Hybridization, Lewis Structure, and Molecular Geometry of BCl3

Boron trichloride is a chemical that can be generated in an industrial setting by chlorinating boron oxide and carbon at 501°C.

B2O3    +    3C    +    3Cl2     ——–>    2BCl3    +    3CO

During the interaction of boron with halogens, corresponding trihalides of boron are formed.

BCl3 has a wide range of applications. Boron trichloride is a chemical that is used to refine aluminium, zinc, magnesium, and copper alloys.

BCl3 is required as a starting ingredient in the manufacture of elemental boron.

This molecule is also involved in the synthesis of organic molecules. It is used to cleave C-O bonds in ethers as a reagent.

Boron trichloride is also used to make electrical resistors and in the production of high-energy fuels and rocket propellants.

Because boron trichloride is so widely used, it’s critical to understand its qualities. So let’s take a deep dive into the realm of boron trichloride in this essay.

What Is a Lewis Structure and How Do I Draw One?

Lewis structure is a representation of electron distribution around atoms used to estimate the number and type of bonds that can be formed.

Now, let’s look at how to draw a Lewis structure properly:-

  1. Determine the total amount of valence electrons in a molecule: The first step is to add up the valence electrons of all the atoms in the molecule.
  2. Draw a skeleton structure around a core atom:
  • Draw a skeleton representation of the molecule using only single bonds. The centre atom is now usually the least electronegative or the one with the most accessible sites.
  1. Place remaining electrons around outside atoms: After forming the skeleton with single bonds, the remaining electrons are placed around the atoms to complete the octet. Start with the electronegative atoms and work your way up to the electropositive atoms.
  2. For each atom, check the octet rule:- Ascertain that all of the atoms satisfy the octet. Otherwise, fill in the blanks with several bonds. Convert one lone pair of an electronegative atom into an electron-deficient atom’s bonding pair.
  3. Determine the formal charge of each atom:
  • Make sure that all atoms have the lowest formal charge feasible without breaking the octet rule. The formal charge is calculated using the following formula:

Although the Lewis dot rule can be used to draw the structures of many molecules, there are some exceptions.

For lanthanides and actinides, Lewis structures are less useful.

Lewis Structure of BCl3

Let’s try to draw the structure of boron trichloride using the Lewis dot rules.

First, we must determine the total number of valence electrons in this molecule.

3 (B)

7 = C l

7*3=21 = 3Cl

As a result, total=21+3=24.

Because boron is now less electronegative, it has become the centre atom. We must create a skeletal structure using just single linkages.

Six valence electrons were employed to form the single bonds in the skeleton, out of a total of 24. As a result, we’re left with 18 electrons to distribute among the Cl atoms.

Each Cl atom will now have six electrons, or three lone pairs.

Finally, we must determine whether all of the atoms satisfy the octet rule. The octet rule is followed by all atoms, as shown in the accompanying graphic.

According to the octet rule, boron requires another 6 electrons in its outermost shell to complete its octet. To gain stability, it shares three single bonds with chlorine.

Chlorine, on the other hand, requires one more electron to complete its octet. To fulfil the octet rule, chlorine forms a connection with the electrons of boron.

Hybridization using BCl3

Sp2 hybridization occurs in boron trichloride. It creates three half-filled Sp2 hybrid orbitals by using one 2s orbital and two 2p orbitals in the excited state.

Let us first study a little about Sp2 hybridization before making any assumptions. As a result, one S orbital and two P orbitals combine to generate three equivalent orbitals in this hybridization. These orbitals are in a plane and are at a 120° angle to one another.

When it comes to boron trichloride, it follows the same path as the other elements and so exhibits Sp2 hybridization.

Boron’s ground state configuration is 1s2 2s2 2p1.

The presence of three unpaired electrons is required to connect with three Cl atoms. With the help of energy, one electron from the 2s orbital is promoted to the 2p sublevel.

As a result, boron’s excited state configuration is 1s2 2s1 2px1 2py1.

In the excited state, boron acquires Sp2 hybridization by using one 2s orbital and two 2p orbitals.

Boron trichloride is formed when it makes three sp-p bonds with Cl atoms. The bonds in this molecule are formed by the half-filled p orbitals of chlorine atoms.

The figure below provides a much clearer representation of the above-mentioned notion!

There is a simple formula for determining hybridization that goes along with this concept.

[V+M-C+A] H = 12

Here,

H stands for hybridization.

V is the number of valence electrons.

M is the number of monovalent atoms.

C is the cation’s charge.

A is the anion’s charge.

It’s Sp hybridization if H= 2.

H= three Sp2 hybridization is what it’s all about.

4 (H) Sp3 hybridization is what it’s all about.

H = 5.0 Sp3d hybridization is what it’s all about.

H= six Sp3d2 hybridization is what it’s all about.

Let’s look at the boron trichloride formula:

[V+M-C+A] H= 12 3 (V)

3 (M)

Because BCl3 is a neutral molecule, C and A would be 0.

As a result, H= 12 [3+3] = 12 * 6

Sp2 hybridization = 3

As a result, it’s evident from the formula that BCl3 is Sp2 hybridised.

Molecular Geometry of BCl3

The molecular geometry of boron trichloride is trigonal planar with a bond angle of 120 degrees, according to VSEPR theory.

The Valence Shell Electron Pair Repulsion Theory (VSEPR) is used to calculate a molecule’s shape and bond angle. The following is the VSEPR chart:

BCl3 is an AX3 type molecule, as shown in the graph. There are 3 bound atoms and 0 lone pairs in this compound. The molecule’s geometry is hence trigonal planar.

This VSEPR graphic also offers us an understanding of how a molecule hybridises.

It’s important to recognise the distinction between molecular geometry and molecular structure/shape.

The geometry of a molecular structure or form includes all electron pairs, whereas the geometry of a molecular structure or shape simply includes the atoms.

In simple terms, molecule structure/shape ignores lone pairs, whereas molecular geometry/electron-pair geometry does.

For compounds that have no lone pair, such as BCl3, the molecular geometry and form will be the same.

BCl3 Molecular Orbital Diagram

The mixing of orbitals in molecules is seen in molecular orbital diagrams.

Let’s have a look at the boron trichloride MO diagram.

The blue hue represents boron’s atomic orbitals, the red colour represents chlorine’s atomic orbitals, and the purple colour represents the molecule’s molecular orbital.

We can observe that the 2s, 2p, and 3p orbitals of boron and the 3p orbital of chlorine overlap. The molecule’s stability is determined by bonding energy.

There is only head-on overlap due to the presence of only sigma bonds.

This describes the boron trichloride molecular orbital diagram.

BCl3’s polarity

Because the charge distribution across the molecule is homogeneous and the molecule’s shape is symmetric, i.e. trigonal planar, the BCl3 molecule is termed non-polar.

As a result, there is no charge polarisation across the BCl3 molecule. You should also read an article about BCl3 polarity for further details.

Conclusion

Boron trichloride is a highly aggressive reagent that, when exposed to alcohol, can produce hydrogen chloride. There are numerous facts and reactions associated with this chemical.

In this post, I attempted to provide all of the important information that you should have before doing any boron trichloride process.

I hope you found the post useful and interesting in some manner.

Read more: Is Br2 a polar or nonpolar substance?

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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