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Question
Among the following compounds, what is the decreasing order of reactivity towards electrophilic substitution? 
Among the following compounds, what is the decreasing order of reactivity towards electrophilic substitution?
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Solution
The correct option is A III > I > II > IV
This is an electrophilic substitution on benzene ring. As the electron density increases on the ring, the reaction becomes faster.
OCH3 group is electron donating by +M effect. CH3 group is also electron donating by +I and hyperconjugation but is weaker than OCH3. CF3 group is electron withdrawing and decreases the electron density on the ring. Hence, reaction is fastest with OCH3 group and is slowest with CF3 group present on the ring.
So, the correct order should be:
III > I > II > IV (Reactivity order towards electrophilic substitution)
Hence, (a) is correct.
This is an electrophilic substitution on benzene ring. As the electron density increases on the ring, the reaction becomes faster.
OCH3 group is electron donating by +M effect. CH3 group is also electron donating by +I and hyperconjugation but is weaker than OCH3. CF3 group is electron withdrawing and decreases the electron density on the ring. Hence, reaction is fastest with OCH3 group and is slowest with CF3 group present on the ring.
So, the correct order should be:
III > I > II > IV (Reactivity order towards electrophilic substitution)
Hence, (a) is correct.
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