Difference Between Lewis Acid and Base

Chemistry would be incomplete without interaction between Lewis Acids and Lewis Bases. The Brønsted-Lowry acid theory has shaped chemistry.

G.N. Lewis, in 1923, suggested that there is a transfer of electrons and not protons. This theory helped chemists forecast a broader range of reactions between Acids and Bases.

Lewis Acid vs Base

The main difference between Lewis Acid and Base is the ability to contribute or take on electrons. Lewis Acid is a chemical division. It has a vacant electronic shell and is competent to acquire an electron set. They form a covalent bond upon obtaining electrons. On the other hand, Lewis base is a chemical division that hands out their lone pair of electrons. Lewis base has a complete electronic shell. They have an extra set of an electron that is not entangled in any bonding.

Lewis Acid vs Base

Lewis Acid has empty shells, and they exist labeled as electrophiles. It is a species lured to an electron-rich core. Lewis Acids have lower energy as they have empty shells.

Mostly all of the cations are part of the Lewis Acid species. If a molecule, ion, or atom happens to have a deficient set of octet electrons, they behave as a Lewis Acid.

Lewis Base has abundant shells, and they exist labeled as nucleophiles, and they have a higher energy level than Lewis’s acids. They rush at a positive charge with their lone set of an electron.

Mostly all of the anions are part of the Lewis Base species. If a molecule, atom, or ion happens to have a Lone set of an electron, they behave as Lewis Base.

Comparison Table Between Lewis Acid and Base

Parameters of ComparisonLewis AcidLewis Base
EnergyChemical species have lower energy.Chemical energy has higher energy.
IonsMostly all of the cations are part of the Lewis Acid species.Mostly all of the anions are part of the Lewis Base species.
Known termElectrophileNucleophile
FeatureShow thermodynamic features when forming adducts.Show Kinetic feature.
Electrons in the outer shellDeficient set of electrons in its outer shellsExtra set of electrons free in its outer shells.

What is a Lewis Acid?

Earlier in 1916, Lewis proposed that atoms get clenched together in a chemical frame by the distribution of electrons. According to Gilbert N. Lewis, Acid can attract an electron set from a second molecule and complete a steady form for one of its atoms.

A Lewis Acid is not automatically a Brønsted–Lowry acid. When a singular electron from each atom is donated, it is called a covalent bond. When one and the other electrons approach arise of one of the atoms, it is called a coordinate bond. 

Lewis Acid gets confined to the trigonal planar classification. They are diverse and react with Lewis Base to shape adducts. Lewis Acid affirms the thermodynamic feature of adduction formation. 

Lewis Acids can get further distinguished based on hardness and softness. Hardness means they are nonpolarizable.

Based on Hardness, Acids: Boranes, alkali metal cations, H+

Based on Softness, Acids: Ni(0), Ag+

Some examples of Simple Lewis Acids:

The organoboranes and boron trihalides are a few of the Simple Lewis Acids. Illustration: BF3 + F− → BF4−

Sometimes Lewis Acid can secure two Lewis Bases:

Example: SiF4 + 2 F− → SiF62− (hexafluorosilicate)

Some examples of Complex Lewis Acids:

Sometimes some of the chemical compounds require additional activation. They need it before producing the adduct when reacting with Lewis Base. 

  1.  Usually, Monomeric BH3 does not exist, and it needs an activation step. The formation of Borane adducts is due to the activation step of diborane degrading. Illustration: B2H6 + 2 H− → 2 BH4− Reaction: [Mg(H2O)6]2+ + 6 NH3 → [Mg(NH3)6]2+ + 6 H2O
  2. Usually, Aluminium trihalides can not exist in the configuration of AIX3. It exists as a polymer and aggregates and degrades by Bases.

Application of Lewis Acids:

  • Friedel-Crafts alkylation 
  • Formation of carbonium ion that is tremendously electrophilic. Illustration: RCl +AlCl3 → R+ + AlCl4−

What is a Lewis Base?

Brønsted–Lowry acid-base theory states that whenever an Acid and Base respond with each other, the acid configures its conjugate base. The Base, on the other hand, interchanges a proton and configures its conjugate acid. 

The Lewis Theory has established based on the electronic structure. Lewis Base can hand out a set of electrons to H+ (proton), and its conjugate base of Brønsted–Lowry acid-base theory gets formed by losing H+.

So looking at Brønsted–Lowry’s acid-base theory and Lewis Theory, a Lewis base can also get classified as a Brønsted–Lowry base. 

Conventional amines (ammonia), pyridine and derivatives, and alkylamines are Lewis Bases. Lewis Base has the highest occupied molecular orbit, and they affirm the kinetic feature of adduction formation.

Lewis Bases can be further distinguished based on hardness and softness. Softness means they are polarizable and larger. 

Hard Bases: Water, Chloride, Ammonia, Amines.

Soft Bases: Carbon Monoxide, Thioethers. 

Application of Lewis Bases:

Electron pair givers that shape compounds by conclusive adapting elements get viewed as Lewis Bases. They are even known as Ligands. Thus the application of Lewis Bases lies widely in the formation of metal catalysts.

As Lewis Bases form many bonds with Lewis Acids, they become multidentate (chelating agents). 

Pharmaceutical companies depend on Chiral Lewis Bases, as they give out chirality on a catalyzer. This property facilitates the formation of asymmetric catalysis that is important for producing pharmaceuticals. 

Main Differences Between Lewis Acid and Base

  1. A Lewis Acid acquires a set of electrons. Lewis Base hands out their pair of electrons.
  2. Lewis Acid has an electron-rich core. Lewis Base rush towards positive charged atoms or molecules.
  3. Lewis Acid has a deficient set of electrons in its outer shells. Lewis Base has an extra set of electrons free from the entanglement of chemical bonding. 
  4. Lewis Acid does not have the highest occupied molecular orbit. Lewis Bases molecular orbit is highly concentrated.
  5. A Lewis acid is not a Brønsted-Lowry acid. Brønsted-Lowry Base can be a Lewis Base.

Conclusion

Lewis Acid accepts a set of electrons, and Lewis Base gives out electrons. Arrhenius Acid and Base, Brønsted Acid and Base can get observed as Lewis Base or Lewis Acid.

The strength of a Lewis Base depends on the pKa factor. If a conjugate acid has a higher pKa factor, it will give rise to a sturdier base. 

Nowadays, few techniques have got developed to forecast Lewis Acidity. Both Lewis Acid and Base are chemical species that interact with each other to form compounds.

They are species that react with each other to form Lewis Adducts. The main point of the reaction is the development of the adduct. 

References

  1. https://pubs.acs.org/doi/pdf/10.1021/cr60311a002
  2. https://www.thieme-connect.com/products/ejournals/html/10.1055/s-2005-869831
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