3.4.12 (HL)—Carbocation stability
- Syllabus
- First assessment 2025
- Objective
- 3.4.12
- Level
- HL
Major Products of Unsymmetrical Addition
HL onlyFor HX or water addition to an unsymmetrical alkene, compare the possible carbocations and select the pathway through the more stable carbocation as the major product.
For an unsymmetrical alkene, draw both protonation routes and compare the carbocations: tertiary is generally more stable than secondary, then primary. Choose the product from the more stable intermediate; do not apply this shortcut to a halonium pathway.
Predicting the Major Addition Product
HL onlyAssessment in practice
Representative question
Question 1
[Maximum number: 2]
Explain which of the two structural isomers is the major product.
2-bromobutane/ CH3CH(Br)CH2CH3
secondary carbocation is more stable than primary carbocation
OR
«secondary» carbocation has greater number of alkyl groups/lower charge on carbon
OR
alkyl groups are more electron releasing/have greater inductive effect «than hydrogen»
Marking guidance:
Do not award M2 for simply stating
Markovnikov's rule.
Electron-Pair Sharing Summary
Retrieve the route: classify nucleophiles and electrophiles, show heterolysis, write substitution and addition mechanisms, map Lewis coordination, compare SN1/SN2, and restore aromaticity in benzene substitution.
Check electron-pair arrow origin and destination, leaving-group departure, intermediate identity, carbocation stability and the assessed mechanism boundary.