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638 CHAPTER 18 
 
Nucleophilic Aromatic Substitution 
 
 
 
Elimination-Addition 
 
 
 
 
 
 
Common Mistakes to Avoid 
 
For most ortho-para directors, we can rely on steric effects that will favor substitution at the para position 
rather than the ortho position: 
 
 
 
The major product results from para attack, and the minor product results from ortho attack. If we want to 
achieve substitution at the ortho position, then we must employ a blocking group: 
 
 
 
With this technique, we use the para-directing effects of the substituent to block (temporarily) the para 
position. This allows us to perform the desired reaction at the ortho position, followed by removal of the 
blocking group. 
 
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