CH 332:Organic Chemistry 2

Chapter 15 Benzene and Aromaticity

Benzene

• Simplest aromatic hydrocarbon• Undergoes completely different

reactions than isolated or nonaromatic conjugated alkenes• Due to its stability!

Benzene• Structure:

– Six-membered ring– Has 3 conjugated π bonds– All C-C bond lengths are equal

• Kekulé structures:

• Comparative heats of hydrogenation:

• Benzene is much more stable than a normal conjugated system

Benzene’s Unusual Stability

Actual: -28.6 kcal/mol

2(-28.6) = -57.2 kcal/mol

3(-28.6) = -85.8 kcal/mol

Actual: -55.4 kcal/mol

Actual: -49.8 kcal/mol

Benzene’s Unusual Stability• Stability of benzene can be

explained by molecular orbital theory– Consider only the 6 p orbitals:

www.chem.ucalgary.ca/courses/351/Carey5th/Ch11/benzene-mo.jpg

– 6 molecular orbitals are formed:

Benzene’s Unusual Stability

The 6 π electrons completely fill all of the bonding molecular orbitals:

AntibondingMOs

BondingMOs

Hückel’s Rule

A molecule is aromatic if it:is cyclic,is planar,is a completely conjugated

compound,and has 4n + 2 electrons

Benzene’s Unusual Stability• Cyclobutadiene is unusually

unstable.• Why?

Antiaromatic Compounds• Consider the molecular orbitals for

cyclobutadiene:

– How many π electrons?– How many π molecular orbitals?

Nonaromatic vs. Antiaromatic

Nonaromatic: a compound that lacks one (or more) of the requirements for aromaticity. Stability similar to comparable acyclic molecule:

(similar stability)

Nonaromatic vs. Antiaromatic

Antiaromatic: a cyclic, planar, completely conjugated compound with 4n electrons. Less stable than comparable acyclic molecule:

less stable

Examples of Aromatic Compounds

• Planar annulenes with 4n + 2 electrons

• [10]-annulene is not planar (nonaromatic)

Annulenes are monocyclic hydrocarbons with

alternating single and double bonds.

Examples of Aromatic Compounds

naphthalene

• Polycyclic aromatic compounds– Note: Hückel’s rules only apply to

monocyclic systems– Even so, fused aromatic rings tend to

form molecules that are aromatic– Electrons are delocalized throughout

the entire molecule

antharacene phenanthrene

Examples of Aromatic Compounds

• Aromatic heterocycles– Heterocycles containing O, N, or S

atoms that have at least 1 lone pair of electrons may be aromatic• Examples:

NNH O

pyridine pyrrole furan

Examples of Aromatic Compounds

• Aromatic heterocycles– Heteroatom must be sp2 to be aromatic

• Example: aromatic or not?

O

O

Examples of Aromatic Compounds

• Charged aromatic compounds– Negatively and positively charged ions

are aromatic if they satisfy all the requirements for aromaticity• Examples:

Nomenclature: Benzene Derivatives

• Monosubstituted benzene derivatives– Substituent named first, followed by

the word benzene• Examples:

NO2 I

Nomenclature: Benzene Derivatives

• Monosubstituted benzene derivatives– Learn these common names:

Nomenclature: Benzene Derivatives

• Disubstituted benzene derivatives– Substituents can be numbered or

named using ortho-, meta-, para- prefixes• Examples:

ClNH2 NO2

OH

CH3

Br

Nomenclature: Benzene Derivatives

• Benzene derivatives with more than 2 substituents– Substituents are numbered

• Example:

O2N NO2

NO2

CH3

Nomenclature: Benzene Derivatives

• Aromatic substituents: benzyl vs. phenyl vs. aryl

Aryl (Ar): any aromatic substituent that is not Bn or Ph