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Chapter 16

CHEM 242 Chapter Notes - Chapter 16: Molecular Orbital Theory, Sodium Cyclopentadienide, Ion


Department
Chemistry
Course Code
CHEM 242
Professor
Jisun Lee
Chapter
16

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Chapter 16 Aromatic Compounds
16-2: The Structure and Properties of Benzene (pg 713)
16-5: Aromatic, Antiaromatic, and Nonaromatic Compounds (pg 722)
- Antiaromatic compounds meet the first three criteria, but delocalization of the pi electrons over the ring
increases the electronic energy (e.g. cyclobutadiene).
- Nonaromatic compounds are cyclic but do not have a continuous, overlapping ring of p orbitals.
16-6: Hükel’s Rule (pg 722)
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- Common aromatic systems have 2, 6, or 10 pi electrons, for N = 0, 1, or 2.
- Antiaromatic systems might have 4, 8, or 12 pi electrons, for N = 1, 2, or 3.
- Cyclooctatetraene is an exception to this rule; it does not show the high reactivity associated with
atiaroatiity, yet it is ot aroati either reatios are typial of alkees; Hükel’s rule siply does ot
apply here eause ylootatetraee assues a oplaar tu oforatio.
- Like cyclooctatetraene, larger annulenes with (4N) systems do not show antiaromaticity because they have
the flexibility to adopt nonplanar conformations; they all react as partially conjugated polyenes
- Aromaticity in the larger (4N+2) annulenes depends on whether the molecule can adopt the necessary planar
conformation.
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16-8: Aromatic Ions (pg 726)
Cyclopentadienyl ions:
- With four pi electrons (a cation), Hckels rule predicts this system to be antiaromatic.
- With six pi electrons (an anion), Hckels rule predicts aromaticity.
- Because the cyclopentadienyl anion (six pi electrons) is aromatic, it is unusually stable compared with other
carbanions.
- Cyclopentadiene is nearly as acidic as water and more acidic than many alcohols. It is entirely ionized by
potassium tert-butoxide:
- The cyclopentadienyl cation is not easily formed. Protonated cyclopenta-2,4-dien-1-ol does not lose water
(to give the cyclopentadienyl cation), even in concentrated sulfuric acid. The antiaromatic cation is simply too
unstable.
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