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Allylic bromination with NBS
- Alkene + NBS, HBr trace, hv
- Usually in carbon tetrachloride
- Free radical mechanism
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Allylic bromination
- Alkene + low conc. Br2, hv
- Free radical mechanism
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Nucleophilic substitution of allylic halides (SN1)
- Alkene + H2O or HOR, heat
- Involves carbocation intermediate
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Nucleophilic substitution of allylic halides (SN2)
Allylic systems more reactive because of stabilization of the transition state
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Electrophilic aromatic substitution
- Reactants: Aromatic + strong E + Base
- Intermediate: Endothermic, nonaromatic (base takes hydrogen adjacent to E)
- Product: Ectothermic, aromatic with E , H-Base
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Bromination of an aromatic
- Reagents: Br2, FeBr3
- Electrophile: Br-Br+-FeBr3
- Product: aromatic ring with Br
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Chlorination of an aromatic
- Reagents: Cl2, AlCl3 or FeCl3
- Electrophile: Cl-Cl+-Al-Cl3 or Cl-Cl+-FeCl3
- Product: aromatic ring with Cl
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Iodination of an aromatic
- Reagents: I2, HNO3
- Electrophile: I+
- Product: aromatic ring with I
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Nitration of an aromatic
- Reagents: HNO3, H2SO4
- Electrophile: O=N+=O
- Product:
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Sulfation of an aromatic
- Reagents: SO3, H2SO4
- Electrophile: SO3
- Product:
 - (After sulfuric acid ion takes H, another sulfuric acid adds H to the benzenesulfonic acid ion)
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Nitro group reduction of an aromatic
- Reagents: Zn or Sn or Fe and HCl
- Intermediate: aromatic ring with N+H3
- Product:
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Friedel-Crafts Alkylation
- Reagents: R-X, AlCl3 or FeBr3
- Electrophile: R+
- Product: Aromatic ring with R substituent (HX + AlCl3)
- To get R+:
- R-X + AlCl3 ⇌ R-X+-Al-Cl3 ⇌ R + X-Al-Cl3
- Base: X-Al-Cl3
Note: rearragements of the R group is possible because of the cation
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Friedel-Crafts Acylation
- Reagents:
- 1)
 and AlCl3 - 2) H2O
- Electrophile: R-C≡O+
- Product:
Note: AlCl 3 binds to O after acylium ion binds to benzene so H 2O is needed in second step to kick Al off
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Ortho-para ratio
- Probability: Ortho favored (2 ortho position vs 1 para position)
- Steric hindrance: Para favored if substituent on ring or electrophile is large
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Orientation when there are two or more substituents
- 1) Reinforce each other's orienting effects
- 2) Oppose each other's orienting effects (activator or better nuc dominates but if they are similar, mixtures result)
- 3) When substituents meta to one another, electrophile usually won't go in between them
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