NOCl or Nitrosyl Chloride is a toxic chemical compound that exists as a yellow colored gaseous compound. This gas is mostly released upon the decomposition of aqua regia. Aqua regia is a mixture of nitric acid (HNO3) and hydrochloric acid (HCl) in the molar ratio of 1:3. Nitrosyl chloride is also known as Tilden’s reagent.
Is NOCl polar or non-polar? NOCl is a polar molecule because the central nitrogen atom in the structure of NOCl has a lone pair of electrons present on it, which results in electron-electron repulsion. This leads to bending in the structure in V-shape, making it asymmetrical. Also, due to the unequal sharing of a charge, a dipole moment of about 1.9 D develops making it a polar molecule.
The molecular mass of NOCl is 65.459 g mol−1 with sp2 hybridization. It is a strong oxidizing agent and also an electrophile.
In gaseous state, the density of this substance is around 2.872 mg mL−1.
How to Distinguish Polar and Non-Polar Molecules
Polar and non-polar molecules are differentiated according to a number of factors. The differences between these two types of molecules can be tabulated as follows:
|BASIS||POLAR MOLECULE||NON POLAR MOLECULE|
|Structure||The asymmetrical arrangement of bonds||Symmetrical bond arrangement|
|Electronegative Difference||The difference in the electronegativity of the atoms is greater than 0.4||The electronegative difference between the atoms making the bond is less than 0.4|
|Type of Bond||Strong H- Bond||Weak Vander Waal forces|
|Dipole Moment||The net dipole of the molecule is considerably greater than 0 D||The net dipole moment is 0 D|
|Physical Properties||Polar molecules have high boiling and melting points||Such molecules have low boiling and melting points|
|Charge Build-up||Negative and positive charges get concentrated i.e. charges accumulate at the poles||No charge build-up anywhere in the entire molecule|
The above table gives an idea of the basis according to which polar and non-polar molecules majorly differ. Having an idea of the classification makes it easier to decide whether a given compound is polar or not.
Why is NOCl a polar molecule?
The Nitrosyl chloride structure is typically a little bent towards the inside. The bond length of the single N–Cl bond is 198pm, and that of the double N=O bond is 114pm. The bong angle is equal to 113°.
The actual structure is different from the Lewis structure because the actual structure takes the effect of lone pair into account.
Due to the presence of lone pair on the Nitrogen atom, there is electron-electron repulsion, which shoves the bonds, leading to a bent structure.
This asymmetrical charge distribution on the central atom distorts the symmetry of the molecule and leads to the origination of a dipole moment.
The reason for this bending is the immense electronegative values of oxygen and chloride. The electronegativity of oxygen and chlorine is 3.44 and 3.16 respectively.
This leads to a strong pull of the electron pair on both sides (because both the elements are highly electrophile), altering the otherwise straight bond.
Below is the geometrical shape of the NOCl molecule
Dipole Moment of NOCl
Due to the electron-electron repugnance, the molecular structure of NOCl is a bowed down, by an angle of 113°.
Subsequently, the polarity of the whole Nitrosyl chloride (NOCl) compound still exists but the overall strength of the dipole moment is somewhat diminished owing to the bending of the bonds.
The dipole moment of Nitrosyl chloride has been determined numerically as 1.84 ± 0.1 Debye. Debye is the SI unit of the dipole moment.
The dipole moment of a compound is the measure of its polarity.
Mathematically, it is defined as the product of charges of two atoms forming a covalent polar bond and the distance between them.
D = Q * R
Lewis structure of NOCl
Lewis structure of a compound shows the arrangement of the electrons around the atoms participating in the bond formation in its molecule.
Nitrogen is the least electronegative element out of the three atoms and takes up the center position in the Lewis structure.
NOCl molecule contains a total of 18 valence electrons.
The formal charge on each atom should be maintained as zero.
The nitrogen forms a double bond with oxygen and a single bond with chlorine leaving behind a lone pair on itself as shown below.
Below is the image of the lewis structure of NOCl
About Nitrosyl Chloride (NOCl)
NOCl is a yellow colored gaseous compound, which is awfully toxic in nature. It is an electrophile compound, which is used as an oxidizing agent in a number of reactions.
Bonding Arrangements of NOCl
NOCl comprises of mainly three elements – Nitrogen, Chlorine, and Oxygen. While deciding the bonding in NOCl, Nitrogen will always be placed in the center because it is the least electronegative element.
- In the structures (a) and (b), there are only six electrons around the central atom
(including the lone pair). But for a stable structure, the octet of the central atom must
- Also, in structure (e), using the lone pairs from both the atoms will result in more than
eight electrons on the central atom, which is not feasible for a stable structure.
