1. Arenes and Aromaticity

2. HydrocarbonsHydrocarbons AromaticAromaticAliphaticAliphatic AlkanesAlkanes AlkynesAlkynes AlkenesAlkenes

3. BenzeneToluene Naphthalene Examples of Aromatic Hydrocarbons HHH HH H CH 3 H H HH H HH H HH HHH

4. Some history 1834 Eilhardt Mitscherlich isolates a new hydrocarbon and determines its empirical formula to be C n H n. Compound comes to be called benzene. 1845 August W. von Hofmann isolates benzene from coal tar. 1866 August Kekulé proposes structure of benzene.

5. Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. Kekulé Formulation of Benzene HHH HH H HHH HH H

6. Structural studies of benzene do not support the Kekulé formulation. Instead of alternating single and double bonds, all of the C—C bonds are the same length. Structure of Benzene Benzene has the shape of a regular hexagon.

7. Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures) Resonance Formulation of Benzene

8. Orbital Hybridization Model of Bonding in Benzene High electron density above and below plane of ring

9. among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2  electrons possess special stability (are aromatic) n 4n+2 0 2 1 6 210 314 418 Hückel's Rule

10. 1) Benzene is considered as the parent and comes last in the name. 2) List substituents in alphabetical order 3) Number ring in direction that gives lowest locant at first point of difference General Points

11. 2-bromo-1-chloro-4-fluorobenzene ExampleExample BrClF

12. Ortho, Meta, and Para alternative locants for disubstituted derivatives of benzene 1,2 = ortho (abbreviated o-) 1,3 = meta (abbreviated m-) 1,4 = para (abbreviated p-)

13. resonance energy = 255 kJ/mol most stable Lewis structure; both rings correspond to Kekulé benzene NaphthaleneNaphthalene

14. Anthracene Phenanthrene resonance energy: 347 kJ/mol 381 kJ/mol Anthracene and Phenanthrene

15. Reactions of Arenes: Electrophilic Aromatic Substitution HE+ EY + HY ++++ ––––

16. Representative Electrophilic Aromatic Substitution Reactions of BenzeneHE+ EY + HY ++++ ––––

17. HE+ EY + HY ++++ –––– Electrophilic aromatic substitutions include: NitrationSulfonationHalogenation Friedel-Crafts Alkylation Friedel-Crafts Acylation

18. Nitration of Benzene H+ + H2OH2OH2OH2O H 2 SO 4 HONO 2 NO 2 Nitrobenzene (95%)

19. Sulfonation of Benzene H+ + H2OH2OH2OH2O heat HOSO 2 OH SO 2 OH Benzenesulfonic acid (100%)

20. Halogenation of Benzene H+ + HBr FeBr 3 Br 2 Br Bromobenzene (65-75%)

21. Friedel-Crafts Alkylation of Benzene tert-Butylbenzene (60%) H+ + HCl AlCl 3 C(CH 3 ) 3 (CH 3 ) 3 CCl

22. Friedel-Crafts Acylation of Benzene 1-Phenyl-1-propanone (88%) H + + HCl AlCl 3 O CH 3 CH 2 CCl CCH 2 CH 3 O