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:
The problem asks to identify which functional group exhibits a negative resonance effect ($-\text{R}$ effect), meaning it actively withdraws electron density away from an attached conjugated $\pi$-system.

Step 2: Key Formula or Approach:

$+\text{R}$ (Positive Resonance) groups: Possess at least one lone pair of electrons on the atom directly linked to the conjugated system, allowing them to donate electron density into the ring/chain.

$-\text{R}$ (Negative Resonance) groups: Typically contain electronegative atoms connected by multiple bonds ($\pi$-bonds) capable of pulling electron pairs toward themselves via delocalization.

Step 3: Detailed Explanation:
Let's evaluate the electronic behaviors of each group: $-\text{Br}$ (Bromine), $-\text{OR}$ (Alkoxy), and $-\text{NHR}$ (Amino) all possess unshared lone pairs on their connecting atoms (Br, O, and N respectively). Consequently, they can push these lone pairs into an adjacent $\pi$-system, exhibiting a $+\text{R}$ effect. The carbonyl group, $-\text{CO}-$, contains a highly polarized carbon-oxygen double bond (C=O). Because oxygen is significantly more electronegative than carbon, it pulls the shared $\pi$-electrons toward itself: $$-\text{C=O} \longleftrightarrow -\xrightarrow{+}{\text{C}}-\text{O}^-$$ When attached to a conjugated system (like a benzene ring), the carbonyl carbon draws electron density out of the system via resonance, which is defined as a $-\text{R}$ (electron-withdrawing resonance) effect.

Step 4: Final Answer:
The functional group exhibiting the $-\text{R}$ effect is $-\text{CO}-$, matching option (A).