Lewis Acid and Bases

When you think of acids, what do you think of? Maybe you think of acidic foods like pineapple or vinegar, or maybe you think of the more technical definition, where a species donates a proton. Well, that definition of an acid is from the BrØnsted-Lowry theory. Today, we are going to be talking about a different theory: the Lewis acid and base theory.

In this article, we will be learning about Lewis acids and bases: what they are, some examples, and how to determine their strength.

• This article covers Lewis acids and bases.
• First, we will define what Lewis acids and bases are.
• Next, we will look at some examples and learn how the Lewis acid-base theory explains how coordination complexes are formed.
• After that, we will learn how to determine the strength of Lewis acids and bases.
• Lastly, we will work on some practice problems.

Lewis definition of acids and bases

In a Lewis acid-base reaction, the base will "attack" the acid and donate an electron pair. This causes a covalent bond to be formed between the two.This general reaction is shown below: Fig. 1:The Lewis base "attacks" the acid, forming a bond

Now let's talk about this reaction on an orbital level. The base's lone pair of electrons (non-bonded electrons), are in its highest occupied molecular orbital (called HOMO). The base is taking these electrons and interacting with the acid's lowest unoccupied molecular orbital (called LUMO). When they interact, they form a bond at a lower energy level, as shown below:

Fig.2: The base's HOMO interacts with the acid's LUMO to form a bond

Lewis acid and bases examples

Now that we know what the general reaction looks like, let's look at some examples:

Fig.3: Examples of Lewis acid-base reactions

In the top example, the fluoride ion (F^-) is our base, which attacks the acid compound boron trifluoride (BF₃). After the reaction, a new B-F bond is formed (shown in red) to make the new compound boron tetrafluoride (BF₄^-).

In the bottom example, ammonia (NH₃) is our base and reacts with the acidic proton/hydrogen ion (H⁺). This forms a new N-H bond and the new compound ammonium (NH₄⁺)

In general, if a species has a negative charge, it will be a base. However, if a species has a positive charge, it will be an acid.

Coordination complexes

Fig.5: Table of electronegativities

The elements the closer to the top right (fluorine:F) of the periodic table are more electronegative, therefore, these elements are stronger electrophiles

For example, here is the trend in electophilicity for group 2:

$$\text{most electrophilic}\,Be^{2+}>Mg^{2+}>Ca^{2+}>Sr^{2+}\,\text{less electrophilic}$$

Strength of Lewis bases

Lewis bases are nucleophiles, so their strength is based on nucleophilicity.

Nucleophilicity is based on four things

1. Charge
2. Electronegativity
3. Resonance/Charge localization
4. Steric hindrance

Let's break this down piece by piece

Charge

Nucleophiles tend to have negative charges. This is because negative charges indicate electron density (i.e. an excess of electrons). Negative charges tell us, "I have extra electrons and need to get rid of them!"

So in general:

$$A^->AH$$

Where A^- is the negative species (conjugate base) of the acid AH.

For example, HS^- is more nucleophilic than H₂S.

Electronegativity

Since electronegativity is the tendency to attract electrons, nucleophiles are stronger when they are less nucleophilic. Basically, highly electronegative species "like" having a more negative charge/high electron density, so they don't want to "give up" their electrons as much.

For example, here is what this trend looks like for our halides (group 17):

$$\text{more nucleophilic}\,I^->Br^->Cl^->F^-\,\text{less nucleophilic}$$

Resonance/Charge localization

Species are more nucleophilic when the charge is localized, rather than delocalized, as shown below:

Fig.6: Localized charges are more nucleophilic

When a charge is delocalized, as shown on the right, the charge is essentially being weakened by being spread out. Since the charge is "weaker", it is less reactive, and therefore less nucleophilic

Steric hinderance

"Sterics" relate to the spatial arrangement of atoms, so "steric hinderance" means "the way the atoms are arranged means they are in the way"

For example:

Fig.7: More R-groups mean more "bulkiness"

Basically, when a nucleophile is "bulky" it makes the reaction slower since it's bulkiness is "in the way". Because of this, bulky=less nucleophilic

Lewis acid and bases practice problems

Now that we've covered a lot about Lewis acids and bases, let's work on some practice problems:

Now let's try a trickier one: