Question

All Brönsted acids are Lewis acids, but the reverse is not true. Give two examples of Lewis acids that are not Brönsted acids.

Short answer

BF₃ and AlCl₃ are Lewis acids that are not Brönsted acids.

Step-by-step solution

Define Brönsted Acid

A Brönsted acid is a substance that can donate a proton (H⁺) to another substance. Examples include hydrochloric acid (HCl) and sulfuric acid (H₂SO₄).

Define Lewis Acid

A Lewis acid is a compound that can accept an electron pair from a Lewis base. It does not necessarily donate protons. Examples include boron trifluoride (BF₃) and aluminum chloride (AlCl₃).

Identify Lewis Acids that are not Brönsted Acids

Since Lewis acids accept electron pairs and are not required to donate protons, we can identify compounds like boron trifluoride (BF₃) and aluminum chloride (AlCl₃) as Lewis acids that do not have protons to donate, making them not Brönsted acids.

Key concepts

Brönsted acids

Brönsted acids are an essential concept in chemistry. They are defined by their ability to donate a proton, commonly known as a hydrogen ion (H⁺), to another substance. This donation process is what primarily characterizes a Brönsted acid.

Classic examples of Brönsted acids include:

• Hydrochloric acid (HCl)
• Sulfuric acid (H₂SO₄)

When a Brönsted acid donates a proton, it becomes its conjugate base. For instance, when HCl donates a proton, it turns into Cl⁻, its conjugate base. This proton donation is a central part of acid-base reactions in chemistry, emphasizing the important role Brönsted acids play.

Proton donation

Proton donation is a fundamental process in which a Brönsted acid releases a proton, or H⁺ ion, to another molecule or ion. This process is integral to what defines a substance as a Brönsted acid.

During this donation:

• The Brönsted acid loses a proton.
• The accepting molecule or ion, termed the base, becomes a conjugate acid.

For example, in the reaction between hydrochloric acid (HCl) and water (H₂O), HCl donates a proton to water, forming the hydronium ion (H₃O⁺) and chloride ion (Cl⁻). Proton donation is crucial for facilitating many chemical reactions, including those in biological systems, like enzyme functions and metabolic pathways.

Electron pair acceptance

Electron pair acceptance is the hallmark of Lewis acids. A Lewis acid is any compound capable of accepting an electron pair from a donor, known as a Lewis base. This can occur without the need for proton donation, distinguishing Lewis acids from Brönsted acids.

Key points about Lewis acid electron pair acceptance:

• Lewis acids are not defined by the presence or absence of protons.
• They generally have empty orbitals, allowing them to accept electron pairs effectively.

This mechanism enables Lewis acids to participate in a wide range of chemical reactions, including the formation of complex compounds and catalytic cycles, demonstrating their versatility in the field of chemistry.

Boron trifluoride

Boron trifluoride (BF₃) is a classic example of a Lewis acid that does not qualify as a Brönsted acid. It does not donate protons but excels in accepting electron pairs due to the electron-deficient nature of boron.

Here's how boron trifluoride acts as a Lewis acid:

• The boron atom in BF₃ has an empty p-orbital.
• It can accept an electron pair from a Lewis base, like ammonia (NH₃).

Due to its capacity to form coordinate bonds by accepting electron pairs, BF₃ finds wide application in industrial processes, such as catalysis and the synthesis of various organic compounds, underscoring its importance beyond a standard acid-base interaction.

Aluminum chloride

Aluminum chloride (AlCl₃) is another prominent Lewis acid that is not a Brönsted acid. It is highly reactive due to its ability to accept electron pairs, rather than donating protons.

Understanding aluminum chloride:

• AlCl₃, especially in its anhydrous form, has an electron-deficient aluminum atom.
• This deficiency allows it to readily accept electrons, making it an efficient catalyst in many chemical reactions.

AlCl₃ is extensively used in the Friedel-Crafts reactions, particularly in the synthesis of aromatic compounds. This utility showcases its role as a strong Lewis acid in facilitating complex chemical transformations, illustrating its widespread use in both industrial and laboratory settings.