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CHAPTER 18 649 
 
 
 
However, we will soon see (immediately after this 
SkillBuilder) that the ortho position on the left is more 
accessible for steric reasons, so substitution at this 
position leads to the major product. 
 
 
 
18.20. 
(a) The directing effects are controlled by the most 
strongly activating group. Therefore, in this case, the 
OH group (a strong activator) controls the directing 
effects. As an activator, the OH group is an ortho-para 
director. However, the para position is already occupied 
(by the methyl group). Therefore, an electrophilic 
aromatic substitution reaction should occur at an ortho 
position. Among the two ortho positions, one of them 
(top) is sterically hindered and we do not expect the 
reaction to occur at that location. Therefore, the reaction 
is most likely to occur at the following position: 
 
 
 
(b) The directing effects are controlled by the most 
strongly activating group. Therefore, in this case, the 
methoxy group (a moderate activator) controls the 
directing effects. As an activator, the methoxy group is 
an ortho-para director. However, the para position is 
already occupied (by the bromine atom). Therefore, an 
electrophilic aromatic substitution reaction should occur 
at an ortho position. Among the two ortho positions, one 
of them (top) is sterically hindered and we do not expect 
the reaction to occur at that location. Therefore, the 
reaction is most likely to occur at the following position: 
 
OMe
Br
O2N
Sterically
hindered
Reaction
occurs here 
 
(c) This aromatic ring lacks an activating substituent. 
Both substituents are deactivators, and they are 
competing with each other (each group directs to the 
positions that are meta to itself). So electronically, the 
four aromatic positions are equally likely to undergo an 
electrophilic aromatic substitution reaction. The 
positions are differentiated from each other when we 
consider steric effects. The larger group (left) is more 
sterically bulky, and therefore blocks the positions 
closest to it. The reaction therefore occurs at one of the 
following locations. 
 
 
These two locations are identical because of symmetry. 
 
 
(d) The directing effects are controlled by the most 
strongly activating group(s). Therefore, in this case, the 
OH groups (strong activators) control the directing 
effects. However, each OH group directs to different 
locations (ortho and para to itself), so all available 
positions are activated. To differentiate between them, 
we note that the following position is too sterically 
hindered for a reaction to occur at this location: 
 
 
 
Among the other two locations, one of them is more 
sterically accessible than the other, so the reaction is 
expected to occur at this location: 
 
 
 
 
 
 
 
 
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