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

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

Then, What could happen to BENZENE ???

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 STABLE !! WHY??? 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 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 Criteria for Aromaticity

Huckel’s Rule Huckel ’ s rule, based on calculations – a plan ar cyclic molecule with alternating double an d 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 el ectrons are delocalized

Aromatic Cation & Anion

Planar, cyclic molecules with 4n pi electrons are m uch ‘LESS’ stable than expected (anti-aromatic) They will distort out of plane and behave like ordin ary alkenes 4- and 8-electron compounds are not delocalized ( single and double bonds) Compounds With 4n pi Electrons Are Not Ar omatic (May be Antiaromatic) Then, What happens to compounds that donot abide by Huckel’rule

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 Since lone pair electrons are in the aromatic ring, protonation destroys aromaticity, making pyrrole a very weak base Pyridine is a relatively weak base compared 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