Lewis Acids and Bases

Lewis acids and bases are defined based on the transfer of electrons. So, according to Lewis theory of acids and bases and their reactions, a Lewis base is an electron-pair donor, and a Lewis acid is an electron-pair acceptor. So, a Lewis base tends to donate an electron-pair to a Lewis acid to form a product with a coordinate covalent bond. This product is known as Lewis adduct.

They are named after American chemist Gilbert Newton Lewis. He introduced a broader concept of acids and bases based on electron transfer. It is called the Lewis concept.

Lewis Acid

They are chemical species with a tendency to accept an electron pair owing to their empty orbitals. They can be referred to as electrophiles as they are electron-deficient chemical species and thus have a strong tendency to accept electrons. The term 'Lewis Acid' was generally used to denote chemical species with a trigonal planar structure and an empty p-orbital. For example, BR₃, where R can be a halide or an organic substituent.

Further, there are also some substances that can be considered both Lewis acids as well as Lewis bases as they can accept and donate electron-pair depending on the reaction. For example, water.

Some examples of Lewis Acids

The following chemical species can act as Lewis acid as they can accept electron pairs:

• H⁺ ions (protons) and onium ions such as H₃O⁺
• D block elements with high oxidation states such as Fe³⁺
• Metal cations like Mg2+ and Li+ that can form coordination compounds with water and act as the ligand behave as Lewis acid as they can accept electron pair
• Carbocations produced by H₃C⁺ and other trigonal planar species

Lewis Base

A chemical species (atomic or molecular) that has the highest occupied molecular orbital can behave as Lewis bases owing to its ability to donate an electron pair to a Lewis acid to form an adduct.

Some common Lewis bases include ammonia, alkyl amines, and more. They are typically anionic in nature, and their base strength varies with the pKa of the parent acid. Lewis bases are classified as nucleophiles as they are rich in electrons and tend to donate electron pairs.

Examples of Lewis Bases

• Pyridine and its derivatives owing to their ability to donate electrons are considered as Lewis bases
• The compounds wherein oxygen, sulphur, selenium and tellurium (group 16 elements of the periodic table) show an oxidation state of -2. For example, water, ketones, etc.
• Anions with an electron pair for example H^- and F^-. Further, complex anions like sulfate anion (SO₄^2-) can also donate electron-pairs
• ?-systems rich in electrons, for example, benzene, ethyne, ethane also tend to donate electrons

Further, many molecules with lone pair of electrons are referred to as Lewis bases owing to their ability to donate electron pairs. For example, CH3^- and OH^-.

Chemical reactions between Lewis Acids and Bases

1. Reaction with the H+ ion
   H⁺, which is a Lewis acid, reacts with H₂O, which is a Lewis base. In this reaction, a proton and a water molecule react with each other to form a hydronium ion (H₃O⁺) as shown below:
   H⁺ + H₂O → H₃O⁺
   H⁺ being a Lewis acid accepts an electron-pair from the oxygen atom and forms a coordinate covalent bond. The Lewis acid formed in this reaction has a +1 charge. Similarly, H⁺ acts as a Lewis acid when it reacts with ammonia (NH₃) to form an ammonium ion (NH₄+). See the reaction below:
   NH₃ + H⁺ → NH₄⁺
   Here, the nitrogen atom donates electron-pair to the proton and an ammonium cation (Lewis adduct) is formed.
2. Reaction of Ag+ and Ammonia
   Here, the reaction takes place between two Lewis bases and one Lewis acid to form an adduct as shown below;
   2NH3 + Ag⁺ → Ag (NH3)₂⁺
   Here, Ag+ is a Lewis Acid and ammonia is a Lewis base. In this reaction, each nitrogen atom gives an electron-pair to Ag⁺ which results in the formation of two separate coordinate covalent bonds.
3. Reaction of Fluoride Ion and Boron Trifluoride
   In this reaction, the coordinate bond is formed between the fluoride anion (F^-) and boron trifluoride (BF₃). The reaction takes place as follows:
   F^- + BF₃ → (BF₄)^-
   In this case, F^- gives and BF₃ accepts electron-pair to form an adduct.

Major Applications of Lewis Acids and Bases

• Lewis acids are used as catalysts in the Friedel-Crafts reaction wherein AlCl₃ receives lone electron-pair from chloride ion to form AlCl₄^-.
• It also forms the electrophilic carbonium ion, which is a strong Lewis acid.
• In organic chemistry, Lewis acids are required in various cationic or pseudo-cationic chemical reactions.
• Lewis acids aid in the modification of the selectivity and the activity of metallic catalysts.

Advantages of Lewis Concept

• It also considers the reactions in which protons don't take part.
• It is more elaborative than other concepts of acids and bases.
• It has included basic characteristics of metallic oxides and acidic characteristics of non-metallic oxides.
• It is very useful when the protonic concept is not applicable.
• It also explains that H⁺ is a Lewis acid and OH^- is a Lewis base.

Limitation of Lewis Concept

It does not tell about the strength of acids and bases as it does not talk about the ionization process. Lewis acids and bases cannot be arranged in any order based on their relative strengths. The strength of an acid or a base depends on the type of reaction in which it participates.

Acids and bases chemical reactions generally occur rapidly, however, there are many Lewis acid and base reactions that are very slow. As a matter of fact, the formation of coordinate covalent bonds (Lewis concept) is a slow process.