Understanding Lewis Acid-Base Theory in Reactions
What Are Lewis Acids and Bases?
Lewis acid
Lewis acid is a species that accepts an electron pair. It is often electron-deficient, meaning it has an incomplete octet or a positively charged center that makes it eager to gain electrons.
Lewis base
Lewis base is a species that donates an electron pair. It is electron-rich, often possessing a lone pair of electrons that can be shared.
Key Characteristics:
- Lewis acids can be neutral molecules (e.g., BF₃) or positively charged ions (e.g., $H⁺$, $Al³⁺$).
- Lewis bases can be neutral molecules (e.g., $NH₃$, $H₂O$) or negatively charged ions (e.g., $OH⁻$, $Cl⁻$).
Lewis acid-base theory is broader than the Brønsted-Lowry definition, as it doesn’t require the transfer of protons ($H^+$) to classify substances as acids or bases.
The Reaction Between BF₃ and NH₃: A Lewis Acid-Base Interaction
Let’s now examine the reaction:
$$
\text{BF}_3 + \text{NH}_3 \longrightarrow \text{F}_3\text{B:NH}_3
$$
Step-by-Step Breakdown:
- Identify the Lewis Acid and Base:
- BF₃:
- Boron trifluoride has an incomplete octet around the boron atom.
- It is electron-deficient and can accept a pair of electrons. Thus, BF₃ is the Lewis acid.
- NH₃:
- Ammonia has a lone pair of electrons on the nitrogen atom, making it electron-rich and capable of donating this pair.
- Therefore, NH₃ is the Lewis base.
- Formation of a Coordination Bond:
- The lone pair on NH₃ is donated to the empty orbital on BF₃, forming a new bond.
- This bond is called a coordination bond, as both electrons come from the same atom (the Lewis base).
- Resulting Product:
- The product, F₃B:NH₃, is a single molecule where the nitrogen atom is bonded to the boron atom through the shared electron pair.
Let’s visualize this with electron-dot structures:
- BF₃:
- Boron is surrounded by three fluorine atoms, but it has only six electrons in its valence shell, making it electron-deficient.
- NH₃:
- Nitrogen is surrounded by three hydrogen atoms and has a lone pair of electrons.
- When NH₃ donates its lone pair to BF₃, the boron atom completes its octet, and a stable F₃B:NH₃ complex is formed.
Why Does This Reaction Happen?
- The driving force behind this reaction lies in the electron deficiency of BF₃.
- Boron, in BF₃, has only six valence electrons, making it highly reactive and eager to accept electrons to achieve a full octet.
- NH₃, with its lone pair, is perfectly suited to fulfill this need. This mutual "give-and-take" forms a strong bond, stabilizing both species.
When identifying Lewis acids and bases in reactions, focus on electron-rich species (bases) and electron-deficient species (acids). Look for lone pairs or incomplete octets!
- In the reaction between BF₃ and NH₃, which species is the Lewis acid and why?
- What type of bond is formed when a Lewis base donates its electron pair to a Lewis acid?
- Can a positively charged ion like $H⁺$ act as a Lewis acid? Why or why not?
