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ENERGY
(-) 27.4kcal/mol
(-) 28.3kcal/mol
(-) 30.3kcal/mol
Heat evolved upon catalytic hydrogenation (DHo)A MEASURE OF ALKENE STABILITY
ALKENE ALKANEH2/Pd-C
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Catalytic hydrogenation of benzene and various cyclohexenes
3 x -28.6kcal/mol
2 x -28.6kcal/mol
Magnitude of aromatic stabilization = 36 kcal/mol
Isolated alkene
Isolated diene
Conjugateddiene
Benzene
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Benzene (C6H6) is not “cyclohexatriene!
Each sp2 hybridized C inthe ring has anunhybridized p orbitalperpendicular to the ringwhich overlaps around thering
1879Landenberg
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BrBr
No reaction
Br2
Br2
FeBr3 /Br2Br
Bonds broken
Bonds made
p bondBr-Br bond
2 C-Br
If reaction occurredp bondBr-Br bond BUT would also loose AROMATIC STABILIZATION
INSTEAD - SUBSTITUTION
ADDITION
EFFECT OF AROMATIC STABILIZATION ON REACTIONS OF BENZENE (and other aromatic compounds)
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Annulenes• Initially, all cyclic
conjugated hydrocarbonswere proposed to bearomatic.
• However, cyclobutadiene isso reactive that it dimerizesbefore it can be isolated.
• And cyclooctatetraeneadds Br2 readily.
• Look at Molecular Orbitals (MO’s) to explain
aromaticity in benzene-likemolecules.
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Constructing Molecular Orbitals• Pi molecular orbitals are the sideways overlap of p
orbitals.• p orbitals have 2 lobes. Plus (+) and minus (-) indicate
the opposite phases of the wave function, notelectrical charge.
• When lobes overlap constructively, (+ and +, or - and -)a bonding MO is formed.
• When + and - lobes overlap, waves cancel out and anode forms; antibonding MO.
node+
+
++
--
--
Energy of isolated p orbitals on C1 and C2
p (BONDING)
p* (ANTI-BONDING)
ENERGY
CONSTRUCTIVE OVERLAP
DESTRUCTIVE OVERLAP C CH
H
H
H
ENERGY
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MO Rules for Benzene• Six overlapping p orbitals must form six molecular
orbitals.• Three will be bonding, three antibonding.• Lowest energy MO will have all bonding interactions, no
nodes.• As energy of MO increases, the number of nodes
increases.• System symmetric so 2 pairs of degenerate orbitals
See 16.3 (Wade)
ENERGY ENERGY
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Energy Diagram for Benzene
The six electrons fill three bonding pi orbitals.All bonding orbitals are filled (“closed shell”), an extremely
stable arrangement (AROMATIC STABILIZATION).
6 atomic orbitals - 6 molecular orbitalsSystem symmetric so 2 pairs of degenerate orbitals
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Energy Diagram for Cyclobutadiene
Following Hund’s rule,two electrons are inseparate orbitalsbecause they are atsame energy.
Most stable if filled withan electron pair (aswith benzene)
If cyclobutadiene adopted a coplanargeometry - two of the molecularorbitals would each have a singleunpaired electron - very unstable.Applies to any (4 n) systemCyclobutadiene is ANTIAROMATIC
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THE ACIDITY OF THE PYRIDINIUM ION
• Heterocyclic aromatic compound.• Nonbonding pair of electrons in sp2 orbital,
so weak base, pKb = 8.8.
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AROMATIC CATIONS AND ANIONS
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O
O
CO2H
N
OHO
H
CO2H
MeO
CO2H
HO
HOOH
NH
H2N
O
O NEt
Et
NH
CF3
acetyl salicylic acidASPIRIN
ibuprofenADVIL
naproxenALEVE
acetaminophenTYLENOL
MANY BENZENE AND NAPHTHALENE DERIVATIVES ARE USEFUL DRUGS
ALBUTEROLbronchodilator
procaineNOVOCAINE
local anaesthetic
FENFLURAMINEappetite suppresant
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Reactive intermediate for attack by nucleophile
OH
H
OHHH
OH
HH
CO2H
OH+H
H
OH
CARCINOGENICITY OF AROMATIC COMPOUNDS
O2 /Cytochrome P450
oxidizing enzymes in the liver
TOLUENE
O2 /Cytochrome P450
oxidizing enzymes in the liver
BENZENE
arene oxide
Base
PHENOL
1,2-hydride shift
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DNA
DNA
mRNA
N
NN
N
NH H
O
O
OP
OO
O
P OOO
PROTEINS
OH+
POLYNUCLEAR AROMATIC HYDROCARBONS
NAPHTHALENEPYRENE
PHENANTHRENEANTHRACENE
3,4-BENZPYRENE
BENZ[a]ANTHRACENE
3,4-BENZPYRENEOXIDE
DNA alkylation by benzpyrene oxide causes errors in reading of genetic code