CHM 326: Exam Mechanisms

• Alkene + NBS, HBr trace, hv
• Usually in carbon tetrachloride
• Free radical mechanism

• Alkene + low conc. Br₂, hv
• Free radical mechanism

• Alkene + H₂O or HOR, heat
• Involves carbocation intermediate

Allylic systems more reactive because of stabilization of the transition state

• Reactants: Aromatic + strong E + Base
• Intermediate: Endothermic, nonaromatic (base takes hydrogen adjacent to E)
• Product: Ectothermic, aromatic with E , H-Base

• Reagents: Br₂, FeBr₃
• Electrophile: Br-Br⁺-FeBr₃
• Product: aromatic ring with Br

• Reagents: Cl₂, AlCl₃ or FeCl₃
• Electrophile: Cl-Cl⁺-Al^-Cl₃ or Cl-Cl+-FeCl₃
• Product: aromatic ring with Cl

• Reagents: I₂, HNO₃
• Electrophile: I⁺
• Product: aromatic ring with I

• Reagents: HNO₃, H₂SO₄
• Electrophile: O=N⁺=O
• Product:
• 

• Reagents: SO₃, H₂SO₄
• Electrophile: SO₃
• Product:
• 
• (After sulfuric acid ion takes H, another sulfuric acid adds H to the benzenesulfonic acid ion)

• Reagents: Zn or Sn or Fe and HCl
• 
• Intermediate: aromatic ring with N⁺H₃
• Product:
• 

• 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

• Reagents:
• 1) and AlCl₃
• 2) H₂O
• Electrophile: R-C≡O⁺
• Product:
• 

Note: AlCl₃ binds to O after acylium ion binds to benzene so H₂O is needed in second step to kick Al off

• Probability: Ortho favored (2 ortho position vs 1 para position)
• Steric hindrance: Para favored if substituent on ring or electrophile is large

• 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