Which of the following compounds is the weakest Bronsted base?

Options

A.
B.
C.
D.

Explanation

To determine which of the following compounds is the weakest Bronsted base, we need to analyze the given compounds based on their ability to accept protons (H⁺ ions). The compounds provided in the question are:

1. Aniline (C6H5NH2)
2. Phenol (C6H5OH)
3. Anisole (C6H5OCH3)
4. Nitrobenzene (C6H5NO2)

### Step-by-Step Solution:

**Step 1: Understand Bronsted Bases**
A Bronsted base is a substance that can accept protons. The strength of a Bronsted base is determined by the stability of the conjugate acid formed after the base accepts a proton.

**Step 2: Analyze Aniline**
Aniline has an amino group (NH2) that can accept a proton to form anilinium ion (C6H5NH3⁺). The conjugate acid is relatively stable due to resonance stabilization from the aromatic ring.

**Step 3: Analyze Phenol**
Phenol has a hydroxyl group (OH) and can accept a proton to form phenolium ion (C6H5OH2⁺). However, the stability of the conjugate acid is less than that of anilinium ion because the oxygen atom is more electronegative and holds onto the proton more tightly.

**Step 4: Analyze Anisole**
Anisole has a methoxy group (OCH3) which is an electron-donating group. It can also accept a proton to form anisoleium ion (C6H5OCH3H⁺). The conjugate acid is stabilized by resonance, but not as effectively as anilinium ion.

**Step 5: Analyze Nitrobenzene**
Nitrobenzene contains a nitro group (NO2), which is a strong electron-withdrawing group. It does not readily accept protons to form a stable conjugate acid. In fact, the nitro group significantly decreases the basicity of the compound.

**Step 6: Compare the Basicity of the Compounds**
- Aniline is a strong Bronsted base.
- Phenol is a weaker Bronsted base than aniline.
- Anisole is also a weak Bronsted base, but slightly stronger than phenol due to the electron-donating effect of the methoxy group.
- Nitrobenzene is the weakest Bronsted base due to the strong electron-withdrawing effect of the nitro group.

### Conclusion:
Among the given compounds, **Nitrobenzene (C6H5NO2)** is the weakest Bronsted base.

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