A Guidebook to Mechanism in Organic Chemistry

Electrophilic and Nucleophilic Substitution in Aromatic Systems

Me,C -£o=o

C 11^

CO+Me,C PhCMe,

A useful synthetic application of Friedel-Crafts acylation is the use

of cyclic anhydrides in a two-stage process to build a second ring on -

to an aromatic nucleus:

O

II

ii

o

C.H.

A1C1,

o

IlOH

H.SO.

O

HCOC1 is very unstable but formylation may be accomplished by

e

protonating carbon monoxide to yield H0=0, i.e. by use of CO,

HC1 and A1C1 3 (the^rattermann-Koch reaction):

C.H„+HC=0

Aid.

PhCHO+H®

DIAZO COUPLING
Another classical electrophilic aromatic substitution is diazo coupling,
in which the effective electrophile has been shown to be the diazonium
-cation:

4 ® 9

Ph-N^N — PhN=N

4J " " "

This is, however, a weak electrophile compared with species such as

®N0 2 and will normally only attack highly reactive aromatic com­

pounds such as phenols and amines; it is thus without effect on the

otherwise highly reactive Ph-OMe. Introduction of electron-

withdrawing groups into the o- or/j-positions of the diazonium cation

Rearrangement of R does not take place as with alkylation, but if

it is highly branched loss of CO can occur leading ultimately to

alkylation rather than the expected acylation: