ACRYLONITRILE: STRUCTURE, PROPERTIES, PRODUCTION, USES - CHEMISTRY - 2024

The acrylonitrile is an organic compound which is condensed formula CH ₂ CHCN. It is one of the simplest nitriles that exist. Chemically and structurally, it is nothing more than the product of the union between a vinyl group, CH ₂ = CH-, and a nitrile, cyano or cyanide group, C≡N. It is a substance with great industrial impact in the world of plastics.

Physically, it is a colorless liquid, with a certain onion smell, and in an impure state it has yellowish tones. It is highly flammable, toxic, and possibly carcinogenic, which is why it is classified as an extremely dangerous substance. Ironically, it makes plastics and everyday toys, such as tupperware and Lego blocks.

Acrylonitrile polymers are present in the plastic of tupperware. Source: Stebulus via Wikipedia.

On industrial scales, acrylonitrile is produced by the Sohio Process, although there are many other less profitable methods by which it can also be synthesized. Almost all the quantity produced is destined to the manufacture of plastics, based on homopolymers, such as polyacrylonitrile, or on copolymers, such as acrylonitrile-butadiene-styrene.

Structure of acrylonitrile

Molecular structure of acrylonitrile. Source: Benjah-bmm27 / Public domain

In the upper image we have the acrylonitrile molecule represented with a model of spheres and bars. The black spheres correspond to carbon atoms, while the white and blue spheres correspond to hydrogen and nitrogen atoms, respectively. The left end, CH ₂ = CH- corresponds to the vinyl group, and the right, to the cyano group, C≡N.

The entire molecule is flat, since the carbons of the vinyl group have sp ² hybridization. Meanwhile, the carbon of the cyano group has sp hybridization, being next to the nitrogen atom in a line that lies in the same plane as the rest of the molecule.

The CN group provides polarity to the molecule, so the CH ₂ CHCN molecule establishes a permanent dipole where the highest electron density is directed towards the nitrogen atom. Therefore, dipole-dipole interactions are present, and they are responsible for acrylonitrile being a liquid that boils at 77 ºC.

CH ₂ CHCN molecules are flat, and it can be expected that in their crystals (solidified at -84 ºC) they will be arranged one on top of the other like sheets or sheets of paper, in such a way that their dipoles do not repel each other.

Properties

Physical appearance

Colorless liquid but with yellowish tones when it has some impurities. When this is the case, they recommend distilling it prior to use. It is volatile and also has a strong onion-like odor.

Molar mass

53.064 g / mol

Melting point

-84 ºC

Boiling point

77 ºC

Flash point

-5 ºC (closed cup)

0 ºC (open cup)

Autoignition temperature

481 ºC

Density

0.81 g / cm ³

Vapor density

1.83 in relation to air.

Vapor pressure

109 mmHg at 25 ºC

Solubility

In water it has a solubility of 74 g / L at 25ºC. Acrylonitrile is also considerably soluble in acetone, light petroleum, ethanol, ethyl acetate, and benzene.

Decomposition

When it decomposes thermally it releases toxic gases of hydrogen cyanide, carbon oxides and nitrogen oxides.

Production

Sohio process

Acrylonitrile can be produced on massive scales by numerous chemical reactions and processes. Of all of them, the Sohio Process is the most used, which consists of the catalytic ammoxidation of propylene. In a fluidized bed reactor, propylene is mixed with air and ammonia at a temperature between 400 and 510 ºC, where they react on solid catalysts.

The reactants enter the reactor only once, the following reaction taking place:

2CH ₃ −CH = CH ₂ + 2 NH ₃ + 3 O ₂ → 2 CH ₂ = CH – C≡N + 6 H ₂ O

Note that oxygen is reduced to water, while propylene oxidizes to acrylonitrile. Hence the name 'ammonia', as it is an oxidation that involves the participation of ammonia.

Alternative processes

In addition to the Sohio Process, other reactions that allow the obtaining of acrylonitrile can be mentioned. Obviously, not all of them are scalable, or at least cost-effective enough, nor do they produce products with high yield or purity.

Acrylonitrile is synthesized starting, again, from propylene, but making it react with nitric oxide on PbO ₂ -ZrO ₂ catalysts in the form of airgel. The reaction is as follows:

4 CH ₃ −CH = CH ₂ + 6 NO → 4 CH ₂ = CH – C≡N + 6 H ₂ O + N ₂

A reaction that does not involve propylene is one that begins with ethylene oxide, which reacts with hydrogen cyanide to become ethylene cyanohydrin; and subsequently, it is dehydrated at a temperature of 200 ºC in acrylonitrile:

EtO + HCN → CH ₂ OHCH ₂ CN

CH ₂ OHCH ₂ CN + Q (200 ºC) → CH ₂ = CH – C≡N + H ₂ O

Another much more direct reaction is the addition of hydrogen cyanide on acetylene:

HC≡CH + HCN → CH ₂ = CH – C≡N

However, many by-products are formed, so the quality of acrylonitrile is inferior compared to the Sohio Process.

In addition to the substances already mentioned, lactonitrile, propionitrile, and propanal or propaldehyde, also serve as starting materials for the synthesis of acrylonitrile.

Applications

Legos are mainly made of ABS plastic, a copolymer in which acrylonitrile is a part. Source: Pxhere.

Acrylonitrile is an essential substance for the plastics industry. Several of its derived plastics are highly known. Such is the case of the acrylonitrile-butadiene-styrene (ABS) copolymer, with which the pieces of the Legos are manufactured. We also have acrylic fibers, made with polyacrylonitrile homopolymer (PAN).

Among other plastics that contain acrylonitrile as a monomer, or that depend on it for their manufacture, we have: acrylonitrile styrene (SAN), acrylonitrile butadiene (NBR), acrylonitrile styrene acrylate (ASA), polyacrylamide, and synthetic rubbers and nitrile resins.

Virtually any natural polymer, such as cotton, can be modified by incorporating acrylonitrile into its molecular structures. Thus, nitrile derivatives are obtained, which makes acrylonitrile chemistry very broad.

One of its uses outside the world of plastics is as a pesticide mixed with carbon tetrachloride. However, due to their terrible impact on the marine ecosystem, such pesticides have been banned or are subject to heavy restrictions.

Risks

Acrylonitrile is a dangerous substance and must be handled with extreme care. Any inappropriate or excessively prolonged exposure to this compound can be fatal, whether by inhalation, ingestion, or physical contact.

It should be kept as far away as possible from any source of heat, as it is a highly flammable and volatile liquid. Nor should it be in contact with water, as it reacts violently with it.

In addition to this, it is a possibly carcinogenic substance, which has been associated with liver and lung cancers. Those who have been exposed to this substance present increases in the levels of thiocyanate excreted in the urine, and of cyanides in the blood as a result of the metabolization of acrylonitrile.

References

1. Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. (10th edition.). Wiley Plus.
4. Wikipedia. (2020). Acrylonitrile. Recovered from: en.wikipedia.org
5. National Center for Biotechnology Information. (2020). Acrylonitrile. PubChem Database., CID = 7855. Recovered from: pubchem.ncbi.nlm.nih.gov
6. Elsevier BV (2020). Acrylonitrile. ScienceDirect. Recovered from: sciencedirect.com
7. Korry Barnes. (2020). What is Acrylonitrile? - Uses & Properties. Study. Recovered from: study.com
8. Pajonk, GM, Manzalji, T. (1993). Synthesis of acrylonitrile from propylene and nitric oxide mixtures on PbO ₂ -ZrO ₂ airgel catalysts. Catal Lett 21, 361–369. doi.org/10.1007/BF00769488