Is BH3 a Lewis acid or a Lewis base? Why?
The correct answer and explanation is:
BH3 (Borane) is a Lewis acid. This is because it has an electron-deficient boron atom that can accept electron pairs.
In Lewis acid-base theory, a Lewis acid is a substance that can accept an electron pair, while a Lewis base is a substance that can donate an electron pair. BH3, or borane, consists of a boron atom bonded to three hydrogen atoms. Boron in BH3 has only six electrons in its valence shell, which is less than the stable octet (eight electrons) it needs to achieve. This makes boron electron-deficient and eager to accept electron pairs from a Lewis base.
The lack of a complete octet on boron is the primary reason why BH3 behaves as a Lewis acid. The electron deficiency on boron makes it highly reactive and able to coordinate with a Lewis base, such as ammonia (NH3) or a hydride ion (H−), to form a stable complex. In these reactions, BH3 accepts electron pairs from the base, filling its electron deficiency.
A classic example of BH3 acting as a Lewis acid is its reaction with ammonia. When BH3 reacts with NH3, the lone pair of electrons on the nitrogen atom of ammonia is donated to the boron atom, forming a complex. This process illustrates the behavior of BH3 as a Lewis acid, accepting electron density to satisfy its electron-deficient state.
In summary, BH3 is a Lewis acid because it has an electron-deficient boron center that can accept electron pairs from a donor, making it an excellent example of Lewis acid behavior in chemical reactions.