Geometry, Hybridization, and Polarity of CH3CN Lewis Structure

Acetonitrile is a colourless chemical liquid with an aromatic odour, commonly known as methyl cyanide. It is mostly created as a byproduct of the acrylonitrile production process. It is employed as a polar aprotic solvent in the chemical synthesis of numerous substances.

It was initially published in 1847 by Jean Baptiste Dumas. It’s also a dangerous air pollutant found in car and industrial emissions.

We’ll look at the Lewis structure of CH3CN, as well as its geometry, hybridization, and polarity, in this article. Keep reading till the very end.

Lewis Structure of CH3CN

Lewis dot symbols or lewis structures are diagrams that show the bonding between distinct atoms in a compound while also detailing the amount of bonds and lone pairs of electrons.

These structures account for the valence electrons present in all of a compound’s atoms. Gilbert N. Lewis first proposed this concept in 1916.

The figure is symbolic, and it uses chemical symbols for elements to represent their atoms, while the dots in the diagram represent the atom’s lone pair of electrons.

We already know that electrons in an atom rotate around the nucleus in defined orbits called shells.

The Rule of the Octet

The valence shell, which is the outermost orbit of an atom, is the shell furthest from the nucleus, and the electrons in this shell are known as valence electrons.

It is considered that in order to achieve stability, an atom will tend to create bonds. The octet rule states that an atom is most stable when it contains eight electrons in its valence shell.

This rule is based on the electrical arrangement of noble gases, which are the most stable elements in the periodic table and have eight electrons in their outermost shell (excluding helium, which has two).

As a result, the atoms create bonds to finish their octet, leading in the production of various compounds.

CH3CN’s Lewis structure is as follows:

Write the hybridization and bonding scheme for NCCH3 using valence bond theory. Is it possible to draw the model with the correct geometry? | Socratic method

The octet for all of the involved atoms is satisfied, as can be seen in the above structure. As a result, it is the proper Lewis structure.

How to Draw the Lewis Structure of CH3CN in Steps

Let’s take a look at the process of creating this Lewis structure one step at a time.

• To construct the Lewis structure for a compound, sum up the valence electrons of all the involved atoms to get the total amount of valence electrons.

In the case of CH3CN, the following is true:

Carbon has a valence electron count of four.

Because there are two carbon atoms, 4 X 2 = 8.

In addition, the number of valence electrons in hydrogen is one.

For a total of three hydrogen atoms 3 = 1 X 3

Finally, for nitrogen, the number of valence electrons equals 5.

As a result, the total number of valence electrons in CH3CN is equal to 8 + 3 + 5.

(16)

• The next step is to select the molecule’s core atom.

Because carbon is the least electronegative species in CH3CN, any of the two carbon atoms can be chosen as the central atom and placed at the core of the molecule, around which all the other atoms are considered to be bound.

• All of the atoms in the molecule are now connected by single bonds.

This step allows us to estimate the number of electrons that one or more atoms in the molecule still require to complete their octet.

• The octet for all hydrogen atoms, as well as the initial carbon atom, is satisfied in the aforementioned structure.

However, the centre carbon atom, carbon-2, as well as the nitrogen atom, require two additional electrons to complete its octet.

• Because both carbon and nitrogen may make multiple bonds, the second carbon atom forms a triple bond with the nitrogen atom, completing the octet for all atoms in the molecule.

In addition, the nitrogen atom has a lone pair of electrons. This is the final Lewis structure for acetonitrile:

Write the hybridization and bonding scheme for NCCH3 using valence bond theory. Is it possible to draw the model with the correct geometry? | Socratic method

• But, hold on, how can we know the structure we’ve designed is correct?

Another concept called formal charge is used to guarantee that we have derived the correct structure for a molecule.

It’s a theoretical concept in which the formal charge of each atom in a molecule is calculated separately, and the diagram is regarded correct if the formal charge value is near to zero, or better yet, zero.

• The following formula is used to compute an atom’s formal charge:

[Total number of valence electrons in Free State] – [Total number of non-bonding electrons – 1/2 (Total number of bonding e–)] Formal Charge = [Total number of valence electrons in Free State] – [Total number of non-bonding electrons – 1/2 (Total number of bonding e–)]

• Next, calculate the formal charge of several CH3CN atoms:

Formal charge = [4] – [0] – 12[8] for carbon atoms.

equals 0

Formal charge = [1] – [0] – 12 [2] for hydrogen atoms.

equals 0

Formal charge = [5] – [2] – 12 [6] for nitrogen atom.

equals 0

Because the formal charge on all of the atoms is 0, the total charge on CH3CN is also 0, indicating that this is the best feasible Lewis structure for the molecule.

Molecular Geometry of CH3CN

A compound’s molecular geometry is the spatial arrangement of its atoms.

The bonding and non-bonding electrons present in that compound surrounding the core atom dictate the three-dimensional placement of its atoms.

