Electrophilic and Nucleophilic Substitution in Aromatic Systems *
MeOx MeO® MeO®0
e
OMe activating thtfo- and /^-positions preferentially leading, there-
for?*le o//>-substitulfon and N0 2 deactivating the o- and /»-positions
preferentially leading, therefore, to /M-substitution by default as this
is the least deactivated position. This, however, is considering the
state of affairs in the starting material, the substrate, whereas the
previous argument compared the several alternative metastable inter
mediates or a complexes. As the formation of the transition state is
the determining step in the reaction, a consideration of the factors
that influence the stability of the related a complex is likely to prove
the more reliable guide as the a complex resembles the transition state
more closely than does the substrate.
This is readily seen with styrene which, considering the substrate
only, might be expected to substitute m- due to electron-withdrawal
(XX):
CH^CH, CH—CH,relative rates of attack on o-, m- and /'-positions and though either
o/p- or m-substitution is usually preponderant, neither alternative is
of necessity exclusive. Thus nitration of toluene has been found to
lead to « 3 per cent of wi-nitrotoluene and of r-butylbenzene to
« 9 per cent of the w-nitro derivative.
A somewhat less satisfactory explanation of a substituent being
predominantly either o/p- or w-directing is provided by