2.3_aromaticity_post

2.3_aromaticity_post - Aromaticity H2/Ni H = -28.6 kcal/mol...

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2.3 Aromaticity 1 Aromaticity cyclohexene 1,3-cyclohexdiene 1,3,5-cyclohexatriene H 2 /Ni H 2 /Ni H 2 /Ni Δ H = -28.6 kcal/mol Δ H calc = 2 x –28.6=-57.2 kcal/mol Δ H obs = - 55.4 kcal/mol Diff = 1.8 kcal/mol Diff = 36 kcal/mol Δ H obs = - 49.8 kcal/mol Δ H calc = 3 x –28.6 = - 85.5 kcal/mol The delocalization of the six π electrons makes the electrons more stable. The difference of 36 kcal/mol is a measure of the added stability of the aromatic system compared to that of the localized system Benzene is unusually stable and not very reactive
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2.3 Aromaticity 2 Molecular Orbitals (MOs) of Benzene 6 atomic p orbitals will combine to form 6 molecular orbitals one node two nodes three nodes Two molecular orbitals that have the same potential energy are called degenerate Antibonding Orbitals Bonding Orbitals Potential Energy π 1 π 3 π 2 π 4 π 6 π 5
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2.3 Aromaticity 3 What Makes a Monocyclic Compound Aromatic The structure must be cyclic and conjugated Each atom in the ring must have an unhybridized p orbital (usually a sp 2 or sp hybridized atom) The unhybridized p orbitals must overlap to form a continuous ring of parallel orbitals. The structure must be planar or nearly planar. Delocalization of the pi electrons over the ring must lower the electronic energy. But are all conjugated monocyclic systems aromatic? Clyclobutadiene Four electrons in four atomic p orbitals, when combined will form 4 π molecular orbitals π 1 π 3 π 2 π 4 bonding orbital nonbonding orbital antibonding orbital Both degenerate MOs have 2 bonding and 2 antibonding for a net bonding order of zero + - - - - - - - - - - - Nodal plane Nodal plane Two nodes No nodes If the pi bonds delocalized it would be a diradical (two unpaired electrons). It is not aromatic and it is very reactive and unstable.
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2.3 Aromaticity 4 Clyclobutadiene Is there an easier way of determining the MOs? π 1 π 3 π 2 π 4 Benzene π 1 π 3 π 2 π 4 π 6 π 5 Clyclooctatetraene bonding orbitals nonbonding orbital antibonding orbitals 4 atomic p orbitals 6 atomic p orbitals 8 atomic p orbitals 4 molecular orbitals orbitals 6 molecular orbitals orbitals 8 molecular orbitals orbitals Polygon rule: States that the MO energy diagram of a regular completely conjugated cyclic system has the same shape as the compound with one vertex (the all bonding) at the bottom. Not in text book
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2.3 Aromaticity 5 Clyclooctatetraene is not aromatic and not planar. Bond lengths alternate from long to short – single to double. Cyclooctatetraene has two nonbonding orbitals each with one electron This is an unstable configuration; cyclooctatetraene adopts a nonplanar conformation with localized p bonds to avoid this instability 1.34 Ǻ 1.48 Ǻ
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2.3 Aromaticity 6 What does it mean: A compound is aromatic?
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This note was uploaded on 07/18/2011 for the course NE 122 taught by Professor Nano during the Winter '09 term at Waterloo.

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2.3_aromaticity_post - Aromaticity H2/Ni H = -28.6 kcal/mol...

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