Nitriles – Structure, Properties Reactions, and Uses.

In organic Chemistry, nitriles are the organic compounds that contain cyanide as the functional group. They are also known as cyano compounds and have the formula −C ≡ N. The inorganic compounds that contain a cyanide group are called cyanides. 

Depending on the oxidation state of propylene, the compound Acrylonitrile undergoes the process of ammoxidation and is processed in many ways in the presence of a catalyst. This compound is a vital component of polymeric substances, including synthetic rubbers, acrylic textiles, and thermoplastic resins. Some nitrile compounds are mainly produced by heating carboxylic acids with ammonia in the presence of catalysts. 

This technique is involved in preparing nitriles like phenyl nitrile and amino nitrile from fats and oils. These are also employed as softening agents in substances like textiles, plastic, synthetic, and also in the manufacture of amines. Nitrile compounds are also derived from the process of amide heating up in the presence of phosphorus pentoxide. We can also reduce primary amines by hydrolyzing lithium to carboxylic acids in the presence of a base or acid or reacting lithium aluminium hydride.

History of Amines

The first compound from the family of nitriles was hydrogen cyanide. It was manufactured for the first time by C. W. Scheele in 1782. J. L. Gay-Lussac managed to produce the very volatile and poisonous pure acid in 1811. Justus von Liebig and Friedrich Wöhler 1832 prepared a nitrile of benzoic acid called benzonitrile. Still, because the yield of the synthesis was minimal, a structure could not be suggested, nor the chemical and physical properties could be determined. Théophile-Jules Pelouze, in 1833 manufactured propionitrile proposing it to be either of hydrocyanic acid or alcohol. 

The first method by which enough substance of the compound could be yielded for chemical research was performed in 1843 by Hermann Fehling by the heating of ammonium benzoate. Fehling determined the structure of the compound by comparing the results to already known data, that is, by studying the heating up of ammonium formate to give hydrogen cyanide. The name “nitrile” was coined by him for the newly formed compound. 

Synthesis of Amines

The main techniques used to produce nitriles like phenyl nitrile and amino nitrile in industries are hydrocyanation and ammoxidation. Stoichiometric amounts of salts are not produced in any of these processes, so they are pretty green.

Ammoxidation

In this process, partial oxidation of a hydrocarbon in the presence of ammonia occurs. This reaction is carried out on a large scale to produce acrylonitrile. 

CH3CH=CH2 + 3⁄2 O2 + NH3 → NCCH=CH2 + 3 H2O

A side product formed as a result of the formation of acrylonitrile is acetonitrile. Various derivatives of phthalonitrile, isobutyronitrile, and benzonitrile are manufactured by ammoxidation. The ammoxidation process is catalysed by the oxides of metals and proceeds through the imines. 

Hydrocyanation

The industrial method by which nitriles are manufactured from alkenes and hydrogen cyanide is hydrocyanation. Homogeneous catalysts are required in the process. The production of adiponitrile is an example of hydrocyanation. 

CH₂=CH−CH=CH₂ + 2 HCN → NC(CH₂)₄CN

From organic halides and cyanide salts

For laboratory scale production, two salt metathesis reactions are popularly performed. By the process of Kolbe nitrile synthesis, the nucleophilic aliphatic substitution of alkyl halides occurs in the presence of cyanides of alkali metals. By the process of Rosenmund-von Braun synthesis, aryl nitriles are manufactured. 

Cyanohydrins

Cyanohydrins are a type of nitriles that are produced in the cyanohydrin reaction by the addition of cyanides of alkali metals to aldehydes. As the organic carbonyl is a polar compound, no catalyst is required in this reaction, unlike alkene hydrocyanation. When trimethylsilyl cyanide is added, o-Silyl cyanohydrins are manufactured. A catalyst is required here. The manufacture of cyanohydrins also occurs in cyanohydrin reactions. For instance, HCN is manufactured from acetone cyanohydrin.

Amide dehydration

• From aldehydes and oximes

Nitriles are popularly produced in the laboratory from aldehydes through aldoximes. Aldoximes are produced when a reaction of aldehydes occurs with hydroxylamine salts at extremely low temperatures. These compounds give nitriles on dehydration due to simple heating. A diverse range of reagents is available to catalyse this reaction, including sulfuryl chloride, triethylamine, and zeolites. Similar reactions are undergone by hydroxylamine-O-sulfonic acid.

• One-pot synthesis from aldehyde

The Van Leusen reaction can be employed to synthesise amines in special cases. Here biocatalysts like aliphatic aldoxime dehydratase are effectively used. 

