Why Substituents Activate or Deactivate a Benzene Ring

Sample Problem 18.4 Draw the products of each reaction and state whether the reaction is faster or slower than a similar 

The lone pair on N makes this group an ortho, para activator. 

This compound reacts faster than benzene. 

The 5+ on this C makes the group a meta deactivator. This compound reacts more slowly than benzene. 

Problem 18.18 Draw the products formed when each compound is treated with HNO3 and H2SO4. State whether the reaction occurs faster or slower than a similar reaction with benzene. 

To understand why some substituents make a benzene ring react faster than benzene itself (activators), whereas others make it react slower (deactivators), we must evaluate the rate-determining step (the first step) of the mechanism. Recall from Section 18.2 that the first step in electrophilic aromatic substitution is the addition of an electrophile (E ) to form a resonance-stabilized carbo-cation. The Hammond postulate (Section 7.15) makes it pos.sible to predict the relative rate of the reaction by looking at the stability of the carbocation intermediate. 

The principles of inductive effects and resonance effects, first introduced in Section 18.6, can now be used to predict carbocation stability. 

D = electron-donor group more stable carbocation 

In other words, electron-donating groups activate a benzene ring and electron-withdrawing groups deactivate a benzene ring towards electrophilic attack. 

We learned in Section 18.6 which groups are electron donating and electron withdrawing. As a result, we know which groups increase or decrease the rate of reaction of substituted benzenes with electrophiles. 

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Why Substituents Activate or Deactivate a Benzene Ring Figure 18.6 Energy diagrams comparing the rate of electrophilic aromatic substitution of substituted benzenes... 

The rate of a subsequent substitution reaction may be either faster or slower than benzene, i.e. the ring may be activated or deactivated. Furthermore, the position of the second substitution may be at any one of three unoccupied positions. If the position of the first substituent is given the number 1, then the second substitution may occur at position 2, 3 or 4. The 5 and 6 positions just mirror the 3 and 2 positions respectively. These positions are often referred to by their old labels ortho, meta and para respectively. It is found experimentally that the position of the second substitution is related to whether or not the ring has been activated or deactivated. That is why these two issues are considered together. 

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