29.2 Characteristic organic reactions
- Syllabus
- 9701–2028–2029
- Topic
- 29.2
- Level
- A2
Learning objectives
Distinguish electrophilic substitution from addition–elimination by its step sequence
| Mechanism term | Attacking species and site | Characteristic sequence | Net result |
|---|---|---|---|
| electrophilic substitution | an electrophile attacks an electron-rich arene π system | electrophile bonds to the ring; H⁺ or another group is lost; the delocalised π system is restored | one ring atom/group is replaced by the electrophile |
| nucleophilic addition–elimination | a nucleophile attacks the δ⁺ carbonyl carbon of an acyl derivative | nucleophile adds and C=O π electrons move to O; C=O reforms as a leaving group departs | one group on the acyl carbon is replaced by the nucleophile |
Read the name as a mechanism history. ‘Electrophilic’ or ‘nucleophilic’ identifies the electron-pair acceptor or donor that attacks; ‘substitution’ describes the overall replacement; ‘addition–elimination’ records two successive stages even though its overall product also looks substituted.
CX6HX6+BrX2AlBrX3CX6HX5Br+HBr
In bromination, the catalyst helps generate a sufficiently strong electrophile. The benzene ring forms a bond to it and then loses H⁺, restoring aromatic delocalisation; this is substitution rather than permanent addition to the ring.
CHX3COCl+HX2OCHX3COOH+HCl
In acyl-chloride hydrolysis, water attacks the carbonyl carbon to form an addition intermediate. The carbonyl reforms as Cl⁻ leaves; proton transfer gives the overall carboxylic acid and HCl products.
Do not classify from the overall equation alone. Electrophilic addition leaves new groups added across a π bond, whereas electrophilic substitution restores the arene π system; addition–elimination contains a real addition intermediate before the leaving group departs.