- So, a lone pair of electrons on either of the terminal atoms should be used to form a
bond. This will give rise to two possible structures- (c) and (d).
- Again, a halogen rarely forms a double bond because, being in group 17, it has space
for only one electron.
- So, the structure (c) will be the most stable one as oxygen (belonging to group 16)
will be able to form a stable double bond.
- This arrangement results in a total of 18 valence shell electrons.
Formation of Nitrosyl Chloride
Nitrosyl chloride is simple in chemical terms and a thermally stable compound. The production and synthesis of NOCl can occur in a number of ways:
- At industrial levels, combining hydrochloric acid (HCl) and nitrosylsulfuric acid results in the materialization of NOCl.HCl + NOHSO4 ———–> H2SO4 + NOCl
- In laboratories, the method of reversible dehydration of nitrous acid by HCl leads to the formation of Nitrosyl chloride.HNO2 + HCl ————> H2O + NOCl
- The reaction of Palladium metal with the aqua regia solution also yields NOCl. This method was first used by Michael Faraday for the production if NOCl.Pd + HNO3 + 3HCl ————> PdCl2 + 2H2O + NOCl
- The most direct method for the manufacturing of Nitrosyl chloride is the direct combination of nitric oxide and chlorine. This chemical reaction can be reversed at a temperature above 100 °C.Cl2 + 2NO ————-> 2NOCl
NOCl Chemical nature and Reactions
In most of the chemical reactions, NOCl comes across as an electrophile (electron pair acceptor) and an oxidant.
The following reactions can be used to explain this acceptor nature of Nitrosyl chloride:
- NOCl + SbCl5 → [NO]+[SbCl6] –The reaction of NOCl with halide acceptors (eg: antimony pentachloride) leads to the generation of nitrosonium salts (i.e. salts with [NO]+ ion).
- NOCl + H2SO4 → ONHSO4 + HClThe reaction of Nitrosyl chloride with sulphuric acid gives nitrosulfuric acid.
- NOCl + AgSCN → AgCl + ONSCNSilver thiocyanate and NOCl react together. In response, it leads to the production of silver chloride and a pseudo-halogen Nitrosyl thiocyanate.It also reacts with Silver cyanide, resulting in the production of Nitrosyl cyanide.
- Mo(CO)6 + 2 NOCl → MoCl2(NO)2 + 6 COPreparation of metal Nitrosyl complexes can be completed by the reaction of metal compounds with NOCl.In the above chemical equation, molybdenum hexacarbonyl and NOCl react together, leading to the formation of a dinitrosyl-dichloride complex.
Properties of NOCl
- Molecular weight = 65.46 g mol−1
- Density = 2.87 mg/ml
- Critical temperature = 167°C
- Melting point = -59.4°C or −74.9 °F
- Boiling point = -5.5°C or 22.01 °F. The b.p. is not as high as elements like water, ammonia, etc. because they are capable of hydrogen bonding. However, it is higher than in many non-polar compounds.
- It appears as a yellow colored gas
- It is a toxic gas. If inhaled, it can prove to be extremely fatal.
- Even though it is a non-combustible material, it accelerates the smoldering of combustible substances.
- NOCl can be decomposed by water to form hydrochloric acid (HCl). It is not completely soluble in water but reacts with water.
Applications of NOCl
- NOCl is a reactive gaseous compound. It is used extensively in organic synthesis for obtaining Nitrosyl derivatives.The addition of NOCl to alkenes and other organic compounds follows Markovnikov Rule.
Example: Addition of NOCl to a Ketene can be depicted by the given chemical equation
H2C=C=O + NOCl → ONCH2C(O)Cl
- Conversion of cyclic amines to alkenes can be achieved by the reaction of amines with Nitrosyl chloride.Example: The reaction of NOCl with Aziridine (a three-membered heterocyclic compound with the formula (CH2)2NH), converts the cyclic compound to ethene, nitrous oxide (N2O), and hydrogen chloride (gaseous HCl).
- At Industrial Levels: The formation of caprolactam involves reaction with NOCl. A photochemical reaction between NOCl and cyclohexane to yield cyclohexanone oxime hydrochloride.
This process makes use of Nitrosyl chloride’s ability to undergo photodissociation and generate NO and Cl radicals. The oxide is then converted to caprolactam.The chemical reaction for the same is as displayed by the equations below.
The atoms in the NOCl molecule differ in their electronegativity as Oxygen and Chlorine atoms are more electronegative than Nitrogen due to which the positive and negative charges are separated. The molecule has a bent V-Shape structure due to which the charge distribution on its atoms is non-uniform and net dipole moment rises across the molecule.
The overall dipole moment of the NOCl molecule is 1.9 D making it a polar molecule.