Other aspects such as binding angles, bond strength, and so on are taken into account by molecular geometry.

The Valence Shell Electron Pair Repulsion (VSEPR) Theory lays out the rules for defining a compound’s molecular shape.

We already know that electrons all have a negative charge and that like charges repel one another.

As a result, the electrons within a molecule repel one another. According to VSEPR theory, the inter-electronic repulsion forces at work inside a molecule dictate the three-dimensional form of the molecule.

These forces are greatest between lone pairs and lone pairs because electrons are free to travel in space, while they are smallest between bond pairs and bond pairs.

According to the VSEPR theory, electrons inside a molecule want to place themselves as far away from other electrons as possible to minimise inter-electronic repulsion, lowering the molecule’s energy and making it more stable.

The number of bond pairs and lone pairs present in a molecule can be determined using the lewis structure.

According to the Lewis structure of CH3CN shown in the previous section, carbon is the molecule’s core atom.

Because this molecule has two carbon atoms, it can take two different shapes depending on which one is chosen as the centre atom.

Looking at the Lewis structure of CH3CN once more:

Write the hybridization and bonding scheme for NCCH3 using valence bond theory. Is it possible to draw the model with the correct geometry? | Socratic method

• When C1 is used as the central atom, the molecular geometry of CH3CN changes to tetrahedral because the central atom is coupled to four distinct groups: three hydrogen atoms and one CN group.

This structure mimics the AX4 formula when written in AXN notation, where A represents the central atom and X represents the bound group.

The number of lone pairs present on the central atom, which is 0 in the CH3CN molecule, is denoted by the letter N in the notation.

The molecule’s electron geometry is also tetrahedral in this example, and the bond angle between distinct atoms is 109.5°.

• If C2 is chosen as the central atom, the CH3CN molecule’s molecular geometry should be linear since the central atom is coupled to two groups: one methyl group (-CH3) and one nitrogen.

This is similar to the AX2 formula in AXN notation. The molecule’s electron geometry is also linear, with a bond angle of 180°.

The following is an AXN notation chart for many general formulas:

Hybridization of CH3CN

Hybridization is the development of hybrid orbitals as a result of the intermixing of orbitals with similar energy levels.

Calculating the steric number of every molecule is used to determine its hybridization.

The steric number, according to VSEPR theory, is the total number of atoms bonded to the centre atom, including lone pairs.

The formula is as follows:

The number of sigma () bonds on the centre atom plus the number of lone pairs on the central atom equals the steric number.

As previously stated, any of the carbon atoms can be chosen as the central atom in the CH3CN molecule, resulting in two hybridization states for this molecule.

• Steric number = 4 + 0 = 4 if C1 is chosen as the centre atom.

As a result, this molecule’s hybridization becomes sp3.

• If C2 is chosen as the centre atom, however, the Steric number is 2 + 0 = 2.

As a result, the molecule’s hybridization becomes sp.

The relationship between steric number and hybridization state is depicted in the graph below:

Steric numberHybridization State
1S
2Sp
3Sp²
4Sp³
5Sp³d
6Sp³d²

Polarity of CH3CN

The presence of two opposite poles in a molecule, i.e. a positive and a negative, is known as polarity.

The difference in electronegativity of the bonding atoms causes these poles to form, with the more electronegative atom attracting the electron more towards itself, resulting in a slight negative charge, while the other atom develops a slight positive charge.

Because of the difference in electronegativity between the carbon and nitrogen atoms, the acetonitrile molecule is polar, with a tiny negative charge on nitrogen and a slight positive charge on the carbon atom.

CH3CN has a dipole moment of 3.5 Debye.

Properties of CH3CN

The table below lists some of the most important features of CH3CN:

Chemical NameAcetonitrile
Chemical FormulaCH3CN
Molecular weight41.053 g/mol
Boiling PointBetween 81&- 83°C
Melting PointBetween -46 & – 44°C

Uses of CH3CN

CH3CN is used in the following ways:

• In the purification of butadienes, as a solvent.

• In the field of high-resolution liquid chromatography.

• When making DNA oligonucleotides.

Conclusion

• CH3CN has the following Lewis structure:

• Because CH3CN has two carbon atoms, it can have two distinct molecular geometries. The shape is tetrahedral if C1 is chosen as the central atom, and linear if C2 is chosen as the central atom.

• The CH3CN molecule can have two hybridization states, similar to the molecular geometry.

The hybridization state for the molecule is sp3 when the C1 atom acts as the central atom, and sp when the C2 atom acts as the central atom.

• The CH3CN molecule has a dipole moment of 3.5 Debye.

Good luck with your studies!!

Read more: Is Hydrogen Bonding Present in NH3?

Misha Khatri
Misha Khatri is an emeritus professor in the University of Notre Dame's Department of Chemistry and Biochemistry. He graduated from Northern Illinois University with a BSc in Chemistry and Mathematics and a PhD in Physical Analytical Chemistry from the University of Utah.

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