Chapter 17 & 18 Discussion

Flat, cyclic, conjugated pi system and 4n + 2 pi electrons

Aromatic if 2, 6, 10, 14, 18 pi electrons…..Anti-aromatic if 4, 8, 12, 16, ….. pi electrons

If neither then not aromatic or anti-aromatic.

Anti-aromatic more stable than expected.

Anti-aromatic – less stable and likely will not form at all.

4 pi electrons. Anti-aromatic

Cyclopropenyl anion

Aromatic?

6 pi electrons, aromatic

Cyclopentadienyl anion

Aromatic?

10 pi electrons aromatic

Lactarius azulene

chamomileYarrow

Azulene

aromatic : cyclic, 10 electrons, flat, conjugated

Naphthalene

Why is naphthalene nonpolar and azulene polar?

Azulenenaphthalene

Aromatic, anti-aromatic or neither?

Aromatic, anti-aromatic or neither?

Depends on conformation. If flat and fully conjugated, antiaromatic!!!!

Lower energy conformation (than anti-aromatic) is not flat

8 pi electrons

NMR

• Proton deshielded by ring current from aromatic pi system to 6-8 ppm

• Carbon peaks between 115-140 ppm

H NMR of aromatics

Singlet 7.26 ppm

Singlet (3H) 2.34 ppm &

Second order spin coupling

• Electrophilic substitution

• Diazonium chemistry

• Nucleophilic substitution

• Misc. modification of substituents

Electrophilic Substitution Reactions of Benzene and Its Derivatives

Halogens are ortho para directors, but are still deactivating compared with benzene

Meta selective: electron withdrawing groups

•electron withdrawing group inductively destabilizes adjacent carbocations in ortho and para cases. • No adjacent carbocation in meta regiochemistry resonance structures

Summary of activation versus deactivation and ortho & para versus meta direction

With multiple substitutents there can be antagonistic or reinforcing effects

Antagonistic or Non-Cooperative

D = Electron Donating Group (ortho/para-directing)W = Electron Withdrawing Group (meta-directing)

Multiple Substituents in antagonistic systems

The most strongly activating substituent will determine the position of the next substitution. May have mixtures.

OCH3

O2N

SO3

H2SO4

OCH3

O2N

SO3H

OCH3

O2N

SO3H

+

=>

Weak step here