Aromaticity

Dr. A.K.M. Shafiqul Islam13.10.08

hydrocarbons

aliphatic aromatic

alkanes alkenes alkynes

Aromaticity

• Aliphatic compounds: open-chain compounds and ring compounds that are chemically similar to open-chain compounds. Alkanes, alkenes, alkynes, dienes, alicyclics, etc.

• Aromatic compounds: unsaturated ring compounds that are far more stable than they should be and resist the addition reactions typical of unsaturated aliphatic compounds. Benzene and related compounds.

Aromatic Compounds

• Aromatic was originally used to described some fragrant compounds in early 19th century.

• Later, aromatic was used for specific chemical behavior – cyclic unsaturated compounds that undergo substitution rather than addition.

• Current: aromatic compounds distinguished from aliphatic compounds by a special electronic configuration.

Benzene• This aromatic hydrocarbon was first discovered in 1825 but its

structure was not generally agreed upon until 1946.

Facts about benzene:• Formula = C6H6

• Isomer number:one monosubstituted isomer C6H5Y knowntwo disubstituted isomers C6H4Y2 known

• Benzene resists addition reaction, undergoes substitution reactions.

• Heats of hydrogenation and combustion are far lower than they should be.

• From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.

KMnOKMnO44 oxidationoxidation no reactionno reaction

BrBr22/CCl/CCl44 additionaddition no reactionno reaction

HIHI additionaddition no reactionno reaction

HH22/Ni/Ni reductionreduction no reactionno reaction

Reagent Cyclohexene Benzene

Benzene + 3 H2, Ni, room temp. NR

Benzene + 3 H2, Ni, 200oC, 1500 psi cyclohexane

Although highly unsaturated, benzene does not react like alkenes, dienes, cycloalkenes, or alkynes (addition reactions) rather it undergoes substitution reactions instead.

Kekule Structure of Benzene

-CH=CH-CH=CH-CH=CH-

HC

HC

CH

CH

CH

HC

• Open chain structure not possible

• Cyclic structure of benzene proposed by Kekule (1872)

Benzene

HH

H

H

H

H

The Lewis representation of benzene suggests that we deal with a six-membered ring of carbon atoms that are held together by alternating single and double bonds.

C: 6 • 4 = 24 valence electronsH: 6 • 1 = 6 valence electrons----------------------------------------Total = 30 electrons

= 15 bonds

Benzene

This implies that we should observe alternatingshort (1.33 A) and long (1.54 A) bond lengths.

Measurements indicate that all bond lengths are the same (1.39 A). Again, we need to draw resonance structures.

Aromatic Compounds

• Aromatic compound: a hydrocarbon that contains one or more benzene-like rings– arene: a term used to describe aromatic compounds– Ar-: a symbol for an aromatic group derived by removing an -H from an

arene– Kekulé structure for benzene (1872)

C

CC

C

CC

H

H

H

H

H

H

A Kekulé structureshowing all atoms

A Kekulé structureas a line-angle formula

• But experiments show that the Kekulé structure is not correct. All C-C bonds are identical and benzene does not undergo addition reactions typical of double bonds.

• A correct description is given by resonance theory or by orbital models – valence bond or molecular orbital.

Benzene• Resonance structure for benzene (1930s)

– the theory of resonance, developed by Linus Pauling, provided the first adequate description of the structure of benzene

– according to the theory of resonance, certain molecules and ions are best described by writing two or more Lewis structures; the real molecule or ion is a resonance hybrid of these structures

– each individual Lewis structure is called a contributing structure– we show that the real molecule is a resonance hybrid of the

two or more Lewis structures by using a double-headed arrow between them

Benzene– here are two contributing structures for benzene

– the resonance hybrid has some of the characteristics of each Lewis contributing structure

– the length of a carbon-carbon bond in benzene, for example, is midway between that of a carbon-carbon single bond and a double bond

Delocalized electrons are not confined between two adjacent bonding atoms, but actually extend over three or more atoms.

Hybrid Resonance Structure

Hydrogenation of cyclohexene and benzene

The Hº for the hydrogenation of benzene is predicted to be -85.8 kcal/mole

Hydrogenation of cyclohexene and benzene

• The Hº for the hydrogenation of benzene was found experimentally to be -49.8 kcal/mole

• This number is much less than calculated

Hydrogenation of cyclohexene and benzene

The Two Criteria for Aromaticity

1. It must have an uninterrupted cyclic cloud of electrons (called a cloud) above and below the plane of the molecule.

2. The cloud must contain an odd number of pairs of electrons.

The Criteria for Aromacity: Hückel's Rule

• A molecule must be cyclic, planar, completely conjugated and contain a particular number of electorn. That is, they are the kind of hydrocarbons treated by Hückel's rule.

Hückel's rule

1. A molecule must be cyclic2. a molecule must be planer3. a molecule must be conjugated4. a molecule must satisfy Hückel's rule and

contain a particular number of electron

Aromaticity and the 4n + 2 Rule• Huckel’s rule (based on calculations) – a planar cyclic molecule with

conjugated double bonds has aromatic stability if it has 4n+ 2 electrons, where n is 0,1,2,3,4 (any integer)

• For n=1: 4n+2 = 6; benzene is stable and the electrons are delocalized

among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2 electrons possess special stability (are aromatic)

n 4n+2

0 2

1 6 benzene!

2 10

3 14

4 18

Hückel's Rule

Applying the Criteria of Aromaticity

• Monocyclic compounds with alternating single and double bonds are called annulenes.

• Cyclobutadiene, benzene, and cyclooctatetraene are examples of annulenes.

Applying the Criteria of Aromaticity

• Cyclopentadiene, cyclopentadienyl cation, and cyclopentadienyl anion.

• Cyclopentadiene and the cyclopentadienyl cation are not aromatic. The cyclopentadienyl anion is aromatic.

• Resonance contributors and resonance hybrid for cyclopentadienyl anion.

• All carbons are equivalent in the cyclopentadienyl anion.

Naphthalene, phenanthrene and chrysene are aromatic

• Naphthalene, phenanthrene, and chrysene

Arometic Heterocyclic Compounds

• Pyridine, pyrrole, furan, and thiophene are heterocyclic aromatic compounds

Orbital structure of pyridine

• A pyridine molecule showing p atomic orbitals on the nitrogen and carbon atoms

Resonance structures of pyrrole

• The lone pair electrons are in a p orbital that overlaps the p orbital of the adjacent carbons, forming a pi bond

Orbital structures of pyrrole and furan

• Schematic of pyrrole and furan molecules showing p atomic orbitals on the ring

Resonance contributors of furan

• The oxygen is sp2 hybridized. One lone pair is in an sp2 orbital. One lone pair is in a p orbital that overlaps the p orbitals of adjacent carbons forming a pi bond