Boron Trichloride is an inorganic compound with the molecular formula (BCl₃). This chemical compound is a colorless gas with a pungent odor and acts as a reagent in organic synthesis. It is highly reactive towards the water. One common question is generally asked about its polarity. So, in this article, I will answer this and cover the fundamentals of the polarity of the BCl3 compound.
So, is BCl3 polar or nonpolar? Boron Trichloride or BCl3 is a nonpolar compound because of its symmetrical structure ie; Trigonal Planar. The B-Cl bond itself is polar because of the difference in electronegativity of Boron(2.04) and Chlorine(3.16) atoms and all three B-Cl bonds lie at 120 degrees to each other. As a result, the dipole moment of each B-Cl bond is canceled out by each other making the net dipole moment zero and the entire molecule nonpolar in nature.
Fumes originating from the chemical compound BCl3 can easily irritate the eyes, along with the mucous membranes. It is corrosive to metals, tissues, and considered toxic in nature.
If kept in prolonged exposure to fire or extreme heat, the containers may rupture itself or even burst out like a rocket violently.
BCl3 is mainly used as a catalyst in chemical manufacture, and soldering fluxes.
When Boron Trichloride reacts with water, it produces boric acid and hydrogen chloride as a substitute compound.
Hydrolysis of Boron Trichloride:
BCl3 + 3H2O → H3BO3 + 3HCl
Note HCl formation!
This happens due to BCl3 being an electron-deficient molecule that easily accepts a pair of the electron from water in Hydrolysis.
Molecular Geometry and Bond Angle of BCl3
Centrally located Boron of this molecule has three valence electrons, which balances out the three chlorines. This causes the molecular configuration for the chlorine atoms to be a part of perfect triangular shape.
This molecule is forming a triangular shape with an angle of 120 degrees splitting up the chlorine atoms with the same angle.
This indicates that the pull of these atoms is perfectly balanced out in this compound. Note that, Boron doesn’t fully complete its octet, but is still able to create a non-polar molecule.
To be polar, a molecule must have an asymmetrical shapeshift in electron density such that an electrical dipole is formed.
No such dipole exists in BCl3 and thus its dipole moment is zero and it is non-polar.
Lewis Structure of BCl3
If we look at the Boron Trichloride’s molecular geometry, it is Trigonal Planar with a bond angle of 120 degrees.
The central atom is distributing symmetric charge throughout the molecule and thus is Nonpolar!
Why BCl3 has zero dipole moment?
B and Cl atoms have different values in terms of electronegativities, hence chlorine (E.N. = 3.00) is more electronegative than the Boron (E.N. = 2.00).
As a result, B–Cl bond forms polar bonds and hence carries a finite dipole moment.
Now, BCl3 is a planar molecular compound in which the three B – Cl molecular bonds are inclined at an angle of 120 degrees.
Thus, the resultant of these two B – Cl bonds is canceled out by equal and opposite dipole moment generated in the B–Cl bond.
Hence, B-Cl molecular bonds have a ZERO dipole moment due to the predicament statements.
Symmetrical geometry is an important factor in determining the nonpolar nature of a compound. One such example is carbon disulfide (CS2). Read out the article regarding the polarity of CS2.
Why NH3 is polar and BCl3 is nonpolar?
In BCl3, Boron has a total of 3 valence electrons. The best configuration for bonding with chlorides that maximizes that distance between these two atoms is trigonal planar.
With all 120 degrees apart with a “pull” that each exerts is balanced out.
Here, NH3, Nitrogen has 5 electrons with 2 lone pairs. The lone pairs effectively create a dipole moment. If a molecule has a dipole moment it ends up being a polar molecule.
NH3’s lone pairs push other covalent bonds downwards, so it forms a trigonal pyramidal shape with 107.5 degrees of angle.
For polarity, one may look if:
- Are different atoms attached to the central atom? if no, it should be polar;
- if yes, check, if the molecular compound has a symmetrical configuration? if no, it should be polar.
