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Chapter 3: Problem 34

The most strongly deactivating substituent in electrophilic aromatic substitution is (a) \(-\mathrm{F}\) (b) \(-\mathrm{C} \equiv \mathrm{N}\) (c) CC(=O)O (d) \(-\mathrm{CF}_{3}\)

Short Answer

Expert verified
(a) -F (b) -C≡N (c) CC(=O)O (d) -CF3 Answer: (b) -C≡N

Step by step solution

01

Evaluate the inductive and resonance effects

Inductive effect: The electron-withdrawing ability of a group through the sigma bonds. More electronegative (electron-withdrawing) substituents are typically more deactivating due to a stronger inductive effect. Resonance effect: Some groups can donate or withdraw electrons through the pi bonds. Electron-withdrawing groups (EWG) have a deactivating resonance effect, while electron-donating groups (EDG) have an activating resonance effect.
02

Assess the fluorine substituent (-F)

Inductive effect: Fluorine is very electronegative and will withdraw electron density through the sigma bonds, making the aromatic ring less electron-rich. Resonance effect: Fluorine can participate in resonance, but it is not as strong of an electron-withdrawing group by resonance as some other substituents.
03

Assess the cyano substituent (-C≡N)

Inductive effect: The -C≡N group is electron-withdrawing through the sigma bonds. Resonance effect: The cyano group has a strong electron-withdrawing resonance effect, which deactivates the aromatic ring for electrophilic aromatic substitution.
04

Assess the carboxylic ester substituent (CC(=O)O)

Inductive effect: This group is electron-withdrawing due to the presence of two oxygen atoms. Resonance effect: The ester group can participate in resonance, but the double bond and lone pairs on the carbonyl oxygen interfere with the aromatic system, which weakens the electron-donation through resonance.
05

Assess the trifluoromethyl substituent (-CF3)

Inductive effect: The -CF3 group is highly electron-withdrawing due to the presence of three fluorine atoms, which are highly electronegative. Resonance effect: The trifluoromethyl group does not contribute to resonance.
06

Compare all substituents and determine the most deactivating one

Comparing all the substituents based on their inductive and resonance effects, we can deduce the following order of deactivation: (b) \(-\)C≡N (strongly deactivating due to both inductive and resonance effects) (c) CC(=O)O (deactivating due to inductive and weaker resonance effects) (d) -CF3 (strongly deactivating due to the inductive effect, but no resonance contribution) (a) -F (moderately deactivating due to inductive effect and weaker resonance effects) The most strongly deactivating substituent in electrophilic aromatic substitution is option (b) \(-\)C≡N.

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