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 is isocyano. 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. 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 is isocyano. 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. The π lone pair of the nitrogen, responsible for the zwitterionic structure, stabilizes the structure and is responsible of the linearity of isocyanides, although the reactivity of isocyanides reflects some carbene character, at least in a formal sense. Thus, both resonance structures are useful representations. 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 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. Ivar Karl Ugi states that 'The development of the chemistry of isocyanides has probably suffered only little delay through the characteristic odor of volatile isonitriles, which has been described by Hofmann and Gautier as 'highly specific, almost overpowering', 'horrible', and 'extremely distressing'. It is true that many potential workers in this field have been turned away by the odour, but this is heavily outweighed by the fact that isonitriles can be detected even in traces, and that most of the routes leading to the formation of isonitriles were discovered through the odor of these compounds.' Isocyanides have been investigated as potential non-lethal weapons. Some isocyanides convey less offensive odours such as malt, natural rubber, creosote, mild cherry or old wood. Non-volatile derivatives such as tosylmethyl isocyanide do not have objectionable odors. While some isocyanides (e.g., cyclohexyl isocyanide) are toxic, others 'exhibit no appreciable toxicity for mammals'. Referring to ethyl isocyanide, toxicological studies in the 1960s at Bayer showed that 'oral and subcutaneous doses of 500-5000 mg/kg can be tolerated by mice'.