Which of the following is least likely to behave as Lewis acid?

Options

A. `NH_3`
B. `BF_3`
C. `OH^-`
D. `H_2O`

Explanation

To determine which of the following substances is least likely to behave as a Lewis acid, we need to understand the definitions of Lewis acids and bases. A Lewis acid is defined as a species that can accept a pair of electrons, while a Lewis base is defined as a species that can donate a pair of electrons.

Let's analyze the options given:

1. **NH3 (Ammonia)**:
- Ammonia has a lone pair of electrons on the nitrogen atom. This allows it to donate that lone pair, making it a Lewis base, not a Lewis acid.

2. **BF3 (Boron Trifluoride)**:
- Boron trifluoride has an incomplete octet, with only three valence electrons. It has vacant p-orbitals that can accept a pair of electrons, making it a Lewis acid.

3. **OH- (Hydroxide Ion)**:
- The hydroxide ion has a lone pair of electrons on the oxygen atom, which allows it to donate that lone pair. Therefore, it behaves as a Lewis base.

4. **H2O (Water)**:
- Water has two lone pairs of electrons on the oxygen atom. It can also donate these lone pairs, thus acting as a Lewis base.

Now, we need to identify which of these options is least likely to behave as a Lewis acid.

- **NH3** is a Lewis base.
- **BF3** is a Lewis acid.
- **OH-** is a Lewis base.
- **H2O** is a Lewis base.

From this analysis, we see that **NH3** is the least likely to behave as a Lewis acid because it primarily acts as a Lewis base due to its lone pair of electrons.

**Final Answer**: NH3 (Ammonia) is least likely to behave as a Lewis acid.

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