Question

Which among following functional groups exhibits $-\text{R}$ effect?

• A. $-\text{CO}-$
• B. $-\text{Br}$
• C. $-\text{OR}$
• D. $-\text{NHR}$

Hint:

An easy trick to recognize a $-\text{R}$ group is to check if the key atom is double- or triple-bonded to a more electronegative atom (e.g., $-\text{C=O}$, $-\text{NO}_2$, $-\text{C}\equiv\text{N}$). These groups act like electron vacuums!

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
We need to identify the functional group that exhibits a negative resonance effect (–R effect), meaning it withdraws electron density from an attached conjugated system.

Step 2: Key Formula or Approach:
+R groups possess lone pairs on the atom directly attached to the π-system, while –R groups contain electronegative atoms connected via multiple bonds capable of pulling π-electrons toward themselves.

Step 3: Detailed Explanation:
–Br, –OR, and –NHR all have lone pairs on their connecting atoms and donate electrons via +R effect. The carbonyl group (–CO–) has a polarized C=O bond where oxygen pulls π-electron density away, making it a –R group.

Step 4: Final Answer:
The group showing –R effect is –CO–, matching option (A).