chapter 17 & 18 discussion
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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