2. Substituted Benzenes      Symmetry and selection rules limit actual transitions: ~185 nm (  ~60,000), “primary band” or “E band” ~204 nm (  ~8,000), “second primary band” or “K band” ~256 nm (  ~200), “secondary band” or “B band”

3. Substituted Benzenes Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

4. Polyaromatics

5. Substituent Effects on Aromatic Absorption 256 nm band is sensitive to electron density of aromatic ring  units = L mole -1 cm -1 Electron density Red = highest Green = moderate

6. pH Effects on Aromatic Absorption Phenoxide ion electrostatic potential map Anilinium ion electrostatic potential map  units = L mole -1 cm -1

7. AromaticCarbonylCompounds Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

8. Woodward Rules for Enones Base values –acyclic or 6-membered ring215 nm –5-membered ring202 nm Additions for –double bond extending conjugation 30 nm –alkyl group or ring residue  10 nm  12 nm   18 nm -hydroxyl group  35 nm  30 nm exocyclic double bond 5 nm

9. Are you getting the concept? Predict the absorption max for the enol of 1,2-cyclopentanedione. 202 (base) 12 (  substituent) 35 (  ) ----- 249 nm Obs. 247 nm

10. Common Solvents Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds