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Lewis Acid and Base

Chemical reactions occur all around us. Every second, there is a chemical change taking place in the universe. This leads to an important question prompting us to wonder what kind of substances react with each other or under which circumstances ordinary substances undergo a chemical reaction. With research and several studies conducted, it was known that only certain substances undergo a chemical reaction.

For instance, soap, when mixed with water, forms lather. These substances can be classified collectively as acids and bases. Moreover, right from our bodies to the Earth’s crust, acids and bases play an integral part in our lives. In this article, we will learn about the different theories of acids and bases and learn about their properties.

Conceptually, acids are defined as substances with a pH lower than 7, while bases are defined as substances with a pH greater than 7. However, the American physical chemist G.N Lewis put forth a different theory in 1923 to define the capabilities of acids and bases. G.N Lewis also further defined the electron-pair theory that helps in understanding the concept of chemical bonding.

According to the Lewis base theory, acids and bases are defined as follows:-

1. Lewis Acids

These are the chemical substances that can accept an electron pair and contain an empty pair orbital. It is also defined as the molecule that contains the lowest unoccupied molecular orbital. Examples of Lewis acids include compounds such as water, Antimony, Arsenic, and even ions like Mg2+, Fe3+, Li+, etc. Other electron-deficient compounds such as enones are also considered examples of Lewis acids.
We can also conclude with the fact that all Lewis acids occur in nature as BR₃.

Here B resembles elements from the periodic table, and R represents either a halide or an organic substituent. It is also known that all Lewis acids contain a trigonal planar in their structure along with an empty p-orbital. Lewis acids mostly contain an empty, vacant orbital. Hence, they are commonly also known as electrophiles.

2. Lewis Bases

These are the chemical substances that contain the capability to donate an electron pair. It can also be defined as the molecule that contains highly localized HOMO, i.e., Highest Occupied Molecular Orbital. Most common examples of Lewis bases include amines, ammonia, etc. In addition, Lewis bases often consist of electron-pair. Hence, they are popularly also known as nucleophiles in various chemical reactions.

We know that Lewis acids are electron-attracting by nature, and Lewis bases are electron-donating by nature. Hence, in a chemical reaction involving Lewis acid and Lewis base, electron pairs are exchanged. This exchange of electrons can be illustrated as follows-

The figure above shows that the bond formed due to the exchange of electron pairs is commonly known as a coordinate covalent bond. In general, the bond is formed when the Lewis acid accepts the electron pair, and the Lewis base donates the electron pair. Due to the nucleophilic nature of the Lewis base, they attack the Lewis acid using their lone pair of electrons. Thus, when the bonding occurs during the chemical reaction, the Lewis acid will use its lowest unoccupied molecular orbital (LOMO). In contrast, the Lewis base utilizes its highest occupied molecular orbital (HOMO).

We can also take the frequent example of a coordinate compound due to Lewis acid, and the base is the reaction taking place when ammonia is mixed with Zn2+.

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Anomaly with Water
Now, as stated before, we can classify elements, ions, or compounds based on their capabilities of electron-donating or receiving nature as Lewis acid and Lewis base. However, certain compounds such as water can act as both Lewis acid and Lewis base. This property of water makes it an amphoteric compound or molecule.

We can understand how water acts as a base with the help of the following illustration:-

2

In the above example, we can see that water accepts a proton given by the hydrochloric acid (HCl) and thus acts as a base. And how water acts as an acid with the help of the following illustration:-

3

In the above example, we can see that water donates a proton (H⁺ ion) to ammonia compound and thus acts as an acid.

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