nitrile
/ NY-trile /
Some molecules end in a sharp little spike of a carbon triple-bonded to a nitrogen, written C≡N. A compound carrying that group is a nitrile. You meet nitriles in acrylic fibres and plexiglass precursors, and the group quietly appears as a useful handle in drug synthesis. It is compact, linear, and surprisingly versatile.
A nitrile is a functional group consisting of a carbon joined to a nitrogen by a triple bond, R-C≡N. The triple bond makes that stretch of the molecule rigid and linear, and because nitrogen is electronegative, the nitrile carbon is electron-poor and electrophilic, while the nitrogen carries a lone pair pointing away from the molecule. The whole group has a strong dipole. Two reactive features stand out: nucleophiles can add to the electrophilic carbon (leading, for instance, to amines or ketones), and the C≡N can be hydrolysed all the way to a carboxylic acid, gaining a carbon in the process. That last point makes the nitrile a favourite trick for lengthening a carbon chain by one.
Nitriles matter as synthetic stepping-stones precisely because the C≡N is so easily transformed into other groups — acids, amides, amines, aldehydes. Worth a careful note on naming and safety: the carbon of the nitrile counts as part of the main chain in IUPAC names (acetonitrile is CH3CN), and although the group is written with the letters of cyanide, an organic nitrile is generally far less acutely toxic than inorganic cyanide salts — but some can release cyanide on metabolism, so they are not harmless.
Acetonitrile, CH3C≡N (often written CH3CN), is a common laboratory solvent. Hydrolysing its C≡N fully gives acetic acid, CH3COOH — note how the nitrile carbon becomes the new acid carbon, lengthening the carbon framework.
Hydrolysing a nitrile to a carboxylic acid keeps the nitrile carbon — a chain-lengthening trick.
The nitrile carbon is counted in the parent chain, so the -nitrile carbon is C1. The group shares its letters with cyanide but most organic nitriles are not as acutely toxic as inorganic cyanide salts.