3.4.11 (HL)—Electrophilic addition mechanisms
- Syllabus
- First assessment 2025
- Objective
- 3.4.11
- Level
- HL
Electrophilic-Addition Mechanisms
HL onlyUse curly arrows from the alkene π bond to the electrophile, then from the intermediate to the nucleophile. Halogens may pass through a halonium intermediate; HX or water may pass through a carbocation.
Begin with an arrow from the alkene π bond to the electrophile. HX or acid-catalysed hydration can form a carbocation, whereas halogen addition uses a bridged halonium ion; the next arrow must start from the nucleophile's pair.
Drawing Addition Mechanisms
HL onlyAssessment in practice
Representative question
Question 1
[Maximum number: 3]
Describe the mechanism of this reaction, using curly arrows to represent the movement of electron pairs.
curly arrow from C=C bond to Br AND curly arrow showing Br-Br bond breaking
carbocation with charge on correct atom
curly arrow from lone pair/negative charge on Br−to C+
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.