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CHEM 267 (7)
Chapter 18

CHEM 267: Chapter 18 Textbook Notes

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CHEM 267
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 Br 2s added across a double bond, a reaction occurs, but when Br2is added with benzene, no reaction occurs because it is much more stable Hückel’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)  Hückel’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 1 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  However, the [6] and [18]annulenes are plana
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