Which of the following is the strongest base ?

Options

A.
B.
C.
D.

Answer

To determine which of the given structures is the strongest base, we need to analyze the ability of each structure to donate a lone pair of electrons. Here’s a step-by-step solution:

### Step 1: Identify the Structures
First, we need to look at the four given structures and identify the functional groups present, particularly focusing on the nitrogen (N) atoms and their lone pairs.

### Step 2: Analyze Resonance Effects
For the first three structures, the nitrogen atom (with a lone pair) is directly bonded to a ring system. This means that the lone pair can participate in resonance with the ring. When a lone pair is involved in resonance, it is less available to act as a base because it is delocalized.

### Step 3: Examine the Fourth Structure
In the fourth structure, there is a methylene group (-CH2-) between the nitrogen atom and the ring. This methylene group prevents the lone pair on the nitrogen from participating in resonance with the ring. As a result, the lone pair remains localized and is more available for donation.

### Step 4: Compare Basicity
Since the lone pairs in the first three structures are involved in resonance and are less available for donation, they are weaker bases. In contrast, the fourth structure has a lone pair that is not involved in resonance, making it more available to accept protons (H+). Therefore, it is the strongest base among the four structures.

### Conclusion
The fourth structure, due to the presence of the methylene group that prevents resonance, has a lone pair that is more available for donation, making it the strongest base.

### Final Answer
The strongest base among the given structures is the fourth structure.

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