PCl5 is the formula for a chemical compound called phosphonium pentachloride. It exists as a colorless, crystalline solid at room temperature and pressure (r.t.p). Phosphonium pentachloride is a compound formed by the combination of two elements i.e., a phosphorus atom to which ‘five’’ atoms of chlorine are attached.
Depending upon the two different elements present, PCl5 can be wrongly perceived as a polar molecule, but that is not the case. It is actually a non-polar molecule. This article will discuss why is PCl5 not polar and all the other related concepts. So, let’s start reading.
Is PCl5 polar or non-polar?
Phosphorous pentachloride (PCl5) is non-polar, because of its symmetrical molecular geometry. Individual P-Cl bonds are polar, but their polarity gets canceled in opposite directions. Thus, it has a net dipole moment of zero.
The non-polarity of phosphorous pentachloride (PCl5) is attributed to its symmetrical, molecular geometry.
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory of chemistry, PCl5 is an AB5 type molecule with a trigonal bipyramidal shape, as you can see in the figure below.
The central phosphorus (P) atom is surrounded by five chlorine (Cl) atoms, with each Cl occupying a corner of the bipyramid. There are five bond pairs and no lone pair around the central phosphorus atom. Thus, forming a D3h symmetry.
All the P-Cl bonds are not equivalent. There exists a difference in electronegativity between the central P (E.N= 2.19) atom and Cl (E.N= 3.16) atoms of each P-Cl bond.
Therefore, P is partial positively charged while each Cl carries a partial negative charge.
In this way, each individual P-Cl bond is polar. But it is due to their unique arrangement that the polarity of 3 P-Cl bonds lying along the T region of the bipyramid gets canceled with the polarity of 2 P-Cl bonds lying along the V region of the bipyramid.
This results into an overall non-polar (PCl5) molecule, with a zero net dipole moment.
Difference between polar and non-polar molecules
Polar molecule
Non-polar molecule
Atoms having a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than 0
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3) and sulfur dioxide (SO2), etc.,
Examples include oxygen (O2), nitrogen (N2), methane (CH4), and phosphonium pentachloride (PCl5).
Factors affecting the polarity of PCl5
The concepts of electronegativity, dipole moment, and geometrical shape are some of the most important factors in determining the polarity of a chemical bond.
It is short of a single electron to complete its octet. 5 Cl atoms thus each share one electron to form a single covalent bond with the central P atom in the PCl5 molecule.
Electronegativity is the ability of an atom to attract the shared pair of electrons from a covalent bond.
The higher electronegativity of chlorine atoms (E.N= 3.16) as compared to a phosphorus atom (E.N= 2.19) means that a Cl atom will more strongly attract the shared pair of electrons from each P-Cl bond than the P atom.
This makes P gain a partial positive charge (Pδ+) due to a deficiency of electrons. On the other hand, the Cl atoms gain a partial negative charge (Clδ-) due to a slight excess of electrons.
This unequal charge distribution between two types of atoms makes each P-Cl bond polar, in a PCl5 molecule.
Dipole moment
A dipole moment is mathematically represented using a symbol µ and has a unit Debye (D).
It is defined as the product of electric charge and the distance between the center of separation of two different constituents of a molecule i.e., the bond length.
It is a vector quantity that means the dipole moment has both a numerical value as well as a direction. The dipole moment equation is represented as equation 1.
Considering the difference in electronegativity between two atoms, each P-Cl bond has a definite dipole moment value of 0.97 D. The direction of this dipole moment is from the less electronegative Pδ+ to the more electronegative Clδ-.
Geometrical shape
The geometrical shape of PCl5 is trigonal bipyramidal, as we discussed already. There are two types of bond angles (90° and 120°) and unequal bond lengths present.
However, it is because of the symmetrical geometric shape of PCl5 that the average dipole moment of three P-Cl bonds represented by a green arrow gets canceled with an average dipole moment of two P-Cl bonds as represented by a red arrow. Therefore, the overall PCl5 has a net dipole moment µ=0 which accounts for its non-polarity.
