15

2.5.2.Friedel-Crafts Acylation

General reaction scheme

[H] = reduction

•
•
•

ACID:Clemmensen (Zn/HCl)
BASE:Wolff-Kishner NH2NH2/base
NEUTRAL: Mozingo via cyclic thioketal followed
Nickel

by

H2/Raney

product is less reactive than the starting material thus:

•

no polyalkylation

also no rearrangements possible because of the reaction mechanism.

Mechanism:

use of acid anhydrides in Friedel-Crafts acylation chemistry.

A case study: The Haworth synthesis of naphthalenes.

2.6.

Directing Effects in Disubstituted Benzenes

(i)

Reinforcing effects

R

R+HX

O

R'

R

R'

+R'-COX

MX3

X = Cl, Br

[H]

O

R'

Cl

O

R'

O

R'

R'

O

H

R'

O

H

R'

O

AlCl3

+AlCl4

acylium ion

slow

r.d.s.

fast

+H+

O

O

O

O

OH

O

HO

O

O

AlCl3

+

Clemmensen
Zn/HCl

more reactive!

HF

Pd/heat

aromatisation

Clemmensen
Zn/HCl

CH3

NO2

SO3H

NO2

NHCOCH3
CN