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Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4...

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Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I
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Page 1: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

Aromatic Compounds- Benzene & Its Family -

Nanoplasmonic Research Group

Organic Chem-istry

Chapter 4Part I

Page 2: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

There are TWO FACTS about BEN-ZENE beyond our expectation

1. Reaction Behaviors Unlike Other Alkenes

2. Unexpected Stabilization Energy

Page 3: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

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

alkenes, BUT

Page 4: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

Then, What could happen to BEN-ZENE ???

Page 5: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

It is Unbelievable, but TRUE

Page 6: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

What about Next ?- Unexpected Stabilization Energy -

Page 7: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

A Measure of Alkene Stability

- Supplementary slide for the previous one -

Page 8: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

The Point is that BENZENE is Unexpected STA-

BLE !!

WHY???

Aromaticity

Page 9: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

• A chemical property in which a conjugated ring of unsatu-rated bonds, lone pairs, or empty orbitals exhibit a stabi-lization stronger than would be expected by the stabiliza-tion of conjugation alone

Aromaticity

1. Follow Huckel’s rule, having 4n+2 electrons in the delocalized cloud2. Are able to be planar and are cyclic3. Every atom in the circle is able to participate in delocalizing the electrons by having a p or-bital or an unshared pair of electrons

Criteria for Aromaticity

Page 10: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

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

Page 11: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

Aromatic Cation & Anion

Page 12: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

• 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 delo-calized (single and double bonds)

Compounds With 4n pi Electrons Are Not Aromatic (May be Antiaromatic)

cyclobutadiene cyclooctatetraene

Then, What happens to compounds that donot abide by Huckel’rule

Page 13: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

What is principle underlie aro-maticity ?

- 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 or-bitals

Page 14: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

Cyclobutadiene and Cyclooctate-traene

* Following Hund’s rule two electrons are in sep-arate orbitals because they are at same energy* If these compounds adopted a coplanar geome-try – two of the molecular orbitals would each have a single unpaired electron – very unstable

Page 15: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

What if heteroatoms exist in the ring ?

Pyridine & Pyrrole

Since lone pair electrons are in the aro-matic ring, protonation destroys aromatic-ity, making pyrrole a very weak base

Pyridine is a relatively weak base com-pared to normal amines but protonation does not affect aromaticity

Page 16: Aromatic Compounds - Benzene & Its Family - Nanoplasmonic Research Group Organic Chemistry Chapter 4 Part I.

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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