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Chapter 18

CHEM267 Chapter Notes - Chapter 18: Annulene, Steric Effects, Antiaromaticity

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Monica Barra

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Chem 267 Chapter 18 Notes Winter 2012
18.3 Structure of Benzene
Each C=C and C-C in benzene is not a double bond or single bond, but each is a “1.5 bond”
18.4 Stability of Benzene
Benzene is unusually stable compared to other cyclic compounds that contain a double bond
o When Br2 is added across a double bond, a reaction occurs, but when Br2 is added with
benzene, no reaction occurs because it is much more stable
ckel’s Rule
Cyclic compounds consisting of alternating single and double bonds do not have the same
reactivity as benzene (aromatic stabilization)
NOTE: the presence of a fully conjugated ring of π electrons is NOT the sole requirement for
aromaticity; the number of π electrons in the ring is also important!
o RULE: an odd number of electron pairs (π bonds) is required for aromaticity
o Ex. benzene has 3 electron pairs (3 π bonds) and is therefore aromatic; whereas
cyclobutadiene has 2 electron pairs (2 π bonds), and is therefore antiaromatic
o When the MOs of antiaromatic compounds are drawn, they contain single, unpaired
electrons in the nonbonding MOs (which are very reactive, and therefore unstable)
ckel’s rule: there must be an odd number of electron pairs (π bonds) to be aromatic
o Must have 4n+2 π electrons (2, 6, 10, 14, 18, etc.)
Simple method of drawing the energy levels of the MOs of conjugated π systems (Frost circle):
18.5 Aromatic Compounds Other than Benzene
Aromatic compounds must satisfy the following 2 criteria:
1. The compound must contain a ring comprised of continuously overlapping p orbitals
2. The number of π electrons in the ring must be a Hückel number
Examples of nonaromatic compounds (don’t satisfy the 1st criterion):
Antiaromatic = cyclic compounds that have 4n π electrons
Annulenes = compounds consisting of a single ring containing a fully conjugated π system
We would expect these 3 annulenes to be all aromatic; however, the
hydrogens of the [10] annulene cause steric hindrance
Therefore, the [10] and [14]annulenes are nonaromatic
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