Electrophilic and Nucleophilic Substitution in Aromatic Systemsits formation and decomposition does not lead to the exchange of
deuterium with any of the hydrogen atoms of the nucleus, showing
that no C—D bond has been formed in the complex.
In the presence of a compound having an electron-deficient orbital,
e.g. a Lewis acid such as A1C1 3 , a different complex is formed, however.
If DC1 is now employed in place of HC1, rapid exchange of deuterium
with the hydrogen atoms of the nucleus is found to take place
indicating the formation of a a complex (II) in which H® or D®, as
the case may be, has actually become bonded to a ring-carbon atom.
The positive charge is shared over the remaining five carbon atoms
of the nucleus via the n orbitals and the deuterium and* hydrogen
atoms are in a plane at right angles to that of the ring:H D
\sAIC1,Q(H)That the n and a complexes with, e.g. toluene and HCI, really are
different from each other is confirmed by their differing behaviour.
ThusHjrmation of the former leads to no colour change and but
little difference in absorption spectrum, indicating that there has been
practically no disturbance in the electron distribution in toluene;
while if AICI3 is present the solution becomes green, will conduct
electricity and the absorption spectrum of toluene is modified,
indicating the formation of a complex such as (II) as there is no
evidence that aluminium chloride forms complexes of the type,
H®A1C1 4 ®.
The reaction may be completed by A1C1 4 ® removing a proton from
the a complex (II)—(IV). This can lead only to exchange of hydrogen
atoms when HQ is employed but to some substitution of hydrogen
by deuterium with DC1, i.e. the overall process is electrophilic
substitution. In theory, (II) could, as an alternative, react by removing
CI® from AICI4® resulting in an overall electrophilic addition reaction
(II) —(III) as happens with the w orbital of a simple carbon-carbon
double bond (p. 141); but this would result in loss of tffe stabilisation
conferred on the molecule by the presence of delocalised n orbitals
involving all six carbon atoms of the nucleus, so that the product, an