Hydrogen cyanide
HCNA triple bond to nitrogen. Just like ethyne, two π bonds at right angles, with nitrogen's lone pair pointing out along the axis.
- C–N 1.16 Å
- C–H 1.08 Å
- ∠HCN 180°
- linear
Hydrogen cyanide has a carbon–nitrogen triple bond, the bond that defines the nitrile family (C≡N). It’s the nitrogen version of ethyne: swap one C–H end of ethyne for a nitrogen with a lone pair and you have HCN.
Both carbon and nitrogen are sp hybridized. Carbon uses its two sp orbitals for the bonds to H and to N; nitrogen uses one sp orbital for the bond to carbon and the other for its lone pair, which points straight out along the axis, away from the molecule. Everything lines up, so HCN is linear, with a 180° H–C–N angle. Turn on Angles & lengths to check.
As in ethyne, each atom keeps two p orbitals at right angles to the axis, and they overlap sideways to make two π bonds. Turn on p orbitals to see both sets, then π electrons to see them merge into a tube around the C≡N axis. Because nitrogen is more electronegative than carbon, that tube is pulled toward the nitrogen end.
Sources
- OpenStax Chemistry 2e, 7.6: Molecular Structure and Polarity
- OpenStax Organic Chemistry, 20.7: Chemistry of Nitriles
3D structure, measurements and electron clouds computed with PySCF (B3LYP/def2-SVP). Computed values can differ from measured ones by a degree or so, or a few hundredths of an ångström.