Orgo II chp. 15 pt.1 - Common Electrophilic substitutions

Alkene vs aromatic rxns

• Alkenes undergo addition

• Arenes undergo electrophilic substitution

What do reagents HF and BF3 do

They join rings together through single bond

• HF: Used for benzene + cyclohexene

• BF3: Used for benzene + cyclohexanol

Common Electrophilic aromatic substitutions (Regagent → substituent)

• halogenation: X2, FeX3 → X

• Nitration: HNO3, H2SO4 → NO2

• Sulfonation: SO3, H2SO4 → SO3H

• Friedel-Crafts alkylation: RCl, AlCl3 → R

• Friedel-Crafts acylation: RCOCl, AlCl3 → RCO

Arenium ion

delocalized cyclohexadienyl cation (AKA Sigma complex)

(Intermediate for electrophilic substitution)

Is arenium ion (sigma complex) formation favorable

No, must have a catalyst

Why are catalysts used in aromatic electrophilic substitution

They lower activation energy needed to react arenium ion

Aromatic halogenation mechanism steps

• Step 1: Br—Br + FeBr3 → Br—Br⁺—Fe^-Br3

• Step 2: Benzene —Br (Arenium ion) + Fe^-Br4

• Step 3: Bromobenzene + H—Br + FeBr3

Draw it out

Aromatic Nitration mechanism steps

• step 1: H₂SO₄ + HNO₃ → H₂NO3 + HSO₄

• step 2: H₂NO₃ → H₂O + NO₂

• Step 3: Benzene + NO₂ → Arenium ion

• Step 4: Arenium ion + H₂O → Nitrobenzene + H₃O⁺

Draw it

Aromatic sulfonation mechanism

• Step 1: SO₃ + H₂SO₄ → SO₃H⁺ + HSO₄^-

• Step 2: Benzene + SO₃H⁺ → arenium ion

• Step 3: arenium ion + HSO₄^- → benzenesulfonic acid + H₂SO₄

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Friedel-Crafts alkylation mechanism steps

• step 1: RCl + AlCl₃ → RCl⁺— Al^-Cl3 → R⁺ + Al^-Cl4

• step 2: Benzene + R⁺ → arenium ion

• step 3: arenium ion + Al^-Cl4 → Alkyl benzene + HCl + AlCl₃

Draw it

Why is Chlorine typically paired with the acetyl group when doing an acylation

Want a good laving group to create the carbocation acetyl group

Two common Acyl groups

• Acetyl group

• Benzoyl group

How do you get acyl chlorides

Halogenate an acid (OH)

Formation of acyl chlorides

• Acetyl group: Acetic Acid + SOCl2 → Acetyl chloride + SO2 + HCl

• Benzoyl group: Benzoic Acid + PCl5 → Benzoyl chloride + POCl3 + HCl

Acylium ion

Resonance hybrid of:

• R—C - - - O⁺

• R—C⁺ - - O

Friedel-Crafts Acylation Mechanism steps

• step 1: CORCl + AlCl₃ → CORCl⁺ — Al^-Cl₃

• step 2: CORCl⁺ — Al^-Cl₃ → acylium ion + Al^-Cl₄

• step 3: benzene + acylium ion → arenium ion

• step 4: arenium ion + Al^-Cl₄ → acetephenone + HCl + AlCl₃

• step 5: acetephenone + AlCl₃ → complex connected by O

• step 6: complex + 3H2O → acetephenone + Al(OH)₃ + 3HCl

Why does acylation not stop after initial acetephenone formation

The reaction will continue to proceed until water is added

Friedel-Crafts Acylation with acid anhydride rxn

benzene + Acetic anhydride —(AlCl₃)→ Acetephenone + Carboxylic acid