Two or More Groups in Conflict

FIGURE 12.39 Size may influence ortho/para ratios. 

We can look at a more complete table (Table 12.3) of these data it is clear that as the size of the substituent increases, the more likely substitution is to take place at the para-position. 

Predict the major site(s) of electrophilic nitration of each of the following molecules  

Both groups are ortho, para-AivecXin and activating, but the resonance donation of the OH group is stronger than the inductive donation of the methyl group, so its directing effect dominates. 

With a biphenyl substrate, we first need to decide which ring is likely to react. Since NMcj is a donor and COOMe withdraws electrons, it is the nitrogen-bearing ring that is more electron rich and hence more reactive. NMcj is ortho, / ara-directing and activating. The other product of ort/zo-substitution is possible but less likely to be formed, as that position would be relatively sterically hindered. 

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