Everything about Isocyanide totally explained
An
isocyanide (also called an
isonitrile) is an
organic compound with the
functional group R-N≡C. The CN functionality is connected to the organic fragment via the
nitrogen atom, not via
carbon as is found in the
isomeric nitriles, which have the connectivity R-C≡N. Hence the prefix
iso. Nitrogen and carbon are connected through a
triple bond with a positive charge on nitrogen and a negative charge on carbon.
Physical properties
Isocyanides are conventional organic compounds in terms of their physical properties. They are susceptible to
polymerization.
In terms of bonding, isocyanides are
isoelectronic with
CO. Thus the C-N-CR
3 (R = hydrogen or an organic group) is linear.
Odour of isocyanides
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 wasn't fully appreciated.
Ugi of
Ugi reaction fame states that "
The development of the chemistry of isocyanides has probably suffered ... 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 odor.” Isocyanides have been investigated as potential
non-lethal weapons.
Tosylmethyl isocyanide (
TOSMIC) is a notable exception (it doesn't smell as bad).
Synthesis of isocyanides
The first isocyanide,
allyl isocyanide was prepared in 1859 by the chemist Lieke from the reaction of
allyl iodide and
silver cyanide. Normally the
alkylation of an alkali metal cyanide gives a nitrile, but the silver ion protects the carbon end of the cyanide. Commonly, isocyanides are synthesized by the reaction of primary
amines with
dichlorocarbene or by
dehydration of a
formamide with
phosphorus oxychloride.
» RNH
2 + CCl
2 + 2 NaOH → RNC + 2 NaCl + 2 H
2O
RNHC(O)H + POCl
3 → RNC + "PO
2Cl" + 2 HCl
The
Hofmann isocyanide synthesis is a
chemical test for primary
amines based on their reaction with
potassium hydroxide and
chloroform as
dichlorocarbene precursors to foul smelling isocyanides.
Another route to isocyanides is by reaction of
organolithium compounds with
oxazoles and
benzoxazoles:
The
benzoxazole gets deprotonated at the 2-position by
n-butyllithium. The lithium compound is in
chemical equilibrium with the
2-isocyanophenolate which can be captured by an
electrophile such as an
acid chloride. Being an
ester the formed isocyanate in the example above behaves uncharacteristically with reportedly a mild
cherry smell.
Another synthetic route towards an isocyanide is 1) condensation of an amine with formic acid, yielding a formamide, and 2) dehydrating this formamide.
Reactions
Isocyanides are stable to strong base (they are often made under strongly basic conditions), but they're sensitive to acid. In the presence of aqueous acid, isocyanides hydrolyse to the corresponding
formamides. However, some isocyanides can polymerize in the presence of acids. Acid-hydrolysis is a convenient method for removing the obnoxiously odiferous isocyanides.
Isocyanides are reactants in two
multicomponent reactions of interest in
organic synthesis: the
Ugi reaction and the
Passerini reaction.
Naturally occurring isocyanides
Several organic molecules extracted from living organisms contain the isocyanide functionality. The first was discovered in 1957 in an extract of the mold
Penicillium notatum Westling. The compound
xanthocillin later was used as the
antibiotic. Since then numerous other isocyanides have been isolated. Most of the marine isocyanides are terpenes, while some of the terrestrial isocyanides originate from α-aminoacids.
Further Information
Get more info on 'Isocyanide'.
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