102
2 Substituted Systems
Cationic intermediates of defined constitution (n-complexes) such as
occur during electrophilic substitution of aromatic compounds are no
longer aromatics and should therefore not be termed arenium- but
... ylium ions. If there is no fundamental preference for an arbitrarily
chosen resonance form (where possible that with the greatest weight)
delocalization of charge (and also of n bonds) can be accounted for simply
by leaving out alliocants.
H
H:6=H
H*H
I e I
0
'
,
- : (±) : vs.
H
H
H ~
H
H
H
H
H
H
CyclohexaPhenylium
dien-l-ylium
dienylium
(Benzenylium)
In cases such as the following one, no possibility yet exists of assigning a
consistent name for the delocalized system; naming must again be arbitrarily based on the most plausible resonance structure.
H>( e)-Me
H -
1-Methylcyclohexa2,5-dien-1-ylium
:>(e )-Me
3-Methylcyclohexa2,4-dien-1-ylium
(Locant -1- can be neglected here since the center of charge is always given
the lowest locant possible.)
H
H
H
9,10-Dihydroanthracen-9-ylium
vs.
e
~
~
H
9-Anthrylium
(Anthracen-9-ylium)
For cationic aromatics too, naming can be based either on specific resonance forms or, in a less explicit way, on the delocalized system as a whole
by omitting any locants.