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Organic Chemistry Chapter 4 Part I. Aromatic Compounds - Benzene & Its Family -. Nanoplasmonic Research Group. There are TWO FACTS about BENZENE beyond our expectation. 1. Reaction Behaviors Unlike Other Alkenes. 2. Unexpected Stabilization Energy.
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Organic ChemistryChapter 4Part I Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group
There are TWO FACTS about BENZENE beyond our expectation 1. Reaction Behaviors Unlike Other Alkenes 2. Unexpected Stabilization Energy
Benzene has 3 double bonds, so we would expect addition reaction as with other alkenes, BUT
What about Next ? - Unexpected Stabilization Energy -
- Supplementary slide for the previous one - A Measure of Alkene Stability
The Point is that BENZENE is Unexpected STABLE !! WHY??? Aromaticity
Aromaticity • A chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone Criteria for Aromaticity 1. Follow Huckel’s rule, having 4n+2 electrons in the delocalized cloud 2. Are able to be planar and are cyclic 3. Every atom in the circle is able to participate in delocalizing the electrons by having a p orbital or an unshared pair of electrons
Huckel’s Rule • Huckel’s rule, based on calculations – a planar cyclic molecule with alternating double and single bonds has aromatic stability if it has 4n+2 pi electrons(n is 0,1,2,3,4) • For n=1: 4n+2 = 6; benzene is stable and electrons are delocalized
Then, What happens to compounds that donot abide by Huckel’rule Compounds With 4n pi Electrons Are Not Aromatic (May be Antiaromatic) • Planar, cyclic molecules with 4n pi electrons are much ‘LESS’stable than expected (anti-aromatic) • They will distort out of plane and behave like ordinary alkenes • 4- and 8-electron compounds are not delocalized (single and double bonds)
What is principle underlie aromaticity ?- Molecular Orbital (MO)!!! - * Six overlapping p orbitals must form six molecular orbitals * Three will be bonding, three antibonding * As energy of MO increases, the number of nodes increases * System symmetric so 2 pairs of degenerate orbitals
Cyclobutadiene and Cyclooctatetraene * Following Hund’s rule two electrons are in separate orbitals because they are at same energy * If these compounds adopted a coplanar geometry – two of the molecular orbitals would each have a single unpaired electron – very unstable
What if heteroatoms exist in the ring ? Pyridine & Pyrrole Pyridine is a relatively weak base compared to normal amines but protonation does not affect aromaticity Since lone pair electrons are in the aromatic ring, protonation destroys aromaticity, making pyrrole a very weak base
How to name aromatic compounds ? Please refer to the text, page 123 Isomeric Structure: ortho- (o-), meta- (m-), para- (p-) As a substituent, phenyl-, benzyl