NH3 has the same N-H covalent bonds attached to Nitrogen, however, the molecule is not symmetrical. Therefore it is a polar molecule.
For BCl3, with 3 valence electrons in group 3, and that Boron is an exception with not filling its octet (8 valence electrons).
BCl3 does fulfill the above two conditions stated as 1) and 2), also as it is a trigonal planar with 120 degrees apart.
Apart from NH3 (Ammonia), there are many similar polar molecules. One of them is PCl3. Check out the article regarding the polarity of PCl3.
What is the Hybridization of BCl3?
BCl3 is the chemical compound with a sp2 hybridization type. In BCl3, Boron as a central atom consists of three bonded atoms of chlorine with no lone pair of electrons left.
Its Steric number if bonding groups are counted comes out to be 3.
However, If we look at the ground state of Boron’s electron configuration in the molecule, it will be 1s2, 2s2, 2p1.
To form bonds with chlorine atoms, boron will need three unpaired electrons. Now, here one electron from the 2s is moved to the 2p level.
Therefore electron configuration is an excited state and is represented as 1s2, 2s2, 2px1, 2py1.
Now, the hybridization of BCl3 occurs where a 2s and two 2p orbitals of boron will participate in the process to form three half-empty sp2 hybrid orbitals.
Each hybrid orbital of sp2 will carry unpaired electrons that’ll overlap with an unpaired electron in Chlorine’s 3p orbital. Hence, Boron will form 3 σsp-p bonds with three atoms of chlorine.
Why does BCl3 have covalent bonds?
The chemical name of BCl3 is boron trichloride which is a combo of boron and chlorine. B and Cl are both nonmetals, so this is a covalent compound.
As B in BCl3 undergoes sp2 hybridization the resultant shape is trigonal planar.
B and Cl form polar bonds but, three such bonds in it have the same bond moments, thus the sum of three vectors comes out to be zero. The molecule becomes nonpolar with covalent bonds.
BCl3 is a non-polar molecule, then why does it form polar bonds?
BCl3 is a nonpolar molecule, yes and the B-Cl bonds are polar due to the electronegativity difference between the elements. Cl has an electronegativity of 3.16 and B has 2.04.
As the bond movement formed in BCl3 cancels out itself. The Cl atoms are identical and they pull the same amount on B electrons.
Hence, Cl atoms will still have a bit more negativity to them but because they are symmetrically arranged the effects will cancel each other out. Hence, it forms polar bonds.
Why is boron trichloride an acid?
The Lewis theory of Acid and base defines acids as chemicals accepting pairs of electrons.
The centrally situated boron atom in BCl3 is electron-deficient thus, enabling the molecule to accept an extra pair of electrons and hence, acts as a Lewis Acid.
What is boron trichloride commercially used for?
Boron trichloride is an initiating material to produce elemental boron.
It is used in the refining of aluminum, zinc, magnesium, and copper alloys to remove traits of nitrides, carbides, and oxides from any molten metal.
It is also used as a soldering flux for alloys of aluminum, tungsten, iron, zinc, and monel metal.
Properties of BCl3
- At room temperature, it exists as a Colorless gas and forms its fumes.
- Its density is around 1.326 g/cm3.
- Its melting point is −107.3 °C or −161.1 °F.
- Its boiling point is around 12.6 °C or 54.7 °F.
- It is easily soluble in compounds like CCl4, ethanol
- As discussed above, its shape is Trigonal planar.
The gas Boron Trichloride is reactive enough to serve as a rocket fuel It is a Lewis Acid with polar covalent bonds and is a non-polar molecular compound. However, it is noted that the chemical compound should be carefully handled because on hydrolysis with water and other alcohols it produces hydrogen chloride(HCl). The molecular geometry of Boron Trichloride (BCl3) is a trigonal planar with asymmetrical charge distribution around the central atom.