NITRILES: PROPERTIES, NOMENCLATURE, USES, EXAMPLES - CHEMISTRY - 2025

The nitriles are those organic compounds that have the CN functional group, to which it is also called as cyano group, or cyanide with respect to inorganic chemistry. Aliphatic nitriles are represented by the general formula RCN, while aromatic nitriles by the formula ArCN.

Although hydrogen cyanide, HCN, and metal cyanide salts are highly toxic compounds, this is not exactly the same with nitriles. The CN group in a carbon skeleton of any type (branched, linear, aromatic, etc.), behaves diagonally different from a cyanide anion, CN ^-.

General formula for an aliphatic nitrile. Source: Benjah-bmm27 via Wikipedia.

Nitriles are widely distributed in the world of plastics, as several of them are derived from acrylonitrile, CH ₂ CHCN, a nitrile with which polymers such as nitrile rubbers, used to make surgical or laboratory gloves, are synthesized. Also, nitriles are present in many natural and pharmaceutical products.

On the other hand, nitriles are precursors of carboxylic acids, since their hydrolysis represents an alternative synthesis method to obtain the latter.

Characteristics and properties

Structure

The molecular structures of nitriles vary as a function of the identity of R or Ar in the RCN or ArCN compounds, respectively.

However, the geometry of the CN group is linear due to its triple bond, C≡N, which is the product of sp hybridization. Thus, the CC≡N atoms are located on the same line. Beyond these atoms, there can be any kind of structure.

Polarity

Nitriles are polar compounds, since the nitrogen of the CN group is very electronegative and attracts electrons towards itself. Therefore, they have higher melting or boiling points than their alkane counterparts.

For example, acetonitrile, CH ₃ CN, is a liquid that boils at 82ºC; while ethane, CH ₃ CH ₃, is a gas that boils at -89 ºC. Note therefore the great effect that the CN group has on intermolecular interactions.

The same reasoning applies to larger compounds: if they have one or more CN groups in their structure, it is quite likely that their polarity increases and they are more akin to polar surfaces or liquids.

Basicity

It might be thought that due to the high polarity of nitriles, they are relatively strong bases compared to amines. However, one must consider the C≡N covalent bonds, and the fact that both carbon and hydrogen have sp hybridization.

The basicity of RCN: it is represented by the acceptance of a proton that usually comes from water:

RCN: + H ₂ O ⇌ RCNH ⁺ + OH ^-

For RCN to protonate: the free pair of electrons on nitrogen must form a bond with the H ⁺ ion. But there is a drawback: nitrogen sp hybridization makes it too electronegative, so much so that it attracts this pair of electrons very strongly and does not even allow it to form a bond.

Therefore, it is said that the electron pair of nitrogen sp is not available, and that the basicity of nitriles is very low. Nitriles are in fact millions of times less basic than amines.

Reactivity

Among the most representative reactions of nitriles we have their hydrolysis and reduction. These hydrolysis are mediated by the acidity or basicity of the aqueous medium, causing a carboxylic acid or a carboxylate salt, respectively:

RCN + 2H ₂ O + HCl → RCOOH + NH ₄ Cl

RCN + H ₂ O + NaOH → RCOONa + NH ₃

In the process, an amide is also formed.

Nitriles are reduced to amines using hydrogen and metal catalysts:

RCN → RCH ₂ NH ₂

Nomenclature

According to the IUPAC nomenclature, nitriles are named by adding the suffix -nitrile to the name of the alkane chain from which it is derived, also including the carbon of the cyano group. Thus, CH ₃ CN is called ethanonitrile, and CH ₃ CH ₂ CH ₂ CN, butanonitrile.

Likewise, they can be named starting from the name of the carboxylic acid, from which the word 'acid' is eliminated, and the suffixes -ico or -oic are replaced by the suffix -onitrile. For example, for CH ₃ CN it would be acetonitrile (from acetic acid); for C ₆ H ₅ CN, it would be benzonitrile (from benzoic acid); and for (CH ₃) ₂ CHCN, 2-methylpropanenitrile.

Alternatively, if the names of the alkyl substituents are considered, the nitriles may be mentioned using the word 'cyanide'. For example, CH ₃ CN would then be called methyl cyanide, and (CH ₃) ₂ CHCN, isopropyl cyanide.

Applications

Nitriles are part of natural products, being found in bitter almonds, in the bones of various fruits, in marine animals, plants and bacteria.

Its CN groups make up the structures of cyanogenic lipids and glycosides, biomolecules that, when degraded, release hydrogen cyanide, HCN, a highly poisonous gas. Therefore, they have an imminent biological use for certain creatures.

It was previously said that CN groups confer a lot of polarity to molecules, and in fact do not go unnoticed when present in compounds with pharmacological activity. Such nitrile medications have been used to combat hyperglycemia, breast cancer, diabetes, psychosis, depression, and other disorders.

In addition to having a role in biology and medicine, industrially they make up a handful of nitrile plastics, with which surgical and laboratory gloves, automotive parts seals, hoses and gaskets are made due to their resistance against corrosion and grease, materials such as tupperware, musical instruments or Lego blocks.

Examples of nitriles

Next, and finally, some examples of nitriles will be listed.

Nitrile rubbers

Molecular structure of the acrylonitrile-butadiene copolymer. Source: Klever via Wikipedia.

Nitrile rubber, from which the aforementioned gloves and grease-resistant materials are made, is a copolymer made up of acrylonitrile and butadiene (above). Note how linear the CN group looks.

Ciamemazine

Molecular structure of ciamemazine. Source: Epop / Public domain

Ciamemazine is an example of a nitrile in the pharmacy area that is used as an antipsychotic, specifically to treat anxiety disorders and schizophrenia. Again, note the linearity of the CN group.

Citalopram

Another nitrile drug is citalopram, used as an antidepressant

Amygdalin

Molecular structure of amygdalin. Source: Wesalius / Public domain

Amygdalin is an example of a cyanogenic glycoside. It is found in bitter almonds, plums, apricots, and peaches. Note how small the CN group looks compared to the rest of the structure; even so, its mere presence is enough to give this carbohydrate a unique chemical identity.

References

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6. Jim Clark. (2016). Hydrolysing nitriles. Recovered from: chemguide.co.uk
7. Ivy Rose Holistic. (2020). Naming nitriles. Recovered from: ivyroses.com
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