Isocyanide: Structure, Formula & Uses

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

In this lesson, we will be learning about a class of organic compounds called isocyanides. Topics to be explored will be their properties, synthesis, reactions, and applications for their use.

A Very Unpleasant Odor

Did you know that some organic molecules have the ability to completely clear a room with their smell? That's right, there are some compounds that smell so strongly that they can cause everyone to seek fresh air. The compounds we are talking about are called isocyanides. Back in the 1800's, Lieke, the first chemist to discover them, stated that isocyanides 'have a penetrating, extremely unpleasant odour; the opening of a flask of allyl isocyanide is enough to foul up the air in a room for several days.'

Even though they don't smell so great, isocyanides still are an important class of organic compounds. Having a foundational understanding of them is beneficial to one's organic chemistry toolbox. This lesson we will be discussing the form and properties of isocyanides, how they are synthesized, reactions they take part in, and what they are used for in terms of application.

Structure and Properties

Let's get started with talking about the structure and properties of isocyanides. An isocyanide is an organic compound that contains a carbon-nitrogen triple bond, with an alkyl or aryl group connected to the nitrogen as well. Isocyanides can be described by two resonance structures, one with a triple bond between the carbon and nitrogen and one with a carbon-nitrogen double bond. Although experiments show that the one with the carbon-nitrogen double bond is a more important resonance contributor, the second structure is also needed due to the linearity of the carbon-nitrogen bond angle (it's near 180 degrees).

Structure and resonance forms of an isocyanide

Synthesis of Isocyanides

Now that we have an idea of what an isocyanide is and what they look like, let's talk about how they can be made synthetically in the lab.

Dehydration of a Formamide

One of the most common methods of making an isocyanide is by the dehydration of an organic formamide. In this reaction, a dehydrating agent is needed and things like phosphoryl chloride, phosgene, and diphosgene have all shown to be effective in triggering the transformation. Consider, as an example, the synthesis of phenyl isocyanide from formaniline using phosphorylchloride (POCl3) as the dehydrating agent.

Synthesis of isocyanide from formaniline using phosphorylchloride

Synthesis from Primary Amines

Another method that's popular for making isocyanides is from primary amines, also called the Hofmann isocyanide synthesis. In this preparation, chloroform reacts with potassium hydroxide to form an intermediate known as a dichlorocarbene, which then goes on to react with the primary amine to form our isocyanide. One drawback of this transformation is that it only works on primary amines. But nonetheless, aniline can be converted to phenylisocyanide in good yield by using a Hofmann synthesis.

Hofmann isocyanide synthesis of phenyl isocyanide

Synthesis from Halides

In terms of a more modern approach to the synthesis of isocyanides, they can be made by taking a benzyl bromide, a silver salt (like silver perchlorate), and trimethylsilyl cyanide (TMSCN) to produce benzyl isocyanide.

Synthesis of benzyl isocyanide from benzyl bromide

Reactions of Isocyanides

Ok so now we know how to make an isocyanide, let's talk briefly about a couple of important reactions they can undergo.

Hydrolysis of Isocyanides

Although isocyanides are stable under basic conditions, if we expose them to aqueous acid they will readily hydrolyze to a formamide. For example, if phenyl isocyanide is reacted with dilute hydrochloric acid, we will make formaniline.

Hydrolysis of phenyl isocyanide to formaniline

Reduction of Isocyanides

Isocyanides can be reduced to their corresponding amines in the presence of lithium aluminum hydride (LiAlH4, a strong reducing agent). For example, dimethylamine can be made in this way from methyl isocyanide.

Reduction of methyl isocyanide to dimethylamine using lithium aluminum hydride

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