Problem 58 - Chemistry The Science in Context

Question

Why is \(\mathrm{BF}_{3}\) a Lewis acid but not a Brensted-Lowry acid?

Step-by-Step Solution

Answer

Explain. Answer: \(\mathrm{BF}_{3}\) is a Lewis acid because it can accept an electron pair due to its empty 2p orbital on the boron atom. However, it is not a Brønsted-Lowry acid because it does not contain any hydrogen atoms and therefore cannot donate a proton (H⁺) to a base.

Step 1: Define a Lewis Acid and a Brønsted-Lowry Acid

A Lewis acid is a substance that can accept a pair of electrons from another atom, while a Brønsted-Lowry acid is a substance that donates a proton (H⁺) to a base.

Step 2: Determine the molecular structure of \(\mathrm{BF}_{3}\)

\(\mathrm{BF}_{3}\) has a central boron atom which is bound to three fluorine atoms, forming a trigonal planar molecule. The boron atom has 3 valence electrons and forms 3 single bonds with the fluorine atoms, also using their valence electrons. Note that Boron has an empty 2p orbital that can accommodate a pair of electrons.

Step 3: Assess if \(\mathrm{BF}_{3}\) is a Lewis Acid

A Lewis acid accepts an electron pair. The boron atom in \(\mathrm{BF}_{3}\) has an empty 2p orbital, allowing it to accept an electron pair from another atom, making \(\mathrm{BF}_{3}\) act as a Lewis acid.

Step 4: Assess if \(\mathrm{BF}_{3}\) is a Brønsted-Lowry Acid

A Brønsted-Lowry acid must donate a proton (H⁺) to a base. \(\mathrm{BF}_{3}\) has no hydrogen atoms in its molecular structure, which means it is incapable of donating a proton. Thus, \(\mathrm{BF}_{3}\) is not a Brønsted-Lowry acid.

Step 5: Conclusion

\(\mathrm{BF}_{3}\) is a Lewis acid because it can accept an electron pair due to its empty 2p orbital on the boron atom. However, it is not a Brønsted-Lowry acid because it does not contain any hydrogen atoms and therefore cannot donate a proton (H⁺) to a base.

Key Concepts

Brønsted-Lowry acid

The Brønsted-Lowry theory is a fundamental concept in chemistry used to distinguish between acids and bases. According to this theory, an acid is any substance that can donate a proton, which is a positively charged hydrogen ion ( H^+ ).

This means that for a substance to be considered a Brønsted-Lowry acid, it must contain hydrogen atoms that can be released as protons.
If there are no hydrogen atoms in the molecule, as is the case with BF_3 , it cannot act as a Brønsted-Lowry acid.

While this concept can sometimes be confusing, it is essential to remember that the defining feature of a Brønsted-Lowry acid is its ability to donate a proton.

BF3 molecular structure

The molecular structure of BF_3 plays a key role in its reactivity as an acid. BF_3 , or boron trifluoride, consists of a central boron atom surrounded by three fluorine atoms. This creates a trigonal planar shape, where the bond angles are approximately 120 degrees.

Boron, in this structure, has three valence electrons and forms three covalent bonds with fluorine atoms. Each bond involves the sharing of an electron from both boron and fluorine.

• This leaves boron with an incomplete octet, having only six electrons instead of the usual eight.
• Additionally, there is an empty 2p orbital on boron that can accept more electrons.

Understanding this structure helps explain the behavior of BF_3 as an acid under the Lewis definition.

electron pair acceptance

Electron pair acceptance is the cornerstone of the Lewis acid concept. A Lewis acid is defined as a compound that can accept an electron pair.

The empty 2p orbital in boron within BF_3 is perfectly suited for this. When BF_3 encounters a molecule or ion with a lone pair of electrons, it can act as an electron pair acceptor.

• The presence of this empty orbital allows BF_3 to form a coordinate covalent bond with a suitable electron donor.
• This interaction is the primary reason why BF_3 behaves as a Lewis acid.

Understanding electron pair acceptance clarifies the unique role BF_3 plays in various chemical reactions, despite not having the capability to act as a Brønsted-Lowry acid.