Ch. 16: Electrophilic Aromatic Substitution

Relative Heats of Hydrogenation (McMurry: Figure 15.2, p. 565) -118 kJ/mol -230 kJ/mol “” -356 kJ/mol -206 kJ/mol Benzene is 150 kJ/mol more stable than hypothetical “1,3,5-cyclohexatriene.” H2H2 cat. HH + heat

Є + electron density Benzene can act as nucleophile/electron donor/Lewis Base. Delocalized π -Electrons

HH HH HH Є + Benzene + Є + B - Energy Reaction Progress :B - Similar to electrophilic addition to alkenes. Electrophilic Aromatic Substitution

+ HH H H HH Є Benzene + Є + B- Energy Reaction Progress + H H H H HH Є :B - Resonance-stabilized cationic reaction intermediate Electrophilic Aromatic Substitution

+ HH H H HH Є + HH H H HH Є H H + H H HH Є Resonance Stabilization

+ HH H H HH Є :B - Benzene + Є + B- Energy Reaction Progress + H H H H HH Є Addition Substitution Substitution similar to E2 reaction Addition similar to S N 1 reaction. Electrophilic Aromatic Substitution Two possible paths to a product

HH H Є HH H-B Benzene + Є + Energy Substitution Product H H H H HH Є B Benzene + Є + Energy Reaction Progress Addition Product Aromatic Product Favored!