18.16
meta-Directing Deactivators: –NO2, –CN, –CHO, –CO2R, –COR, –CO2H
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Q1: Why are meta-directing groups called deactivating groups?
Meta-directing groups are deactivating because they withdraw electrons from the aromatic ring, decreasing its reactivity toward electrophilic substitution. For example, nitration of nitrobenzene occurs 100,000 times slower than benzene due to the deactivating nitro group. Electron-withdrawing groups destabilize the carbocation intermediate formed during the reaction, raising the activation energy barrier and slowing the overall reaction rate.
Q2: How do meta directors affect the carbocation intermediate in electrophilic aromatic substitution?
Meta directors destabilize the resonance-stabilized carbocation intermediate by withdrawing electrons from the aromatic ring. This electron withdrawal increases the transition state energy, creating a higher activation energy barrier compared to benzene. The destabilization of the carbocation is the key factor that makes electrophilic aromatic substitution slower for meta-substituted compounds.
Q3: Why do meta-directing groups produce meta-substituted products as the major product?
Although meta directors deactivate the ring at all positions, deactivation is stronger at the ortho and para positions than at the meta position. This differential deactivation makes the meta position relatively more reactive, favoring meta-substituted products as the major product. The ortho and para positions are so strongly deactivated that substitution at these positions is significantly slower.
Q4: What is the relationship between electron-withdrawing groups and directing effects?
All electron-withdrawing groups are meta directors that deactivate the aromatic ring. These groups include nitro, cyano, aldehyde, carboxylic acid, and carboxylic ester substituents. By withdrawing electron density from the ring, these groups reduce reactivity toward electrophilic substitution and direct incoming electrophiles to the meta position rather than ortho or para positions.
Q5: How does the activation energy barrier differ between benzene and nitrobenzene?
Nitrobenzene has a significantly higher activation energy barrier than benzene because the electron-withdrawing nitro group destabilizes the carbocation intermediate. Energy diagrams show that this increased transition state energy directly correlates with the dramatically slower reaction rate. The higher barrier explains why nitrobenzene nitration is approximately 100,000 times slower than benzene nitration.
Q6: What is the rate-determining step in electrophilic aromatic substitution with meta directors?
The rate-determining step is the formation of the resonance-stabilized carbocation intermediate when the electrophile adds to the aromatic ring. Meta directors destabilize this carbocation by withdrawing electrons, increasing the energy required to form it. This destabilization raises the activation energy barrier, making the carbocation formation step significantly slower and controlling the overall reaction rate.
Q7: How do meta-directing deactivators compare to ortho-para-directing groups in reactivity?
Meta-directing deactivators withdraw electrons and decrease ring reactivity, slowing electrophilic aromatic substitution at all positions. In contrast, ortho-para-directing groups either activate the ring by donating electrons or deactivate it less severely. The fundamental difference is that meta directors destabilize carbocation intermediates through electron withdrawal, whereas ortho-para directors either stabilize them or deactivate more weakly.