Other methods of nitrile synthesis

• Diethylaluminum cyanide is the commercial source for the cyanide group. It can be manufactured from HCN and trimethylaluminum. It has also been employed in nucleophilic addition to ketones. 
• Base hydrolysis of trichloromethyl aryl ketimines gives aromatic nitriles. The reaction is known as Houben-Fischer synthesis. 
• Oxidation of primary amines also gives nitriles. The use of Trichloroisocyanuric acid, potassium persulfate, and anodic electrosynthesis are commonly involved in the process. 
• Nitriles are also manufactured from aryl carboxylic acids in a reaction called Letts nitrile synthesis.

Occurrence of Nitriles

Nitriles are found naturally in a wide variety of animal and plant sources. More than 100 types of nitriles occurring in nature have been identified from marine and terrestrial sources. These compounds are commonly found in fruit pits, specifically almonds. These are also produced when Brassica crops like Brussels sprouts, cauliflower, and cabbage are cooked. These crops generate nitriles through the process of hydrolysis. A cyanohydrin called mandelonitrile is produced by ingesting almonds. This compound also releases hydrogen cyanide and causes cyanogenic glycoside toxicity.

Currently, more than thirty nitrile-containing pharmaceuticals are being marketed for a wide variety of medicinal indications. The nitrile group in these preparations is robust, remaining unmetabolized and passing out of the body in an unchanged form. Different types of pharmaceuticals that contain nitriles exist. For example, vildagliptin and anastrozole (used in the treatment of breast cancers). In many cases, nitrile has the same functionality as the substrates, while in other cases, the nitrile enhances solubility in water or reduces the susceptibility to metabolism by the liver. Several drugs contain a functional nitrile group. 

Properties

Following are the physical and chemical properties of nitriles:

Physical properties

• Nitriles are colourless compounds that are solid or liquids at room temperature having unique odours.
• Their boiling points range between 82-118 °C.
• Nitriles show strong Van der Waals dispersion forces and also dipole-dipole movements among the molecules.
• They also exhibit high electronegativity and polarity.
• They are considered highly soluble in water, and the solubility decreases with the chain length.

Chemical properties

Nitrile undergoes reactions in different ways. These include alkylation, reduction, and hydrolysis. Here you understand all the different varieties of nitrile reactions in a clear way, especially their hydrolysis. Various sets of reactions are shown by nitriles as discussed below:

• Hydrolysis: Nitriles can only hydrolyze when an acid or a base is present. Carboxylic acids and carboxamides are produced by hydrolysis of nitriles efficiently in the presence of a base or an acid. 
• Reduction: Reduction of nitriles to primary or tertiary amines occurs when they are treated with lithium aluminium hydride. The process of reduction happens in the presence of some catalysts.
• Alkylation: The formation of nitriles occurs due to the alkylation of nitriles. Nucleophilic addition reactions like Friedel–Crafts acylation are other reactions.

Uses

The use of nitriles is seen in various industrial and medical processes as given below

• Nitriles are employed in the production of seals, nitrile gloves, and hoses as they show resistance to chemicals.
• They are employed as an antidiabetic medication employed in the treatment of breast cancers.
• The nitrile compound known as pericyazine is employed in the treatment of opiate dependence as an antipsychotic.
• This compound is seen in many animal and plant sources.
• They are used in applications of oil-resistant substances and also for low-temperature uses. 
• They are also used in hydraulic systems, automotive systems, and the manufacture of aircraft parts.
• Even these products can be used in the production of amines. 

Conclusion

Nitriles such as phenyl nitrile and amino nitrile and their derivatives like iso nitrate are important compounds in organic Chemistry. Their chemical reactions have a wide range of applications. We hope that this article helps you gain a fundamental understanding of this group of compounds, helps you understand nitrile meaning, and enables you to answer most of the questions related to the topic.

Frequently Asked Questions

Q) What happens when amides are dehydrated?

A) When primary amides are dehydrated, the result is the formation of nitriles. Thionyl chloride and phosphorus pentoxide are the common reagents used in this reaction. In a similar reaction, nitriles are produced from secondary amides by the process of von Braun amide degradation. Here, cleavage of one C-N bond occurs. 

Q) What is the Sandmeyer Reaction?

A) In the laboratory, the preparation of aromatic nitriles occurs from aniline as a raw material. This preparation occurs through diazonium compounds and is called the Sandmeyer reaction. Transition metal cyanides are required for this reaction. 

Q) What are the main points about nitrile?

A) Nitrile can be defined as an organic chemical containing the Cyano functional subunit or group. The structure of nitrile is CN- where there is a triple bond between the atoms of carbon and nitrogen. The formula of nitrile in general terms is RCN, where R represents the organic group or subunit.