The functional group that shows negative resonance effect is:
- –NH\(_2\)
- –OH
- –COOH
- –OR
The Correct Option is C
Approach Solution - 1
To determine which functional group shows a negative resonance effect, we need to understand the concept of resonance effects in organic chemistry. The resonance effect refers to the impact that substituents or functional groups can have on the electron distribution within a molecule.
The negative resonance effect, or -R effect, occurs when a substituent withdraws electrons through resonance. This usually happens when the substituent has a group capable of delocalizing electrons away from the ring or system it's attached to.
Let's analyze the given options:
- –NH2: The amine group is known for donating electrons through resonance due to the lone pair of electrons on the nitrogen. This results in a positive resonance effect (+R).
- –OH: The hydroxyl group also donates electrons through resonance due to the lone pair on the oxygen atom, resulting in a positive resonance effect (+R).
- –COOH: The carboxylic acid group is capable of withdrawing electrons through resonance because the carbon in C=O can participate in resonance, pulling electron density away from the system to which it's attached. Therefore, it shows a negative resonance effect (-R).
- –OR: The alkoxy group (-OR) behaves similarly to –OH in terms of electron donation due to the lone pair on oxygen, showing a positive resonance effect (+R).
Based on the analysis, the correct answer is that the –COOH group shows a negative resonance effect because the carboxyl group can delocalize electron density away from the molecular structure it's attached to.
Therefore, the correct option is: –COOH.
Approach Solution -2
Explanation: The negative resonance or electron-withdrawing resonance effect (also denoted as $-R$ effect) occurs when a functional group withdraws electron density through resonance from the conjugated $\pi$-system of a molecule.
Carboxylic Acid Group (--COOH): This group exhibits a strong $-R$ effect due to the presence of a highly electronegative oxygen atom doubly bonded to the carbon atom, which can pull electron density away from the conjugated system. As a result, it decreases the electron density within the conjugated system and stabilizes negative charge via resonance.
Other Options:
--NH$_2$ (Amino group) and --OH (Hydroxyl group): These groups exhibit a positive resonance ($+R$) effect due to the availability of lone pairs on nitrogen or oxygen, which can donate electron density through resonance.
--OR (Alkoxy group): Similar to --OH, the --OR group also exhibits a positive resonance effect due to the lone pairs on oxygen, making it an electron-donating group through resonance.
Conclusion: Among the given options, the --COOH group is the one that shows a negative resonance effect due to its electron-withdrawing nature.
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