Which of the following circled hydrogen is most acidic?

Options

A.
B.
C.
D.

Explanation

To determine which of the circled hydrogens is the most acidic, we need to analyze the stability of the carbanions formed when each hydrogen is removed. The more stable the carbanion, the more acidic the corresponding hydrogen is. Here’s a step-by-step solution:

### Step 1: Identify the Circled Hydrogens
First, we need to look at the structures provided in the options and identify the circled hydrogens. Each option will have a different hydrogen that we will analyze for acidity.

### Step 2: Remove the Hydrogen to Form Carbanions
For each option, we will consider the removal of the circled hydrogen (as H⁺) to form a carbanion (C⁻). This carbanion will carry a negative charge.

### Step 3: Analyze the Stability of the Carbanions
We will evaluate the stability of the carbanions formed in each option:

- **Option A**: When the hydrogen is removed, the resulting carbanion can delocalize its negative charge over two benzene rings through resonance. This resonance stabilization makes the carbanion relatively stable.

- **Option B**: Similar to Option A, the carbanion formed here also has resonance with two benzene rings. However, the presence of nitrogen does not contribute significantly to the stability due to the lack of a vacant d-orbital.

- **Option C**: Upon removal of the hydrogen, the carbanion can resonate with two benzene rings and also form a partial double bond with phosphorus due to its vacant d-orbital. This additional resonance and the ability to form a double bond significantly enhance the stability of the carbanion.

- **Option D**: The carbanion formed here can also resonate with two benzene rings, but it lacks the additional stabilization factors present in Option C.

### Step 4: Compare the Stability
After analyzing each option, we find that:
- Option A and B have similar resonance stabilization.
- Option D has similar resonance stabilization but lacks additional factors.
- Option C has the most stabilization due to resonance with two benzene rings and the ability to form a double bond with phosphorus.

### Conclusion
Thus, the most acidic hydrogen is found in **Option C**, as the carbanion formed is the most stable due to the presence of a vacant d-orbital on phosphorus, allowing for additional resonance stabilization.