BF3 is a strong Lewis acid. Why. Is back bonding doesn't decreases its Lewis acidic character?

Question

Accepted Answer

BF3 is electron deficient molecule and has an empty p-orbital, so it can accept a pair of electrons, making it a Lewis acid. A Lewis acid can accept a pair of electrons from atom of same or different molecules (known as Lewis base).

Lewis acid strength of BF3 decrease due to 2p pi - 2p pi back bonding between the 2p orbitals of boron and flourine.

Answer

Yes it decreases its acidic character Order of Acidic chararcter in Boron Halides
BF3 is less than all

Answer

BF3 is a strong Lewis acid because it has an empty p orbital on the boron atom, which can accept an electron pair from a Lewis base. This ability to accept an electron pair allows it to form a coordinate covalent bond with a Lewis base, making it a strong Lewis acid.

The Lewis Acid-Base Theory:
The Lewis acid-base theory, proposed by Gilbert N. Lewis, defines an acid as a substance that can accept an electron pair (Lewis base), and a base as a substance that can donate an electron pair (Lewis acid). According to this theory, the strength of a Lewis acid depends on its ability to accept an electron pair.

Empty p Orbital in BF3:
In the case of BF3, boron is the central atom surrounded by three fluorine atoms. The boron atom has an empty p orbital, which is available to accept an electron pair. This empty orbital is the key feature that makes BF3 a strong Lewis acid.

Back Bonding:
Back bonding refers to the process in which the filled p orbital of a Lewis base overlaps with the empty p orbital of a Lewis acid. This results in the formation of a coordinate covalent bond. However, back bonding does not decrease the Lewis acidic character of BF3. Here's why:

1. Electron Density on Fluorine: The electronegativity of fluorine is higher than that of boron. As a result, the fluorine atoms in BF3 tend to attract electron density towards themselves, making the boron atom electron deficient. This electron deficiency enhances the Lewis acidity of BF3.

2. Empty p Orbital: The empty p orbital on the boron atom is still available to accept an electron pair, even in the presence of back bonding. Back bonding does not fill this empty orbital but rather forms a coordinate covalent bond with the filled p orbital of the Lewis base. Therefore, the Lewis acidic character of BF3 is not diminished by back bonding.

3. Strength of Back Bonding: Back bonding is generally weaker than the formation of coordinate covalent bonds between a Lewis acid and a Lewis base. The energy required to break a coordinate covalent bond is higher than that required to break a back bonding interaction. This indicates that the Lewis acidic character of BF3 is primarily determined by its ability to accept an electron pair through the empty p orbital.

In conclusion, BF3 is a strong Lewis acid due to the presence of an empty p orbital on the boron atom, which allows it to accept an electron pair from a Lewis base. Back bonding does not decrease its Lewis acidic character because the empty p orbital remains available for accepting electron pairs, and the strength of back bonding is generally weaker than the formation of coordinate covalent bonds.

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