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

ADDITION Bonds Bonds

2

2 No reaction

Br

Br

exothermic

would be

Br broken made Br

bond (65 kcal/mol)) 2 C-Br

Br-Br (2 x 70 (50 kcal/mol) kcal/mol)

bond 2 C-Br Br-Br bond endothermic also would

loose AROMATIC STABILIZATION

INSTEAD - SUBSTITUTION

3 / 2FeBr Br Br

p

p

Aromatic compounds like benzene undergo SUBSTITUTION Instead of ADDITION

For bromination E+ = Br-Br + FeBr3

Energy Diagram for Bromination

ELECTROPHILIC SPECIES IN EAS

Br2/FeBr3Br

HNO3/H2SO4NO2

D2O/D2SO4D

O

R

ClAlCl3

R Cl

OAlCl3

OR Cl2/AlCl3 for Cl

FeBr3BrBr

O N O

D

RO

RO

ELECTROPHILE

nitronium ion

deuterium ion

carbocation

acylium ion

+Nitronium O=N=O

Benzene +

Acylium CH3C=O

Summary of Directing Effects

Four categories:I. Strongly activating, ortho/para directing (eg. -OMe)II. Weakly activating, ortho/para directing (eg. -CH3)III. Weakly deactivating, ortho/para directing (eg. -Cl,)IV. Deactivating, meta directing (eg. -NO2)

I. Strongly activating, ortho/para directing (LP electrons)(eg. -OMe, Inductive and Resonance effects)

NH2 OMe Anisole Aniline

I. Strongly activating, ortho/para directing(eg. -OMe, Consider inductive and resonance effects)

II. Weakly activating, ortho/para directing (eg. -CH3 Consider inductive effects)

Stability of Intermediate defines outcome

Intermediate is more stable if nitration occurs at the ortho or para position.

Carbocation stability!

II. Weakly activating, ortho/para directing (eg. -CH3 inductive)

III. Weakly deactivating, ortho/para directing(eg. -Cl, Inductive/Resonance)

(III. Weakly deactivating, ortho/para directing eg. -Cl,Consider inductive and resonance effects)

IV. Deactivating, meta directing (eg. -NO2, inductive effects) NO2

NO2O2N

IV. Deactivating, meta directing (eg. -NO2, inductive)

SYNTHETIC STRATEGIES WITH AROMATIC COMPOUNDS

STERIC EFFECTS IN ELECTROPHILIC AROMATIC SUBSTITUTION