Question

Consider the reaction $$ \mathrm{BF}_{3}+\mathrm{NH}_{3} \longrightarrow \mathrm{F}_{3} \mathrm{~B}-\mathrm{NH}_{3} $$

Short answer

BF3 and NH3 react to form a coordinate covalent bond, resulting in \( \mathrm{F}_3\mathrm{B}-\mathrm{NH}_3 \).

Step-by-step solution

Understand the Reaction

The given reaction is a typical example of a Lewis acid-base reaction. In this reaction, boron trifluoride \( \mathrm{BF}_3 \) acts as a Lewis acid (electron pair acceptor), and ammonia \( \mathrm{NH}_3 \) acts as a Lewis base (electron pair donor).

Identify the Electron Pair Donor and Acceptor

In the reaction \( \mathrm{BF}_3 + \mathrm{NH}_3 \to \mathrm{F}_3\mathrm{B}-\mathrm{NH}_3 \), boron in \( \mathrm{BF}_3 \) has an incomplete octet and can accept an electron pair. The nitrogen atom in \( \mathrm{NH}_3 \) has a lone pair of electrons that it can donate to the boron atom.

Form the Coordinate Covalent Bond

The lone pair on the nitrogen atom in \( \mathrm{NH}_3 \) is donated to the boron atom in \( \mathrm{BF}_3 \), forming a coordinate covalent bond. This creates the adduct \( \mathrm{F}_3\mathrm{B}-\mathrm{NH}_3 \), where nitrogen now shares its lone pair with boron.

Write the Product

The final product of the reaction is \( \mathrm{F}_3\mathrm{B}-\mathrm{NH}_3 \), which is the stabilized adduct with a complete octet around the boron atom. This new compound has a more stable electronic configuration.

Key concepts

Electron Pair Donor

In a Lewis acid-base reaction, the electron pair donor is the substance that provides a pair of electrons to form a bond. In the reaction between boron trifluoride \( \mathrm{BF}_3 \) and ammonia \( \mathrm{NH}_3 \), ammonia serves as the electron pair donor. The nitrogen atom in \( \mathrm{NH}_3 \) has a lone pair of electrons that is not involved in bonding with hydrogen atoms. This lone pair is free to be shared with other atoms, making it an ideal electron pair donor.

• Ammonia \( \mathrm{NH}_3 \) has a stable molecular structure with a lone pair of electrons.
• The ability of nitrogen to donate this pair differentiates ammonia as a Lewis base.
• Donating its electron pair allows ammonia to bond with electron-deficient atoms like boron in \( \mathrm{BF}_3 \).

In this context, the donation of the electron pair from nitrogen to boron completes the bonding requirements, compensating for boron's electron deficiency.

Electron Pair Acceptor

The electron pair acceptor in a Lewis acid-base reaction is the component that receives an electron pair to form a bond. In the reaction \( \mathrm{BF}_3 + \mathrm{NH}_3 \rightarrow \mathrm{F}_3\mathrm{B}-\mathrm{NH}_3 \), boron trifluoride \( \mathrm{BF}_3 \) acts as the electron pair acceptor.

The reason \( \mathrm{BF}_3 \) accepts electrons lies in the boron atom's electronic structure:

• Boron has three valence electrons and forms three covalent bonds with fluorine in \( \mathrm{BF}_3 \), leaving it with an incomplete octet.
• Due to its electron deficiency, boron eagerly accepts a lone pair of electrons from a donor like \( \mathrm{NH}_3 \).

By accepting the electron pair, boron fulfills its tendency to achieve a stable electronic configuration, resulting in the formation of a more stable compound.

Coordinate Covalent Bond

A coordinate covalent bond, also known as a dative bond, occurs when both electrons in a bond originate from the same atom. In the interaction between \( \mathrm{BF}_3 \) and \( \mathrm{NH}_3 \), the coordinate bond is vital. Here, the nitrogen atom in \( \mathrm{NH}_3 \) donates a lone pair of electrons to the electron-deficient boron in \( \mathrm{BF}_3 \).

This process can be broken down into the following steps:

• The lone pair on nitrogen provides the two electrons that enter a shared electron environment with boron.
• The donating atom, nitrogen, effectively becomes part of the new molecule \( \mathrm{F}_3\mathrm{B}-\mathrm{NH}_3 \), without changing its oxidation state.
• This type of bond is significant because it stabilizes molecules and complexes with deficient atoms, leading to the formation of stable compounds.

In summary, a coordinate covalent bond represents a typical bond formation mechanism in Lewis acid-base reactions, marked by one atom furnishing both bonding electrons.

Incomplete Octet

In chemical bonding, an incomplete octet refers to atoms that do not have eight electrons surrounding them. This concept is crucial to understanding the behavior of some Lewis acids like boron trifluoride \( \mathrm{BF}_3 \).

Here are key aspects of the incomplete octet:

• Boron, in \( \mathrm{BF}_3 \), possesses only six electrons through its bonds with fluorine, making it electron-deficient.
• An incomplete octet makes boron a perfect electron pair acceptor, as it seeks additional electrons to achieve stability.

By reacting with an electron pair donor such as \( \mathrm{NH}_3 \), boron can complete its octet through the formation of a new bond. This interaction and property of having an incomplete octet elevate boron's reactivity and propensity to form Lewis acid-base adducts.