Organic chemistry
Electrophilic aromatic substitution, radicals, pericyclic reactions
Unify aromatic substitution, radical-chain chemistry, and concerted pericyclic reactions through their distinct electron-flow rules and selectivity principles.
IntuitionIntuition: preserve, transfer, or reorganize electrons
Aromatic substitution replaces a ring hydrogen while recovering aromatic stabilization. Radical chains move one electron at a time through reactive intermediates. Pericyclic reactions reorganize a cyclic array of orbitals in one concerted event; orbital symmetry and geometry govern whether the pathway is allowed.
SchoolSchool level: three patterns, different bookkeeping
Definition: Electrophilic aromatic substitution (EAS)
An electrophile forms a σ bond to an aromatic ring, temporarily giving a nonaromatic σ-complex; loss of H⁺ restores aromaticity.
A radical is a species with an unpaired electron. Homolytic bond cleavage shares one electron with each fragment, unlike heterolysis, which gives an ion pair. A chain reaction has initiation, propagation, and termination steps.
UndergraduateUniversity: mechanisms and selectivity
| Reaction family | Electron description | Key selectivity question | — | — |
|---|---|---|---|---|
| EAS | two-electron arrows; aromaticity temporarily lost | substituent directing effects and activation | ||
| Radical chain | fishhook arrows; one electron per arrow | relative rates of competing H abstraction/addition | ||
| Pericyclic | cyclic, concerted orbital reorganization | thermal/photochemical conditions and stereospecificity |
Example
Why does benzene bromination require a Lewis-acid catalyst, and why is the ring aromatic again in the product?
Solution
FeBr3 polarizes/activates Br2 to provide a stronger electrophile. The initial σ-complex is nonaromatic; loss of a proton restores the conjugated six-π-electron ring, making deprotonation strongly favorable.
AdvancedAdvanced: orbital symmetry and spin
Woodward–Hoffmann analysis follows phase relationships of interacting frontier orbitals along a cyclic transition structure. Electrocyclic thermal rules predict conrotatory motion for 4n π electrons and disrotatory motion for 4n+2; photochemical excitation reverses these correlations for the idealized case. These rules are specific to concerted pericyclic pathways, not ordinary stepwise reactions. Radical selectivity instead reflects bond strengths, polar effects, spin, and chain kinetics.
ResearchResearch: predicting selectivity beyond textbook rules
References
- Organic Chemistry · Jonathan Clayden, Nick Greeves, Stuart Warren, 2012
- The Conservation of Orbital Symmetry · Robert B. Woodward, Roald Hoffmann, 1970