Question

Which of the following group shows $-R$ effect?

• A. $-OH$
• B. $-OCOR$
• C. $-NH_2$
• D. $-CN$
• E. $-OR$

Hint:

Groups with multiple bonds to electronegative atoms (like $-CN, -NO_2$) show $-R$ effect.

Answer 1

The Correct Option is D

In organic chemistry, the concept of the mesomeric effect (or resonance effect) is crucial in understanding the behavior of functional groups attached to a conjugated system. The mesomeric effect, denoted as the \(+R\) or \(-R\) effect, refers to the electron-withdrawing or donating resonance effect exerted by substituents.

The \(-R\) effect refers to groups that withdraw electrons through resonance. This effect typically occurs when a group is attached to a conjugated system and can pull electron density away from the system.

Let's analyze each option provided:

1. \(-OH\): The hydroxyl group generally donates electrons via resonance due to the lone pairs on the oxygen, thus showing a \(+R\) effect.
2. \(-OCOR\): This group also tends to donate electrons due to resonance with adjacent pi systems, showing a \(+R\) effect.
3. \(-NH_2\): The amine group donates electrons because of the lone pairs on nitrogen, typically showing a \(+R\) effect as well.
4. \(-CN\): The cyano group is known for its strong \(-R\) effect. In the cyano group, the nitrogen atom can participate in resonance by pulling electron density towards itself, thus exhibiting the \(-R\) effect.
5. \(-OR\): Similar to hydroxyl, this group donates electrons due to oxygen's lone pairs, showing a \(+R\) effect.

Based on this analysis, the correct option that shows the \(-R\) effect is the cyano group \(-CN\). Other groups primarily exhibit the \(+R\) effect by donating electrons into the conjugated system.