Nitrosyl chloride

Nitrosyl chloride is the chemical compound NOCl. It is a yellow gas that is most commonly encountered as a decomposition product of aqua regia, a mixture of hydrochloric acid and nitric acid. The related nitrosyl halides nitrosyl fluoride, NOF, and nitrosyl bromide, NOBr, are also known.

Structure and synthesis
Although its formula is written NOCl, the structure is better represented ONCl. A double bond exists between N and O (distance = 1.16 Å) and a single bond between N and Cl (distance = 1.69 Å). The O-N-Cl angle is 113°.

NOCl can be prepared by the direct combination of chlorine and nitric oxide. Alternatively, nitrosylsulfuric acid and HCl also affords the compound:
 * HCl +  NOHSO4  &rarr;  NOCl  +  H2SO4

It also arises from the combination of hydrochloric and nitric acids according to the following reaction:
 * HNO3 +  3 HCl  &rarr;  Cl2  +  2 H2O  +  NOCl

Although this mixture was used for dissolving gold for a long time the first description of the gas was done by Edmund Davy in 1831.

Reactions

 * NOCl reacts with halide acceptors to give the nitrosyl cation, [NO]+. This species is available as NOBF4.
 * NOCl reacts with water to release HCl.
 * Heating NOCl above 100 C gives back Cl2 and NO.
 * In organic synthesis, NOCl is often employed. It adds to alkenes to afford &alpha;-chloro oximes.

Safety
NOCl is toxic and irritating to the lungs.