R3.4.7 Lewis acid-base reaction (Higher Level Only)

Lewis Acid–Base Reactions: Coordination Bond Formation

Let’s examine the reaction between ammonia (${\text{NH}}_3$) and boron trifluoride (${\text{BF}}_3$):

1. The Players:
   • ${\text{NH}}_3$: A Lewis base with a lone pair of electrons on nitrogen.
   • ${\text{BF}}_3$: A Lewis acid because boron is electron-deficient (it has an incomplete octet).
2. The Interaction:
   • The lone pair on nitrogen in ${\text{NH}}_3$ is donated to the empty $2p_z$ orbital on boron in ${\text{BF}}_3$.
   • This forms a coordination bond, resulting in ${\text{F}}_3\text{B}\to {\text{NH}}_3$, where the arrow indicates the direction of electron donation.

Representing the Reaction with Lewis Structures

Here’s how to represent this reaction step-by-step:

1. Draw the Lewis structure for ${\text{BF}}_3$, showing boron with three single bonds to fluorine and an empty orbital.
2. Draw the Lewis structure for ${\text{NH}}_3$, showing the lone pair on nitrogen.
3. Use an arrow to show the lone pair from ${\text{NH}}_3$ moving toward the boron atom in ${\text{BF}}_3$.
4. Draw the resulting structure, ${\text{F}}_3\text{B}\to {\text{NH}}_3$, with the arrow representing the coordination bond.

Nucleophiles and Electrophiles: The Link to Lewis Acids and Bases

In both organic and inorganic chemistry, Lewis acids and bases are often referred to as electrophiles and nucleophiles, respectively:

• Nucleophile (Lewis base): A species that donates an electron pair to form a bond. It is "nucleus-loving" and seeks positively charged or electron-deficient areas.

• Electrophile(Lewis acid): A species that accepts an electron pair to form a bond. It is "electron-loving" and seeks electron density.

When a nucleophile reacts with an electrophile, a new bond is formed.

Common Mistakes to Avoid

Reflection