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Chapter 11: Problem 232

Explain why the \(-\mathrm{CF}_{3},-\mathrm{NO}_{2}\), and -CHO groups should be meta-orienting with deactivation.

Short Answer

Expert verified
The \(-CF_3\), \(-NO_2\), and \(-CHO\) groups are meta-orienting and deactivating in electrophilic aromatic substitution reactions due to their electron-withdrawing nature, which decreases the overall electron density in the benzene ring and makes it less reactive towards electrophilic attack. Their orientation preference arises from the resonance structures of the intermediates, which show that the meta position has the lowest electron density and therefore the most stable intermediate for electrophilic attack.

Step by step solution

01

Identifying the effect of the groups on electron density

First, we need to understand the nature of these groups. All three groups, \(-CF_3\), \(-NO_2\), and \(-CHO\), are electron-withdrawing groups (EWGs). They withdraw electron density from the benzene ring, leading to a decrease in electron density and making the ring less susceptible to electrophilic attack.
02

Examining the resonance structures

Next, we'll analyze the resonance structures of these groups with the benzene ring. When an electrophile attacks the benzene ring, the stability of the intermediate formed will be important for determining the regioselectivity. Draw the resonance structures for these groups attached to the benzene ring, and observe how the electron density is distributed in the ortho, meta, and para positions.
03

Understanding the deactivation effect

Since these groups are EWGs, they have a deactivating effect on the benzene ring, making it less reactive toward electrophilic attack. They stabilize the intermediate formed during the attack by sharing their electron-withdrawing ability. The increased stability of the intermediate means that it is less likely to react with an electrophile, leading to slower overall reaction rates.
04

Importance of meta-orientation

Considering the resonance structures of the ring with these groups attached, we find that the electron density is lowest at the meta positions. When an EWG is present on the ring, the intermediate is stabilized when the electrophile attacks the meta position because the electron-withdrawing effect of the group is not disturbed. In contrast, attacking the ortho or para position will disrupt the electron-withdrawing effect of the group, destabilizing the intermediate and making it less likely to form.
05

Conclusion

Based on the electronic properties of the \(-CF_3\), \(-NO_2\), and \(-CHO\) groups, we can conclude that they are meta-orienting during electrophilic aromatic substitution reactions. Their electron-withdrawing nature decreases the overall electron density in the benzene ring, making them deactivating groups. Additionally, the orientation preference arises from the resonance structures of the intermediates, which show that the meta position has the lowest electron density and therefore the most stable intermediate for electrophilic attack.

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Most popular questions from this chapter

\(2,2^{\prime}-\) Dinitrobiphenyl is to be synthesized. Outline the method of preparation from iodobenzene. No other organic reagent may be used. Any inorganic reagent may be employed.

Give structures and names of the principal products expected from the ring monobromination of each of the following compounds. In each case, tell whether bromination will occur faster than with benzene itself. (a) iodobenzene (b) sec-butylbenzene (c) acetophenone \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3}\right)\) (d) phenetole \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\right)\) (e) diphenylmethane \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{C}_{6} \mathrm{H}_{5}\right)\) (f) benzotrifluoride \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CF}_{3}\right)\)

Starting with the definition of partial rate factor, derive an expression relating \(\mathrm{p}^{\mathrm{G}} \mathrm{f}\) to the rate of substitution para to \(\mathrm{G}\) in \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{G}\).

Certain activated benzene rings can be chlorinated by hypochlorous acid, HOCL, and this reaction is catalyzed by \(\mathrm{H}^{+}\). Can you suggest a possible function of \(\mathrm{H}^{+}\) ?

Prepare a ketone from each of the following precursors by the Friedel-Crafts acylation method.
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