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A short class with Sarah and her students. The other ways are under More.
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Here are CH3CN and CH3NC. Count the atoms in each.
Both have two carbons, three hydrogens and one nitrogen, so both are C2H3N.
So what is the difference?
In CH3CN the methyl group is bonded to the carbon of the CN group. In CH3NC it is bonded to the nitrogen.
Right. What do we call compounds like these?
Functional isomers. One is a cyanide, or nitrile, and the other is an isocyanide.
Exactly. Everything else in this topic follows from which end of the CN group is attached.
In CH3–N≡C, how many bonds does nitrogen make?
One to the methyl group and three to carbon, so four bonds and no lone pair.
Nitrogen normally makes three bonds. What does a fourth bond do to its formal charge?
It makes it plus one, like nitrogen in the ammonium ion.
And the terminal carbon, with three bonds and a lone pair?
Minus one. So it is CH3–N⁺≡C⁻.
Good. Chemists also draw it as CH3–N=C:, with no charges. The real molecule lies between the two forms.
Why does bromoethane give a nitrile with KCN but an isocyanide with AgCN?
KCN gives free cyanide ions, which attack through carbon. In AgCN the carbon is already bonded to silver, so the nitrogen attacks.
Now reduce each product. What do you get?
The nitrile gives propan-1-amine, a primary amine. The isocyanide gives N-methylethanamine, a secondary amine.
Why the difference?
In the nitrile, the CN carbon joins the chain. In the isocyanide, it becomes a methyl group on the nitrogen.
Perfect reasoning from structure.
R is bonded to carbon in a cyanide (R–C≡N) but to nitrogen in an isocyanide (R–N≡C).
the cyano carbon of a nitrile is sp hybridised, so R–C≡N is linear, with one σ and two π bonds.
R–N⁺≡C⁻ is a resonance hybrid with R–N=C:, with a lone pair on the terminal carbon.
nitriles count the CN carbon (ethanenitrile), while isocyanides use the prefix isocyano- (isocyanomethane).
nitriles give acids and primary amines; isocyanides give an amine and methanoic acid on hydrolysis, and secondary amines on reduction.
CH3CN (ethanenitrile) and CH3NC (isocyanomethane) share the formula C2H3N but boil at about 82 °C and 60 °C respectively.
1-Bromopropane with KCN gives butanenitrile, CH3CH2CH2CN; with AgCN it gives 1-isocyanopropane, CH3CH2CH2NC.
Reducing ethanenitrile gives ethanamine (CH3CH2NH2), while reducing isocyanomethane gives N-methylmethanamine (CH3NHCH3).
Potassium sets the carbon free; silver holds carbon, so nitrogen it will be.
Twins called Nitrile and Isonitrile carry identical backpacks labelled C2H3N. Nitrile holds the carbon strap and walks calmly; Isonitrile holds the nitrogen strap, carries a negative charge on its free end and smells so bad that everyone knows when it is near.
Cyanide: Carbon Connects. Isocyanide: In through Nitrogen.
N.I.C.E. - Nitriles Include the CN Carbon; isocyanides Exclude it (they use the prefix isocyano-).
Term
IUPAC name of CH3CN
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Hybridisation and shape of the cyano carbon in a nitrile
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Charged structure of an isocyanide
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Why does AgCN give mainly isocyanides?
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Product of reducing an isocyanide RNC
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An organic cyanide, R–C≡N, in which the alkyl or aryl group is bonded to the carbon of the CN group.
An organic compound R–N≡C, in which the group is bonded to the nitrogen; also called an isonitrile or carbylamine.
A nucleophile that can attack through either of two different atoms, as CN⁻ can through carbon or nitrogen.
The mixing of one s and one p orbital to give two orbitals at 180°, which produces a linear shape.
The charge given to an atom in a structure by comparing its bonds and lone pairs with its number of valence electrons.
Compounds with the same molecular formula but different functional groups, such as CH3CN and CH3NC.
Having the same number of electrons, as CN⁻, N2 and CO do (14 each).
Cyanides (nitriles, R–C≡N) and isocyanides (isonitriles, R–N≡C) are isomers that differ in which atom of the CN group is joined to the carbon chain. This lesson explains their bonding and naming, and how their structures decide how they are made and how they react.
In a cyanide the alkyl group is bonded to the carbon of the –C≡N group, which is sp hybridised, so the R–C≡N unit is linear. In an isocyanide the alkyl group is bonded to nitrogen, which carries a positive formal charge, while the terminal carbon carries a lone pair and a negative charge; the structure is written R–N⁺≡C⁻ and is a resonance hybrid with R–N=C:. Cyanides are named as alkanenitriles, counting the carbon of the CN group, and isocyanides with the prefix isocyano- or as alkyl isocyanides. The two classes are functional isomers, and their different structures explain why KCN gives cyanides while AgCN gives isocyanides, and why their hydrolysis and reduction products differ.
Both classes contain one carbon atom and one nitrogen atom joined by a triple bond. The difference is the point of attachment: in a cyanide, also called a nitrile, the alkyl or aryl group R is bonded to the carbon (R–C≡N); in an isocyanide, also called an isonitrile or carbylamine, R is bonded to the nitrogen (R–N≡C). A cyanide and an isocyanide with the same R have the same molecular formula: CH3CN and CH3NC are both C2H3N.
The single question "which atom of the CN group is attached to R?" explains the names, the isomerism, the choice between KCN and AgCN, and the different products of hydrolysis and reduction. The preparation and reactions are covered in detail in the next two lessons.