Predict Markovnikov addition using carbocation stability
Explain how alkyl inductive effects stabilise carbocations and predict the major product of HX addition.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Apply the stated reagents and conditions, balance equations, and keep mechanistic claims within the named examples. Written practice is self-checked
Before you begin
- Use displayed and structural formulas, covalent bond polarity, balanced equations, and the organic reaction terms from Topic 13.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which simple carbocation is generally most stable?
What you will learn
- Compare primary, secondary and tertiary carbocation stability
- Explain the electron-releasing inductive effect
- Predict the major HX addition product
More alkyl groups stabilise a positive carbon
An alkyl group can release electron density through sigma bonds toward a nearby electron-deficient carbon. This inductive effect disperses some positive charge and stabilises a carbocation.
For comparable simple alkyl carbocations, stability increases primary < secondary < tertiary. This ordering is a relative trend; do not treat a carbocation as a stable isolated product in the overall reaction.
Markovnikov orientation follows the more stable intermediate
In addition of HX to an unsymmetrical alkene, the orientation that forms the more stable carbocation is favored. For propene, adding H to the terminal carbon gives a secondary carbocation at carbon 2; X⁻ then attacks carbon 2.
The major product from propene + HBr is 2-bromopropane. State both the intermediate stability reason and the product; avoid using a memorised rule without checking which intermediate forms.
Pause and explain
Why is the secondary carbocation pathway favored for propene + HBr?
Worked example
Predict the major product of HBr addition to propene and explain the orientation.
Show the worked solution
- Place H on the terminal alkene carbon to form a secondary carbocation at C2.
- The alternative orientation gives a primary carbocation, which is less stable.
- Br⁻ attacks C2.
Answer CH₃CH=CH₂ + HBr → CH₃CHBrCH₃; 2-bromopropane is favored because the route forms a more stable secondary carbocation rather than a primary one.
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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 CH₃⁺, CH₃CH₂⁺, (CH₃)₂CH⁺ and (CH₃)₃C⁺ by increasing alkyl substitution and stability.
Give me a hint
Count alkyl groups attached to the positive carbon.
Compare my reasoning
- CH₃⁺ has zero alkyl groups.
- Ethyl and isopropyl cations are primary and secondary.
- The tert-butyl cation is tertiary and most stabilised.
CH₃⁺ < CH₃CH₂⁺ < (CH₃)₂CH⁺ < (CH₃)₃C⁺, from least to most stable in the simple alkyl series.
Look for these in your work
- Substitution counted
- Order correct
- Trend described
Predict the major product of HCl addition to but-1-ene.
Give me a hint
Compare the carbocation after H adds to either alkene carbon.
Compare my reasoning
- Adding H to terminal C1 leaves positive charge at secondary C2.
- The alternative puts positive charge at primary C1.
- Cl⁻ attacks C2 in the favored route.
2-chlorobutane is the major product by the more stable secondary-carbocation pathway.
Look for these in your work
- Both orientations considered
- Intermediate stability applied
- Product named
A student says the alkyl group donates a complete electron pair to a carbocation. Refine the explanation.
Give me a hint
Distinguish a polarisation effect through sigma bonds from a new covalent bond.
Compare my reasoning
- The inductive effect is a partial shift of electron density through sigma bonds.
- It does not transfer a whole pair as a separate reagent.
- This partial donation stabilises the positive centre.
Alkyl groups exert an electron-releasing inductive effect through sigma bonds; they do not donate a complete electron pair as a separate species.
Look for these in your work
- Partial effect described
- No full transfer claimed
- Stabilisation linked
Common mistakes
- Claiming the more substituted carbocation is less stable
- Placing the halogen on the carbon that gives the primary intermediate as the major pathway
- Saying inductive effect means full electron transfer from an alkyl group
What can you explain now?
Which simple carbocation is generally most stable?
Compare with the explanation
Tertiary
Three alkyl groups donate electron density toward the electron-deficient carbon.
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Make it stick
How does carbocation stability explain the major product from HX addition to an unsymmetrical alkene?
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