3.4.13 (HL)—Electrophilic substitution in benzene
- Syllabus
- First assessment 2025
- Objective
- 3.4.13
- Level
- HL
Electrophilic Substitution of Benzene
HL onlyBenzene attacks the charged electrophile E+, forms a charged intermediate, then loses H+ to restore aromaticity. The formation of E+ is outside this assessed mechanism boundary.
The first step temporarily disrupts aromatic delocalization; loss of H⁺ then restores the ring and replaces H by E. An addition product would leave aromaticity broken, which explains why the assessed outcome is substitution.
Drawing Benzene Electrophilic Substitution
HL onlyAssessment in practice
Representative question
Question 1
[Maximum number: 4]
Explain the mechanism for the nitration of benzene, using curly arrows to indicate the movement of electron pairs.
Official mechanism shown in the figure.
Accept mechanism with corresponding Kekule structures.
Do not accept a circle in M2 or M3.
Accept first arrow starting either inside the circle or on the circle.
If a Kekule structure is used, the first arrow must start on the double bond.
M2 may be awarded from a correct diagram for M3.
M4: Accept C6H5NO2+H2SO4 if HSO4− is used in M3.
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.