive effects
Relate alkyl electron release to positive-charge stabilisation in primary, secondary and tertiary carbocations.
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 carbocation is more stable in the simple alkyl series?
What you will learn
- Describe the inductive effect of alkyl groups
- Rank simple carbocation stability
- Connect stability to SN1 pathway favorability
Alkyl groups release electron density
An alkyl group can push electron density through σ bonds toward a nearby electron-deficient carbon. This partial electron-releasing inductive effect spreads some positive charge and stabilizes a carbocation.
For simple comparable alkyl carbocations, stability generally increases primary < secondary < tertiary as the number of alkyl groups increases.
Stability changes a pathway preference
SN1 requires a carbocation intermediate, so a more stable carbocation can make that route more favorable. Tertiary halogenoalkanes commonly react by SN1; primary often react by SN2.
The inductive effect is not a complete electron transfer or a new bond from an alkyl group. It is a partial shift of electron density through σ bonds; compare structures under otherwise similar conditions.
Pause and explain
What is the inductive effect of an alkyl group in this context?
Worked example
Explain why a tertiary halogenoalkane is more likely than a primary one to react by SN1.
Show the worked solution
- SN1 begins with C–X bond ionisation and carbocation formation.
- Three alkyl groups release electron density toward a tertiary positive carbon.
- The tertiary carbocation is more stable than the primary alternative.
Answer Alkyl groups exert an electron-releasing inductive effect, stabilising the tertiary carbocation needed for SN1, so tertiary halogenoalkanes commonly favor that pathway.
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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.
Rank CH₃CH₂⁺, (CH₃)₂CH⁺ and (CH₃)₃C⁺ in increasing simple carbocation stability.
Give me a hint
Count the alkyl groups attached to the positive carbon.
Compare my reasoning
- CH₃CH₂⁺ is primary.
- (CH₃)₂CH⁺ is secondary and (CH₃)₃C⁺ is tertiary.
- More alkyl groups stabilize the electron-deficient carbon.
CH₃CH₂⁺ < (CH₃)₂CH⁺ < (CH₃)₃C⁺.
Look for these in your work
- Classes identified
- Order correct
- Reason linked
Explain how the inductive effect supports tertiary SN1 reactivity.
Give me a hint
Connect σ-bond electron release to the first SN1 intermediate.
Compare my reasoning
- SN1 ionisation gives a carbocation.
- Three alkyl groups release partial electron density toward its positive centre.
- This stabilises the tertiary carbocation relative to a primary one.
The electron-releasing inductive effect of three alkyl groups stabilises the tertiary carbocation formed in the slow SN1 step.
Look for these in your work
- SN1 intermediate named
- Inductive direction stated
- Stability comparison made
A student writes that a methyl group donates a whole electron to C⁺. Correct the wording.
Give me a hint
Distinguish partial polarization through σ bonds from an electron-transfer reaction.
Compare my reasoning
- An alkyl group shifts electron density partially through σ bonds.
- No complete electron transfer is implied.
- The partial donation helps stabilize the positive carbon.
An alkyl group exerts a partial electron-releasing inductive effect through σ bonds; it does not transfer a whole electron.
Look for these in your work
- Partial shift stated
- σ bonds named
- Effect linked
Common mistakes
- Describing inductive effect as full electron transfer
- Ranking primary carbocations as most stable
- Treating the pathway preference as an absolute rule independent of conditions
What can you explain now?
Which carbocation is more stable in the simple alkyl series?
Compare with the explanation
Tertiary rather than primary
More alkyl groups stabilize a positive carbon through an inductive effect.
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Make it stick
How does alkyl substitution stabilize a carbocation and affect the likelihood of SN1?
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