Chapter 12 Conjugation and Aromaticity

 

 

            Aromatic structures (rings)

 

            - many natural products (amino acids and proteins)

            - many dyes

            - graphite

            - some molecules that exhibit liquid crystalline behavior

- some aromatic compounds are highly carcinogenic some are not carcinogenic at all

 

Aromatic compounds have the characteristic of being unusually stable and undergoing different reactions from nonconjugated and conjugated alkenes.

 

Examples of how aromatic compounds react differently from alkenes.  (Benzene is used since it is the archetypal aromatic ring.)

 

1.  Hydrohalic acids

electrophilic addition

 

                                                            1,2- and 1,4-addition (electrophilic)

 

 

2.  Halogenation

 

 electrophilic addition

 

                                                            1,2- and 1,4-addition (electrophilic)

 

 

electrophilic aromatic substitution

 

 

3.  Reaction of Strong Acids

 

 

 

                                                            electrophilic aromatic substitution

 

 

 

4.  Oxidation by KMnO4

 

 

 

 

5.  Hydrogenation

 

 

very slow reaction

 

6.  Diels-Alder Reaction

 

 

 

 

 

 

 

 

 

           

                                   

Which resonance form contributes more?  Do you expect a difference in bond lengths?

 

 

 

All C atoms in benzene are sp2 hybridized.  Overall benzene is planar.

 

 

p orbitals can overlap as in  conjugated alkenes, except there is a ring of overlapping orbitals

results in a continuous p electron cloud above and below ring for 3 p bonds and 6 p electrons are delocalized over 6 C atoms

           

                                                           

                                                             

                                                                   

                              

                                                           

 

The extensive delocalization predicted by the resonance forms explains the structure of benzene and also its increased stability relative to acyclic conjugated compounds such as 1,3,5-hexatriene.

 

 

 

Molecular Orbitals of Benzene

 

            - MO’s also explain increased stability of benzene

 

            - MO’s also explain structure of benzene

 

 

These are all p orbitals.  The s bonding MO’s are lower in energy.  The s* orbitals are all higher in energy.  Remember though that it is the HOMO and LUMO and orbitals close in energy to them that are most important.

 

MO

New nodes

Bonding

Antibonding

Net

A

0

6

0

6

B

1

4

2

2

C

1

2

0

2

D

2

2

4

-2

E

2

0

2

-2

F

3

0

6

-6

 

 

 

If this accurately portrays the relative energies of the MO's, which compound would be more stable?

 

 

 

Experimental Characteristics of Aromatic Compounds

 

1.  approximately equal C-C bond distances

 

2.  planar

 

3.  unusual chemical stability

 

4.  undergo electrophilic aromatic substitution (Chapter 13)

 

5.  In NMR spectroscopy, the presence of a diamagnetic ring current is observed.  d for aromatic-H = 7 - 8 ppm  (Chapter 14).

 

 

Structural Requirements for Aromaticity

 

Experimental evidence has shown that molecules that are considered aromatic have the following characteristics in common.

 

aromatic characteristics = aromaticity

 

1.  Aromaticity requires conjugated multiple bonds.

 

                       

                        aromatic                                                         not aromatic

                        unusual stability                                             no special stability

 

2.  Aromaticity requires fully conjugated cyclic structures.

 

             

not aromatic, not cyclic                     conjugation doesn’t extend over entire ring,

                                                            not aromatic

 

However

                                    cyclooctatetraene                  cyclobutadiene

                                    not aromatic                           not aromatic

                                    no special stability                extremely unstable

 

 

 

 

 

 

3.  Hückel’s Rule : Planar monocyclic p compounds with (4n+2) p electrons are aromatic (where n = 0, 1,2).

(Here n is an integer, not the number of p electrons.  (4n+2) is the number of p electrons.  Huckel’s Rule just states that planar monocyclic p compounds with 2, 6, 10, 14, etc. electrons are aromatic)

 

n

p electrons

0

2

1

6

2

10

3

14

4

18

 

                                   

Planar monocyclic p compounds with (4n) p electrons are unstable and highly reactive.  They are called antiaromatic.

 

 

n

p electrons

0

0

1

4

2

8

3

12

4

16

 

 

        6 p electrons, aromatic

 

 

 

 4 p electrons, antiaromatic

 8p electrons, antiaromatic, should be very unstable, but actually a stable molecule, avoids antiaromaticity by adopting a nonplanar geometry where p orbitals do not overlap

 

 

 

 

Are these compounds aromatic or antiaromatic?

 

 

 

 

 Tropylium cation (shown with a counter ion): still a relatively reactive compound, but much more stable compared to other conjugated carbocations

 

Cyclopentadienyl anion: still a highly reactive and basic anion, but more stable and less basic than other conjugated carbanions

 

unusually acidic for a hydrocarbon, pKa = 16

 

                                                H-CºC-H                    pKa = 25

 

                                                CH3CH2-OH               pKa = 15.9