Have you heard of the term ‘aqua regia’? If you are a student of chemistry, I bet you have. It is a 1:3 molar ratio of the mixture of nitric acid and hydrochloric acid.
Today, in this article, we will talk about one of the components of aqua regia: Nitrosyl Chloride (NOCl).
NOCl is a yellowish red gas having a molar mass of 65.46 g/mol and a density of 2.872 mg/ml. It has strong oxidizing properties and also acts as a strong electrophile. It is highly water-reactive.
It can be synthesized in the laboratory via reversible dehydration of HNO2 or nitrous acid. The reaction is as follows:
HNO2 + HCl —–> H2O + NOCl
Nitrosyl chloride is used for the industrial production of caprolactam ((CH2)5C(O)NH). It is a highly toxic gas and also accelerates the burning of combustible materials.
When we study a certain molecule or chemical composition, it is important to learn about the science behind its formation.
Atoms of different kinds of elements come together and associate to form molecular structures that form chemical compounds. This study of atomic attraction that results in new products is, therefore, an interesting and noteworthy chapter in chemistry.
The attractive force that holds atoms together is known as a chemical bond.
Hence, we will discuss the nature of the chemical bonding of NOCl in this article. To do this, we will first talk about the concept of Lewis Structure.
Do you know what valence shell electrons refer to? They refer to the electrons present in the outermost shell of an atom of an element.
If we represent these electrons as dot notations surrounding the corresponding atoms inside a molecule and point out the type of bond formed between them, we can get a simplified diagram of chemical bonding. This is known as Lewis Structure.
Lewis Structure is therefore the 2D diagrammatical representation of the distribution of electrons around atomic elements in a molecule. It is also known as the electron-dot structure.
Lewis Structure of NOCl
Step 1: First take a molecule of NOCl. A nitrosyl chloride molecule consists of one atom of nitrogen, one atom of chlorine, and one atom of oxygen.
Step 2: Now, we will find out the total number of valence electrons present in one NOCl molecule. To do so, have a look at the periodic table.
Nitrogen has an atomic number of 7 and belongs to group 15. It has 5 valence electrons.
Oxygen has an atomic number of 8 and belongs to group 16. It has 6 valence electrons.
Chlorine has an atomic number of 17 and belongs to group 17. It has 7 valence electrons.
the total number of valence electrons in NOCl molecule = 5 + 6 + 7 = 18.
Step 3: We will now find out the central atom in the molecule. According to the general rule barring some exceptions, the element having the least electronegativity value takes the central position.
Here, Nitrogen forms the central atom.
Step 4: We will draw a sketch where we will denote the valence electrons as dot notations and find out the nature of bond formation.
Step 5: Here, the octet rule comes into play.
The octet rule states that the main group elements of the periodic table have a tendency to attain the octet valence shell configuration of the noble gas elements. As we can see in the above sketch, Oxygen and Chlorine have achieved octet configuration whereas Nitrogen is two electrons short of achieving the same.
Let us shift two electrons from oxygen towards nitrogen to form a double bond.
Note: Sharing of two electrons denote a single bond, four a double bond, and so on.
Now, all three atoms have achieved octet configurations in the above diagram.
Step 6: We cannot be sure whether the sketch we have drawn is the right Lewis Structure diagram. For that, we need to check the formal charge values.
In NOCl, the formal charge of N = 5 – 0.5*6 – 2 = 5 – 3 – 2 = 0.
Formal charge of O = 6 – 0.5*4 – 4 = 6 – 2 – 4 = 0.
Formal charge of Cl = 7 – 0.5*2 – 6 = 7 – 1 – 6 = 0.
Since all the atomic elements are present in their least possible formal charge values, we can safely conclude that we have got our required lewis Structure diagram for Nitrosyl Chloride.
There is a double bond between N and O and a single bond between N and Cl.
NOCl Molecular Geometry
In the above section, we found out the 2-dimensional graphical representation of NOCl. However, the Lewis Structure concept has its own limitations:
It cannot predict the 3-dimensional molecular geometry of any given molecular structure.
It fails to explain the nature of chemical bonds in detail or give us any clear idea about the actual atomic arrangement in space.
Here, we will take the help of the Valence Shell Electron Pair Repulsion (VSEPR) theory.
We already know about the valence electrons and how they form bonded and lone pairs around atoms inside a molecule. The negatively charged electrons form a dense cloud around the atomic nuclei.
According to the VSEPR model, since electrons are like charges they have a tendency to repel each other. In order to minimize this repulsive tendency, the atoms go farther away from each other and try to stabilize the molecular structure.
We will use the VSEPR model to explain the molecular geometry of NOCl.
In NOCl, the central atom is Nitrogen.
According to VSEPR theory, we can use AXnEx notations to find out the 3D molecular shapes.
Here, A = central atom, X = surrounding atoms, and E = lone pair of electrons around the central atom.
In NOCl, A is Nitrogen, we have one O and one Cl around Nitrogen, ∴ ‘n’ = 2, and there are two lone electrons i.e. 1 lone pair around the central atom, ∴ ‘x’ = 1.
The required VSEPR notation for nitrosyl chloride is AX2E1.
Here, we have a diagram giving us the VSEPR chart:
According to the chart, we get a bent molecular structure for NOCl.
If we look at the molecule, the O=N and the Cl-N bonds stretch to form a linear structure. However, the lone pair on top of central Nitrogen pushes down the other atoms to give us a smaller bond angle and a bent molecular geometry.
The bond angle is around 113 degrees.
If you are studying the chemical bonding of a molecule, you will definitely come across the term ‘ hybridization’.
Orbital hybridization is one of the most noteworthy concepts to explain the nature of bonding. Here, the atomic orbitals of the same atomic element combine and fuse to form hybridized orbitals.
For example, one s and two p orbitals of a carbon atom come together and combine to form three sp2 hybrid orbitals which take part in forming single bonds with hydrogen and the other carbon atom in an ethylene molecule.
To discuss the type of hybridization in NOCl, we will first incorporate an important terminology: Steric Number.
Steric Number is defined as the sum of the number of sigma bonds surrounding the atom and the number of lone pairs of electrons on the atom.
Note: A single bond contains a sigma bond, a double bond contains a sigma bond and a pi bond.
Steric number = Number of atoms bonded to central atom inside a molecule + Number of lone pair of electrons attached to the central atom
Steric number of Nitrogen in NOCl = 2 + 1 = 3.
This denotes a hybridization of sp2.
So, we can say that the central atom of nitrosyl chloride has an sp2 hybridization.
Polarity is an interesting topic of chemistry. This property is closely linked to another useful concept: electronegativity.
Electronegativity: It is defined as the tendency of an atom to gain negatively charged electrons.
Polarity: It is related to the distribution of charges inside a compound and is defined as the separation of electric charges resulting in a net dipole moment.
A bond is polar when there is a considerable electronegativity difference between the atoms at both ends (usually more than 0.4-0.5). In a molecule, when there is asymmetry i.e. the molecule does not have a linear or symmetrical geometry, the dipoles do not get canceled out and we have a polar molecule.
Let us now find out whether NOCl is polar or non-polar:
N has an electronegativity value of 3.04, O value is 3.44 and that of Cl is 3.16.
Both the O and Cl end are highly electrophilic and hence tend to pull electrons towards them. Also, the lone pair makes the geometry of NOCl bent. The asymmetry of the molecule and unequal charge distribution result in a polar molecule.
You must also read the article I wrote specifically for the polarity of NOCl.
In this article, we have discussed in detail the Lewis Structure, VSEPR theory to predict molecular geometry, orbital hybridization, and polarity of NOCl molecule.