Based on these three factors we can categorize other different types of molecules as either polar or non-polar.
As a catalyst for organic synthesis for example synthesis of acetyl cellulose; the plastic film base used in photography
As an intermediate compound in water treatment and pesticide manufacturing
For the synthesis of pharmaceutical drugs such as antibiotics; penicillin and cephalosporin.
As a reagent in metallurgy
FAQ
Is PCl5 polar?
The bonds present in PCl5 are polar.
But the overall (PCl5) molecule is nonpolar because of its symmetric structure that results in the cancellation of dipole moment.
The net dipole moment for PCl5 is zero and this show, its nonpolar nature.
Why is PCl5 a non-polar compound even though it has polar bonds present?
The individual bonds in PCl5 are polar because of the electronegativity difference between them, but these polar bonds are arranged in such a way that their polarities get canceled by each other, resulting in an overall (PCl5) nonpolar molecule.
What is the molecular geometry of PCl5 and how does it affect the polarity?
“PCl5 has trigonal bipyramidal structure. The trigonal bipyramidal structure makes the arrangement as that opposite bonds cancel the dipole moment of each other hence the PCl5 is nonpolar.”
What is the hybridization and bond angle in PCl5?
The bond angle in PCl5 are 90° and 120° and the hybridization of P in PCl5 is dsp3 hybridized.
What is the dipole moment of PCl5?
Polar compounds have a dipole moment greater than 0 and nonpolar molecules have a dipole moment equal to 0. For PCl5, the Net dipole moment (µ) =0, Hence, it is a nonpolar molecule.
What types of bonds are present in PCl5?
The bond present in PCl5 are types of polar covalent bonds. Because the electronegativity of Phosphorus (P) is 2.19 and that of Chlorine (Cl) is 3.16.
∴ The difference in electronegativity between phosphorous and chlorine is (3.16 – 2.19) = 0.97
According to pauling scale, the electronegativity value lie between 0.4 to 1.7 shows polar covalent bond nature.
Physicochemical properties of PCl5
Molecular formula of PCl5 = 208.2 g/mol
PCl5 has a high melting point (160.5°C) due to its crystalline packing in solid-state.
PCl5 exists as a colorless solid at r.t.p and emits a pungent odor. It may appear yellowish-green in color in case of an HCl contamination.
The oxidation state of phosphorus in PCl5 is +5. Five Cl atoms each with an oxidation state of -1 produce an overall negative charge equal to -5.+5 cancels with -5, therefore, no charge is present on PCl5, complementing its non-polar nature.
Chemical reactions of PCl5
Self-ionization reaction:
In the presence of polar solvents, PCl5 may undergo a self-ionization reaction. When a high PCl5 concentration is added to a polar solvent; 2 PCl5 molecules dissociate into PCl4+ and PCl6– ions.
It is because of this self-ionization that the solid-state of PCl5 is sometimes represented using an ionic formula: PCl4+PCl6–
Hydrolysis reaction:
Hydrolysis is the breakdown of PCl5 by the addition of a water molecule. PCl5 hydrolysis yields different oxides of phosphorus (POCl3 and H3PO4) and hydrogen chloride (HCl).
Chlorination reaction:
It involves the substitution of an O or OH with a Cl from PCl5. For e.g., carboxylic acids (R-COOH) can undergo chlorination in the presence of phosphonium pentachloride to yield a corresponding acyl chloride (R-COCl).
Similarly, alcohols (R-OH) react with PCl5 and receive a Cl to get converted into their corresponding alkyl chlorides (R-Cl).
Summary
Phosphorous pentachloride (PCl5) is a nonpolar molecule. It has a symmetrical, trigonal bipyramidal geometry with bond angles of 90° and 120°.
The net dipole moment in PCl5 is zero which shows its non-polarity nature.
The bond present in PCl5 is polar because of the electronegativity difference between them.
There are 5 P-Cl polar covalent bonds present in PCl5.
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