Which of the following is not a resonating structure for the phenoxide ion ?

Q: Which of the following is not a resonating structure for the phenoxide ion ?

Which of the following is not a resonating structure for the phenoxide ion ?

Question

Accepted Answer

The phenoxide ion is the conjugate base of phenol, formed by the removal of a hydrogen ion (H⁺) from the hydroxyl group of phenol. This ion is stabilized by resonance, which involves the delocalization of the negative charge over the aromatic ring. Let's analyze why Option 4 is the correct answer and why the other options are incorrect.

1. **Option 4: Structure with negative charge on oxygen**  
   This option is the correct answer because the phenoxide ion itself is represented with a negative charge on the oxygen atom. This is not a resonating structure but rather the original structure of the phenoxide ion before resonance occurs. Resonance structures involve the movement of electrons and the delocalization of charge, which means the negative charge is spread over the carbon atoms in the ring, not remaining solely on the oxygen.

2. **Option 1: Structure with negative charge on ortho-carbon**  
   This is a valid resonating structure for the phenoxide ion. In resonance, the negative charge can be delocalized to the ortho positions (positions adjacent to the oxygen atom) of the benzene ring. This delocalization helps stabilize the ion by spreading the charge over a larger area.

3. **Option 2: Structure with negative charge on para-carbon**  
   Similarly, this is also a valid resonating structure. The negative charge can be delocalized to the para position (opposite the oxygen atom) of the benzene ring. This is part of the resonance stabilization process, where the charge is distributed across different atoms in the ring.

4. **Option 3: Structure with positive charge on oxygen**  
   This option is incorrect because it contradicts the nature of the phenoxide ion. The phenoxide ion is negatively charged due to the loss of a hydrogen ion, and the oxygen atom carries a negative charge initially. A structure with a positive charge on oxygen would not represent a resonating structure of the phenoxide ion.

In conclusion, Option 4 is the correct answer because it represents the initial state of the phenoxide ion rather than a resonating structure. The other options describe valid resonance forms where the negative charge is delocalized over the aromatic ring, contributing to the stability of the phenoxide ion.