Aromatic Compounds- Benzene & Its Family -

Nanoplasmonic Research Group

Organic Chem-istry

Chapter 4Part I

There are TWO FACTS about BEN-ZENE 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

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

It is Unbelievable, but TRUE

What about Next ?- Unexpected Stabilization Energy -

A Measure of Alkene Stability

- Supplementary slide for the previous one -

The Point is that BENZENE is Unexpected STA-

BLE !!

WHY???

Aromaticity

• 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

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

Aromatic Cation & Anion

• 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

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

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

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

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