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Lecture

Chapter 15 notes

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Department
Chemistry
Course Code
CHMB42H3
Professor
Wanda/ Lana

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15.1 Aromatic Compounds Are Unusually Stable  Benzene is very stable o Has very high delocalization energy  Recall: Delocalization energy is a measure of how much more a compound with delocalized electrons is than it would be with localized electrons  More so than other compounds with delocalized energies  Aromatic compounds – compounds with unusually large resonance energies o These are particularly stable 15.2 The Two Criteria for Aromaticity  The Two Criteria for Aromaticity: o 1. Must have an uninterrupted cyclic cloud of π electrons (called a π cloud) above and below the plane of the molecule  Therefore:  Molecule must be cyclic  Every atom in the ring must have a p orbital  Each p orbital must overlap with the p orbitals on either side of it o Thus, the molecule must be planar o 2. The π cloud must contain an odd number of pairs of π electrons  By Huckel’s rule (see below)  For a compound to be aromatic, it must be cyclic and planar and have an uninterrupted cloud of π electrons. The π cloud must contain an off number of pairs of π electrons.  Huckel’s rule (4n + 2 rule) – Rule states that for a planar, cyclic compound to be aromatic, its uninterrupted π cloud must contain (4n + 2) π electrons, where n is any whole number o Rule requires that an aromatic compound have 1, 3, 5, 7, 9 and so on pairs of π electrons  Ie. An odd number of pairs of π electrons 15.3 Applying the Criteria for Aromaticity  Annulenes – Monocyclic hydrocarbons with alternating single and double bonds o Cyclobutadiene – 2 pairs of π electrons o Cyclooctatetraene – 4 pairs of π electrons o Benzene – 3 pairs of π electrons  Benzene is the only aromatic compound listed from the above  Cyclobutadiene and cyclooctatraene have an even number of pairs of π electrons o Aromaticity requires an odd number  Cyclooctatraene has bond angles of 135° o Aromaticity requires the ring to be planar  Cyclopentadiene is not aromatic o Does not have an uninterrupted ring of p orbital bearing atoms 3  One of its ring atoms is sp  Only sp and sp carbons have p orbitals  Cyclopentadienyl cation is not aromatic o Has an uninterrupted ring of p orbital-bearing atoms o However, has an even number of pairs of π electrons  Cyclopentadienyl anion is aromatic o Has an uninterrupted ring of p orbital-bearing atoms  The lone-pair electrons are π electrons by resonance o π cloud contains an odd number of pairs of π electrons 15.4 Aromatic Heterocyclic Compounds  Heterocyclic compounds can also be aromatic o A cyclic compound in which one or more of the ring atoms is an atom other than carbon  heteroatom – The non-carbon atom  Most common heteroatoms are N, O and S  Pyridine’s lone-pair electrons on the nitrogen atom are not π electrons  Pyrole’s lone-pair electrons on the nitrogen atom are π electrons by resonance 15.5 Some Chemical Consequences of Aromaticity  Cyclopentadiene has high pa of 15 o Very acidic for a hydrogen bonded to an sp carbon o Resulting anion is aromatic  High stability, therefore readily loses proton  Cycloheptatrienyl bromide is usually in its ionic form o Resulting cation (from the loss of Br) is aromatic 15.6 Antiaromaticity  Antiaromatic compounds are highly unstable o Fulfills the first criterion for Aromaticity, but does not
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