Which among the following is a Lewis acid?

Options

A. `H_2O`
B. `B_2H_6`
C. `NH_3`
D. `CH_3OH`

Answer

To determine which among the given compounds is a Lewis acid, we need to first understand the definition of a Lewis acid. A Lewis acid is defined as an electron pair acceptor. This means that it has the ability to accept a pair of electrons from a donor (Lewis base) during a chemical reaction.

Let's analyze the given compounds one by one:

1. **Water (H2O)**:
- Water has two lone pairs of electrons on the oxygen atom. Since it has available electrons, it does not seek to accept more electrons, making it a Lewis base rather than a Lewis acid.

2. **Diborane (B2H6)**:
- Diborane is known to be electron deficient. The boron atoms in diborane have vacant p-orbitals and can accept electron pairs. Therefore, diborane qualifies as a Lewis acid.

3. **Ammonia (NH3)**:
- Ammonia has a lone pair of electrons on the nitrogen atom. It can donate this pair of electrons to form bonds, which makes it a Lewis base, not a Lewis acid.

4. **Methanol (CH3OH)**:
- Methanol also has a lone pair of electrons on the oxygen atom. Similar to water and ammonia, it can donate electrons, classifying it as a Lewis base rather than a Lewis acid.

After analyzing all the compounds, we conclude that the only Lewis acid among the options provided is diborane (B2H6).

### Final Answer:
**Diborane (B2H6) is a Lewis acid.**