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Chapter 22: Problem 17

Propose an explanation for the fact that the trifluoromethyl group is almost exclusively meta directing.

Short Answer

Expert verified
The trifluoromethyl group (-CF3) is meta-directing in aromatic substitution reactions due to its electron-withdrawing effect on the aromatic ring through the inductive effect. This causes a decrease in electron density in the ring, resulting in the destabilization of ortho and para intermediates during electrophilic aromatic substitution. At the meta position, the positive charge formed during the reaction can be stabilized by diffusing the charge away from the electron-withdrawing group, leading to a more stable intermediate and favoring meta substitution.

Step by step solution

01

Understand the electronic effects of the trifluoromethyl group

The trifluoromethyl group (-CF3) has a withdrawing effect on the aromatic ring due to the inductive effect. Fluorine atoms are highly electronegative and pull electron density away from the carbon atom in the group, leading to a polarization of the C-F bonds and a net positive charge on the carbon atom. This positive charge results in a reduced electron density around the carbon atom and consequently, the electron density in the aromatic ring decreases due to this inductive effect.
02

Determine the intermediate formations during electrophilic aromatic substitution

When an electrophilic aromatic substitution reaction occurs on an aromatic ring with a substituent, three resonance structures can be formed: ortho, meta, and para substitution. These resonance structures correspond to the site where the new group will be added to the ring relative to the existing substituent. The main factor to determine which structure to form is the stability of the intermediate formed during the reaction.
03

Compare the stability of the ortho, meta, and para intermediates

The order of stability of intermediates in an electrophilic aromatic substitution reaction is directly influenced by the electron-donating or electron-withdrawing nature of the existing substituent. A group that has an electron-donating effect on the aromatic ring will stabilize the intermediates formed by resonance, favoring ortho and para substitution. On the other hand, a group that has an electron-withdrawing effect will generally destabilize the intermediates formed by resonance, favoring meta substitution.
04

Explain the meta-directing nature of the trifluoromethyl group

As we have seen, the trifluoromethyl group has an electron-withdrawing effect on the aromatic ring due to the inductive effect. This withdrawing effect destabilizes ortho and para intermediates, as the positive charge formed in these positions does not have a strong resonance stabilization. At the meta position, the positive charge formed during the reaction can be stabilized by the diffusion of charge away from the electron-withdrawing group. This stabilization leads to the formation of a more stable intermediate, and meta substitution is favored. Therefore, the trifluoromethyl group is meta-directing.

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