Problem 24 - Chemistry An Atoms First Approach

Question

In terms of orbitals and electron arrangements, what must be present for a molecule or an ion to act as a Lewis acid? What must be present for a molecule or an ion to act as a Lewis base?

Step-by-Step Solution

Answer

For a molecule or an ion to act as a Lewis acid, it must have a vacant orbital to accept electron pairs from a Lewis base and a tendency to stabilize and accept the donated electron pair due to high electronegativity or a positive charge. Conversely, for a molecule or ion to act as a Lewis base, it must have lone pairs of electrons available for donation to a Lewis acid and a tendency to donate these electron pairs, often due to lower electronegativity or higher electron pair availability.

Step 1: Definition of Lewis Acid and Lewis Base

According to the Lewis theory, a Lewis acid is defined as an electron pair acceptor, while a Lewis base is defined as an electron pair donor. In this context, it means that a Lewis Acid has to accept electron pairs from a Lewis base to establish a chemical bond.

Step 2: Necessary Conditions for Lewis Acid

For a molecule or an ion to act as a Lewis Acid: 1. It should have a vacant orbital to accept electron pairs from a Lewis Base. This empty orbital can be caused by the central atom carrying a positive charge or having an incomplete octet. 2. It should have a tendency to stabilize and accept the donated electron pair, meaning it should have high electronegativity or carry a positive charge.

Step 3: Necessary Conditions for Lewis Base

For a molecule or an ion to act as a Lewis Base: 1. It should have lone pairs of electrons available to donate to a Lewis Acid. Usually, these are present on the more electronegative atoms in the molecule or ion. 2. It should have a tendency to donate these electron pairs to form a bond with a Lewis Acid, usually due to a lower electronegativity of the atom holding the electron pair, or in some cases, a higher electron pair availability.

Step 4: Example of Lewis Acid and Lewis Base Interaction

A classic example of a Lewis acid-base interaction can be observed in the reaction between boron trifluoride (BF3) and ammonia (NH3). Here, boron trifluoride (BF3) serves as the Lewis acid with a vacant orbital, and ammonia (NH3) acts as the Lewis base with a lone pair of electrons on the nitrogen atom. The electron-donating and electron-accepting capacities of the molecules make it possible for the nitrogen atom in ammonia to donate its lone pair of electrons to the boron atom in boron trifluoride, thus forming an adduct as shown in the following equation: \(BF_3 + NH_3 \rightarrow [BF_3 - NH_3]\) This equation demonstrates the interaction between Lewis acids and bases in terms of orbitals and electron arrangements.