3.4.10 (HL)—Leaving group effects
- Syllabus
- First assessment 2025
- Objective
- 3.4.10
- Level
- HL
Leaving Groups and Rate
HL onlyLeaving-group identity affects substitution rate through carbon–halogen bond enthalpy and polarity: a weaker, more easily broken bond generally permits faster departure.
For comparable halogenoalkanes, the weaker C–I bond usually makes iodide a better leaving group and substitution faster than for chloride. Explain the trend with bond enthalpy and the actual rate-determining bond change, not polarity alone.
Explaining Leaving-Group Effects
HL onlyAssessment in practice
Representative question
Question 1
[Maximum number: 2]
Explain why CH3CHIC(CH3)3 reacts faster than CH3CHBrC(CH3)3.
C-I «bond» is weaker than C-Br «bond»
due to large atomic radius of I
OR
I is a better leaving group «than Br »
OR
activation energy of reaction is lower
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.