Arenes and Aromaticity

HydrocarbonsHydrocarbons AromaticAromaticAliphaticAliphatic AlkanesAlkanes AlkynesAlkynes AlkenesAlkenes

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

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 August W. von Hofmann isolates benzene from coal tar August Kekulé proposes structure of benzene.

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

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.

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

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

among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2  electrons possess special stability (are aromatic) n 4n Hückel's Rule

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

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

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-)

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

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

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

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

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

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

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

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

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

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