ENERGY

(-) 27.4kcal/mol

(-) 28.3kcal/mol

(-) 30.3kcal/mol

Heat evolved upon catalytic hydrogenation (DHo)A MEASURE OF ALKENE STABILITY

ALKENE ALKANEH2/Pd-C

Catalytic hydrogenation of benzene and various cyclohexenes

3 x -28.6kcal/mol

2 x -28.6kcal/mol

Magnitude of aromatic stabilization = 36 kcal/mol

Isolated alkene

Isolated diene

Conjugateddiene

Benzene

Benzene (C6H6) is not “cyclohexatriene!

Each sp2 hybridized C inthe ring has anunhybridized p orbitalperpendicular to the ringwhich overlaps around thering

1879Landenberg

BrBr

No reaction

Br2

Br2

FeBr3 /Br2Br

Bonds broken

Bonds made

p bondBr-Br bond

2 C-Br

If reaction occurredp bondBr-Br bond BUT would also loose AROMATIC STABILIZATION

INSTEAD - SUBSTITUTION

ADDITION

EFFECT OF AROMATIC STABILIZATION ON REACTIONS OF BENZENE (and other aromatic compounds)

Annulenes• Initially, all cyclic

conjugated hydrocarbonswere proposed to bearomatic.

• However, cyclobutadiene isso reactive that it dimerizesbefore it can be isolated.

• And cyclooctatetraeneadds Br2 readily.

• Look at Molecular Orbitals (MO’s) to explain

aromaticity in benzene-likemolecules.

Constructing Molecular Orbitals• Pi molecular orbitals are the sideways overlap of p

orbitals.• p orbitals have 2 lobes. Plus (+) and minus (-) indicate

the opposite phases of the wave function, notelectrical charge.

• When lobes overlap constructively, (+ and +, or - and -)a bonding MO is formed.

• When + and - lobes overlap, waves cancel out and anode forms; antibonding MO.

node+

+

++

--

--

Energy of isolated p orbitals on C1 and C2

p (BONDING)

p* (ANTI-BONDING)

ENERGY

CONSTRUCTIVE OVERLAP

DESTRUCTIVE OVERLAP C CH

H

H

H

ENERGY

MO Rules for Benzene• Six overlapping p orbitals must form six molecular

orbitals.• Three will be bonding, three antibonding.• Lowest energy MO will have all bonding interactions, no

nodes.• As energy of MO increases, the number of nodes

increases.• System symmetric so 2 pairs of degenerate orbitals

See 16.3 (Wade)

ENERGY ENERGY

Energy Diagram for Benzene

The six electrons fill three bonding pi orbitals.All bonding orbitals are filled (“closed shell”), an extremely

stable arrangement (AROMATIC STABILIZATION).

6 atomic orbitals - 6 molecular orbitalsSystem symmetric so 2 pairs of degenerate orbitals

Energy Diagram for Cyclobutadiene

Following Hund’s rule,two electrons are inseparate orbitalsbecause they are atsame energy.

Most stable if filled withan electron pair (aswith benzene)

If cyclobutadiene adopted a coplanargeometry - two of the molecularorbitals would each have a singleunpaired electron - very unstable.Applies to any (4 n) systemCyclobutadiene is ANTIAROMATIC

THE ACIDITY OF THE PYRIDINIUM ION

• Heterocyclic aromatic compound.• Nonbonding pair of electrons in sp2 orbital,

so weak base, pKb = 8.8.

AROMATIC CATIONS AND ANIONS

O

O

CO2H

N

OHO

H

CO2H

MeO

CO2H

HO

HOOH

NH

H2N

O

O NEt

Et

NH

CF3

acetyl salicylic acidASPIRIN

ibuprofenADVIL

naproxenALEVE

acetaminophenTYLENOL

MANY BENZENE AND NAPHTHALENE DERIVATIVES ARE USEFUL DRUGS

ALBUTEROLbronchodilator

procaineNOVOCAINE

local anaesthetic

FENFLURAMINEappetite suppresant

Reactive intermediate for attack by nucleophile

OH

H

OHHH

OH

HH

CO2H

OH+H

H

OH

CARCINOGENICITY OF AROMATIC COMPOUNDS

O2 /Cytochrome P450

oxidizing enzymes in the liver

TOLUENE

O2 /Cytochrome P450

oxidizing enzymes in the liver

BENZENE

arene oxide

Base

PHENOL

1,2-hydride shift

DNA

DNA

mRNA

N

NN

N

NH H

O

O

OP

OO

O

P OOO

PROTEINS

OH+

POLYNUCLEAR AROMATIC HYDROCARBONS

NAPHTHALENEPYRENE

PHENANTHRENEANTHRACENE

3,4-BENZPYRENE

BENZ[a]ANTHRACENE

3,4-BENZPYRENEOXIDE

DNA alkylation by benzpyrene oxide causes errors in reading of genetic code