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Organic Chemistry Chapter 4 Part I

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 Chemistry Chapter 4 Part I

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  1. Organic ChemistryChapter 4Part I Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group

  2. There are TWO FACTS about BENZENE beyond our expectation 1. Reaction Behaviors Unlike Other Alkenes 2. Unexpected Stabilization Energy

  3. Benzene has 3 double bonds, so we would expect addition reaction as with other alkenes, BUT

  4. Then, What could happen to BENZENE ???

  5. It is Unbelievable, but TRUE

  6. What about Next ? - Unexpected Stabilization Energy -

  7. - Supplementary slide for the previous one - A Measure of Alkene Stability

  8. The Point is that BENZENE is Unexpected STABLE !! WHY??? Aromaticity

  9. 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

  10. 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

  11. Aromatic Cation & Anion

  12. 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)

  13. 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

  14. 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

  15. 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

  16. 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

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