Nomenclature and Structure of Cyanides and Isocyanides

Cyanides (also called nitriles or alkyl cyanides) and isocyanides (also called

carbylamines or isonitriles) are built on the same two-atom $-\text{C}{\equiv}\text{N}$

unit and share the molecular formula $\text{R}-\text{CN}$, yet they are genuinely

distinct constitutional isomers, differing in exactly which atom of that unit is bonded

to the rest of the organic skeleton.

Cyanide (nitrile): $\text{R}-\text{C}{\equiv}\text{N}$. The organic group $\text{R}$

is bonded to the CARBON of the cyano group. IUPAC names a nitrile by adding the suffix

'-nitrile' to the name of the parent chain (INCLUDING the nitrile carbon in the count):

$\text{CH}_3\text{CN}$ is ethanenitrile (common name acetonitrile, or methyl cyanide),

$\text{CH}_3\text{CH}_2\text{CN}$ is propanenitrile (or ethyl cyanide).

Isocyanide (carbylamine): $\text{R}-\text{N}{\equiv}\text{C}$. The organic group

$\text{R}$ is instead bonded to the NITROGEN of the cyano group, with carbon left as the

terminal atom. Isocyanides are named by adding 'isocyanide' or 'carbylamine' after the

alkyl group's name: $\text{CH}_3\text{NC}$ is methyl isocyanide (or methyl carbylamine).

Structure and bonding. In a nitrile, the terminal carbon is $sp$-hybridised, forming

one $\sigma$ and two $\pi$ bonds to nitrogen (a genuine carbon-nitrogen triple bond,

directly comparable to $-\text{C}{\equiv}\text{C}-$), with nitrogen's lone pair pointing

outward along the molecular axis, away from the rest of the molecule. In an isocyanide,

by contrast, the terminal carbon is DIVALENT in the classical sense -- bonded only to

nitrogen by a formal triple bond that is best described using a resonance structure

carrying a carbene-like lone pair on the terminal carbon and a formal negative

charge on carbon / positive charge on nitrogen ($\text{R}-\overset{+}{\text{N}} {\equiv}\overset{-}{\text{C}}$ as the major resonance contributor) -- this terminal …