Nitrosyl Chloride1


[2696-92-6]  · ClNO  · Nitrosyl Chloride  · (MW 65.46)

(electrophilic and radical nitrosating agent for synthesis of a wide range of nitroso compounds, diazotization, and deamination; addition to alkenes yields chloronitroso derivatives; at high temperatures will chlorinate alkanes)

Physical Data: mp -61.5 °C; bp -6.4 °C; d (-12 °C) 1.417 g cm-3; d (gas) 2.99 g L-1.

Solubility: sol alcohols, ethers, CHCl3; hydrolyzed in water to give HNO2, HNO3, NO, and HCl.

Form Supplied in: readily available as an orange-yellow gas in cylinders, easily condensed to a deep red liquid; often more convenient to generate ClNO in situ from alkyl nitrites and HCl in ether, ethanol, or other organic solvents, or in water from HNO2 and HCl (or H+ and Cl-).

Handling, Storage, and Precautions: ClNO gas is highly irritating to the skin, eyes, and mucous membranes; very irritating odor; can cause fatal pulmonary edema; use in a fume hood; can react violently with acetone.


ClNO acts as a powerful electrophilic nitrosating agent,2 reacting with primary amines to give diazonium ions, secondary amines to give nitrosamines (eq 1), alcohols to give alkyl nitrites, thiols to give thionitrites, and carbonyl compounds to give (usually) keto oximes.3 The product of C-nitrosation can be the monomeric nitroso compound (blue), dimeric nitroso compound (white), or the tautomeric oxime, depending on substrate structure. Cyclic ketones often yield dioximes. ClNO is the effective reagent when diazotization, etc., is carried out in hydrochloric acid4 (using Sodium Nitrite) due to the equilibrium given in eq 2, for which the equilibrium constant at 25 °C is 1.1 × 10-3 L2 mol-2.5

The use of ClNO dissolved in organic solvents (or generated in situ from alkyl nitrites and HCl) has some advantage synthetically over acidic aqueous solutions of HNO2, for reactions of compounds (for example, amines or amides) which only have a limited solubility in water.

ClNO is a very reactive species and reacts with aromatic amines of pKa >~4 at the encounter limit.6 BrNO is, as expected, less reactive but bromide ion (and thiocyanate ion) catalysis of nitrosation in water is more pronounced because of the much larger equilibrium constant for BrNO formation.

Ring opening of cyclic ketones can occur by reaction with ClNO and EtOH in liquid SO2 (eq 3).7

With alkenes, addition occurs to give chloronitroso products (eq 4),8 a reaction which has some synthetic utility, for example in aziridine synthesis9 (after reduction of the NO group to NH2 and ring closure). Generally, ClNO addition follows the Markovnikov rule for unsymmetrical alkenes (and is interpreted as proceeding via a nitroso carbocation intermediate). This reaction was much used in the past to characterize terpenes.10 With some bicyclic systems (including norbornene and norbornadiene), there is evidence of syn addition via a four-center transition state.11 Generally good yields for this reaction can be obtained by using NaNO2 and HCl.

Nitroso products can also be obtained photochemically via a radical pathway.12 An example is shown in eq 5. Under some conditions, gem-chloronitroso products are formed (eq 6). The former reaction is used industrially with cyclohexanecarboxylic acid in the manufacture of ε-caprolactam.

Peroxycarboxylic acids react with ClNO to give C-nitroso compounds.13 Organometallic compounds such as alkyl (and aryl) Grignard reagents14 also give nitroso compounds with ClNO.


At higher temperatures than those used for nitrosation, ClNO will act as a chlorinating agent, particularly for alkanes, again probably by a radical pathway. Chlorinations have also been achieved in other solvents; for example, that of sulfoxides in CHCl3-pyridine (eq 7)15 and the conversion of thioamides to imidoyl chlorides (eq 8).16

1. Beckham, L. J.; Fessler, W. A.; Kise, M. A. CRV 1951, 48, 319.
2. Williams, D. L. H. Nitrosation; Cambridge University Press: Cambridge, 1988; p 10.
3. Touster, O. OR 1953, 7, 327.
4. Hammett, L. P. Physical Organic Chemistry; McGraw-Hill: New York, 1940; p 294.
5. (a) Schmid, H.; Hallaba, E. M 1956, 87, 560. (b) Schmid, H.; Fouad, M. G. M 1957, 88, 631.
6. Crampton, M. R.; Thompson, J. T.; Williams, D. L. H. JCS(P2) 1979, 18.
7. Rogic, M. M.; Vitrone, J.; Swerdloff, M. D. JACS 1977, 99, 1156.
8. Kadzyauskas, P. P.; Zefirov, N. S. RCR 1968, 37, 543.
9. Kemp, J. E. G. COS 1991, 7, 474.
10. Tilden, W. A.; Stenstone, W. A. JCS 1877, 554.
11. Meinwald, J.; Meinwald, Y. C.; Baker, T. N. JACS 1963, 85, 2513.
12. Pape, M. Fortschr. Chem. Forsch. 1967, 7, 559.
13. Labes, M. M. JOC 1959, 24, 295.
14. Müller, E.; Metzger, H. CB 1956, 89, 396.
15. Loeppky, R. N.; Chang, D. C. K. TL 1968, 5415.
16. Kantlehner, W. COS 1991, 6, 526.

D. Lyn H. Williams

University of Durham, UK

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