Question Details

Which of the following resonating structure is the most stable?

Options

${CH}^{+} - CH = CH - N^{+} (= O) - O^{-}$

$CH = CH - {CH}^{+} - N^{+} (= O) - O^{-}$

$CH - {CH}^{+} - {CH}^{+} - N^{+} (= O) - O^{-}$

${CH}^{+} - CH = CH - N^{2+} (- O^{-}) (- O^{-})$

Show Answer

Correct Answer :

Option A

${CH}^{+} - CH = CH - N^{+} (= O) - O^{-}$

Solution :

The correct answer is:

${CH}^{+} - CH = CH - N^{+} (= O) - O^{-}$

Step-by-step Explanation:

To determine the relative stability of the given resonating structures, we apply the standard rules of resonance stability:
1. Charge Separation and Repulsion: Resonating structures with like charges (positive-positive or negative-negative) on adjacent atoms are highly unstable due to strong electrostatic repulsion.
2. Location of Positive Charge Relative to Electron-Withdrawing Groups: A positive charge (carbocation) is highly destabilized when it is placed directly adjacent to a strongly electron-withdrawing group, such as the positively charged nitrogen atom ( $- N^{+} O_{2}$ ) group.

Let's analyze each option based on these criteria:

Option 1:
${CH}^{+} - CH = CH - N^{+} (= O) - O^{-}$
In this structure, the positive charge on the carbon atom is at the terminal position, separated from the positively charged nitrogen atom ( $N^{+}$ ) by two carbon atoms ( $- CH = CH -$ ). Because the positive charges are separated by multiple bonds, electrostatic repulsion is minimized, making this the most stable structure.

Option 2:
$CH = CH - {CH}^{+} - N^{+} (= O) - O^{-}$
Here, the carbocation ( ${CH}^{+}$ ) is directly adjacent to the positively charged nitrogen atom ( $N^{+}$ ). The placement of two positive charges on adjacent atoms causes severe electrostatic repulsion, making this structure highly unstable.

Option 3:
$CH - {CH}^{+} - {CH}^{+} - N^{+} (= O) - O^{-}$
This structure features adjacent carbocations ( ${CH}^{+} - {CH}^{+}$ ) as well as a carbocation adjacent to the positively charged nitrogen. The multiple adjacent positive charges lead to extreme destabilization.

Option 4:
${CH}^{+} - CH = CH - N^{2+} (- O^{-}) (- O^{-})$
This structure contains a highly charged nitrogen atom ( $N^{2+}$ ) along with two negative charges on the oxygens. The greater degree of charge separation and high formal charges make it significantly less stable than Option 1.

Thus, Option 1 is the most stable resonating structure because it minimizes electrostatic repulsion between positive charges by keeping them separated.