Isocyanide

An isocyanide (also called isonitrile or carbylamine) is an organic compound with the functional group -N≡C. It is the isomer of the related cyanide (-C≡N), hence the prefix iso. The organic fragment is connected to the isocyanide group via the nitrogen atom, not via the carbon. They are used as building blocks for the synthesis of other compounds.

The C-N distance in isocyanides is very short, 1.158 Å in methyl isocyanide. The C-N-C angles are near 180°.

Akin to carbon monoxide, isocyanides are described by two resonance structures, one with a triple bond between the nitrogen and the carbon and one with a double bond between. Surprisingly, the second one, with a carbenic character is the more important. Nevertheless, the π lone pair of the nitrogen, responsible of the zwitterionic structure, stabilizes the structure and is responsible of the linearity of isocyanides. Isocyanides are best shown as a mixture of both resonance structures. They are susceptible to polymerization.

Isocyanides exhibit a strong absorption in their IR spectra in the range: 2165–2110 cm⁻¹

The electronic symmetry about the isocyanide ¹⁴N nucleus results in a slow quadrupolar relaxation so that ¹³C-¹⁴N nuclear spin coupling can be observed, with coupling constants of ca. 5 Hz for the isocyanide ¹³C nucleus and 5–14 Hz for the ¹³C nucleus which the isocyanide group is attached to.

Their disagreeable odour is legendary. To quote from Lieke, "Es besitzt einen penetranten, höchst unangenehmen Geruch; das Oeffnen eines Gefässes mit Cyanallyl reicht hin, die Luft eines Zimmers mehrere Tage lang zu verpesten, ..." (It has a penetrating, extremely unpleasant odour; the opening of a flask of allyl [iso]cyanide is enough to foul up the air in a room for several days). Note that in Lieke's day, the difference between isocyanide and nitrile was not fully appreciated.

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