What are Lewis acids and bases?

Lewis acids are defined as the compounds that have a strong tendency to receive an additional pair of electrons from the Lewis bases and the Lewis bases donate one pair of electrons. This acid base reaction forms an adduct, and adduct is a compound that have a coordinate covalent bond and both the electrons are provided by only one atom. It is very common to find an electron-deficient molecules which have one deficient octet of electron around one atom. These molecules have a tendency to acquire an octet electron configuration, and this is done by reaction with one atom that have a lone pair of electron.

According to the concept of Brønsted–Lowry of acid and bases, a base is defined as any specie that can receive a proton, and acids are any species that can donate protons. The definition proposed by Lewis was an alternative to the previous definition, which focused on pair of electrons. According to Lewis, any species that can donate a pair of electron is known as Lewis base, and any species that can accept a pair of electron is known as Lewis acid. All the Brønsted–Lowry bases, which are proton acceptors, can also donate a pair of electrons. Some examples of the Brønsted–Lowry bases can be: OH⁻, H₂O, and NH₃. This proves that the definition that has been given by Lewis is not contradictory to the definition of Brønsted–Lowry of acid and bases. The definition given by Lewis also expands the definition of acids, so that it includes substances other than H⁺ ion.

Some molecules such as BCl₃ are  electron deficient molecules, have less than an octet of electrons around one atom and these substances have a strong tendency for gaining of an additional electrons pair and this achieved by reacting with other substance that contain a lone pair of electron. The definition of Lewis is less obstructive than both the Brønsted–Lowry and the Arrhenius definition, which grew out of the observation from him of the tendency. A general Brønsted–Lowry acid-base reaction that is depicted using the Lewis electron symbols is provided below:

H⁺ + :B →H : B⁺

Lewis acid Electron pair acceptor + Lewis base electron pair donor

The H⁺ proton has no valence electrons and can be considered as a Lewis acid because it can accept a lone pair of electron to form a bond. The proton is known to react with bases. For an example, the neutral compounds of boron, aluminium and other element of Group 13, that possess only six valence electrons, and have a very strong affinity for electron pairs. These compounds are potent Lewis acids that react with an electron-pair donor such as ammonia in order to form an acid–base adduct, which is a covalent bond. An example is given below:

BF₃ + NH₃  →F₃B : NH₃

Lewis acid + Lewis base→Acid-base adduct

What is coordinate covalent bond?

The coordinate covalent bond is also known as dative bond and these type of bonds are formed when electron pairs are shared between two atoms. The attraction of the electron pairs by both the nuclei helps in holding the atoms together. During the formation of a simple covalent bond, one electron is supplied to the bonds by each atom and in the case of coordinate covalent bonds it may not be the case. In the case of co-ordinate covalent bonds, the electron pair comes from one atom only.

An example of Co-ordinate covalent bond

The reaction between ammonia and hydrogen chloride

NH₃ + HCl→NH₄Cl

The NH₄⁺ are formed due to the transfer of a hydrogen ion from the hydrogen chloride to the lone pair of electrons on the ammonia molecule.

A dative covalent bond is formed by the attachment of the fourth hydrogen in the ammonium ion, NH₄⁺, this because there is a transfer of the hydrogen nucleus from the chlorine to the nitrogen. The electron of the hydrogen is left behind on the chlorine in order to form a negative chloride ion.

How to identity Lewis acids and bases from a chemical reaction

Example 1: BH₃ + (CH₃)₂S → H₃B : S(CH₃)₂

The given things are the reactants and the products.

Solution: In the above given example, in BH_3, there are six valence electrons in boron. Thus, in this case it is electron deficient and can accept a pair of electrons. Like the oxygen, the sulfur atom in the compound (CH₃)₂S has two lone pairs of electrons. So, the (CH₃)₂S compound is capable of donating on electron pair to the boron atom of BH₃. Thus, in this example, the Lewis acid is BH₃ and the Lewis base is (CH₃)₂S.

Example 2: CaO + CO₂ → CaCO₃

The given are the reactants and the products.