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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.
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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.
Cyclopropenyl anion 4 pi electrons. Anti-aromatic
Cyclopentadienyl anion 6 pi electrons, aromatic
Azulene Yarrow chamomile 10 pi electrons aromatic Lactariusazulene
Naphthalene aromatic : cyclic, 10 electrons, flat, conjugated
naphthalene Azulene Why is naphthalene nonpolar and azulene polar?
Aromatic, anti-aromatic or neither? 8 pi electrons Depends on conformation. If flat and fully conjugated, antiaromatic!!!! Lower energy conformation (than anti-aromatic) is not flat
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 orthopara 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& paraversus 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.
