My book states addition of $\ce{NOCl}$ to propylene results in 2‐chloro‐1‐nitrosopropane:

addition of nitrosyl to propylene

Would it follow syn or anti addition?

According to Mallya and Nagendrappa [1], the reaction would follow syn addition:

Cycloalkenyltrimethylsilanes add nitrosyl chloride stereo- and regio- selectively. The addition is syn, with chlorine and nitrosyl group adding to C-1 and C-2 respectively.

I am unsure whether it's applicable to my problem. Moreover, I was under the impression that the addition would be anti due to steric repulsions.


  1. Mallya, M. N.; Nagendrappa, G. Nitrosyl Chloride Addition to Cycloalkenyltrimethylsilanes. An Unexpected Reversal of Regiochemistry of Addition, and Synthesis of β-Aminosilanes. Arkivoc 2007, 2006 (14), 155–174. DOI: 10.3998/ark.5550190.0007.e17.
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    $\begingroup$ You might be better with a cyclic alkene such as cyclopentene as it is possible to have cis and trans products which will give clues to how the reaction occurs $\endgroup$ May 27, 2022 at 8:59

2 Answers 2


Syn or anti addition is difficult to resolve with propene because the final product admits rotation about the single bond. However, if the rotation is constrained or the relative positions of the functional groups are marked by chirality, then we see anti addition. See this summary of reaction mechanisms, step 41.

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More is involved than steric favoritism here. When the nitrosyl cation adds to the alkene, it picks up an electron pair on the nitrogen which can then back-bond to the carbocation center (cf. the bromonium ion formed with electrophilic bromination). The electron-accepting orbital is then antibonding with respect to this linkage and thus displaced to the back side of the carbon. So the chloride ion will attack the carbon there to give the anti product.


The most important fact about this question is the stereochemistry of nitrosyl chloride $(\ce{NOCl})$ addition to an alkene. The OP has wondered whether it follows syn- or anti-addition.

The answer is not simple one word answer. According to the Ref.1, the nitrosochlorination of alkenes is shown to possess features common to other reactions of electrophilic addition, yet the stereochemistry of the addition depends on the structure of the alkene. For example, the addition of $\ce{NOCl}$ to norbornene follows cis-stereochemical course while that to $\Delta^9$-octalin follows trans-stereochemical course. However, the Ref.2 suggests that stereochemical outcome of the reaction also depends upon solvents used in the reaction. For instance, the stereochemistry of the addition of nitrosyl chloride to cyclohexene in liquid sulfur dioxide is not the same when the identical reaction is performed in $\ce{CH2Cl2}$ or in $\ce{CHCl3}$ (Ref.2).

In later studies of same reaction in liquid sulfur dioxide, the trans-stereochemical out come of the dimeric product has established by NMR spectral data (Ref.3).

Structural influence on this reaction was also established on the $\ce{NOCl}$ addition on 3,4,6-tri-O-acetyl-D-glucal, which has predominantly showed cis-addition (Ref.4 & 5):

3,4,6-Tri-O-acetyl-D-glucal reaction with NOCl

The structural effect is also evedence in oxymercuration: The oxymercuration of D-glucal has given the corresponding cis-product while that of its 3,4,6-triacetate has given the corresponding trans-product (Ref.6).


  1. P. P. Kadzyauskas and Nikolai S. Zefirov, "Nitrosochlorination of Alkenes," Russian Chemical Reviews 1968, 37(7), 543 (DOI: https://doi.org/10.1070/RC1968v037n07ABEH001665).
  2. Masaji Ohno, Masaru Okamoto, and Kenkichi Nukada, "Dependence of steric course of nitrosyl chloride addition to cyclohexene on solvents," Tetrahedron Lett. 1965, 6(45), 4047-4052 (DOI: https://doi.org/10.1016/S0040-4039(01)99612-1).
  3. B. W. Ponder, Thomas E. Walton, and William J. Pollock, "Addition of nitrosyl chloride to cyclohexene," J. Org. Chem. 1968, 33(10), 3957-3959 (DOI: https://doi.org/10.1021/jo01274a065).
  4. R. U. Lemieux, T. L. Nagabhushan, and S. W. Gunner, "Synthesis of 3,4,6-Tri-O-acetyl-2-oximino-α-D-hexopyranosides," Can. J. Chem. 1968, 46(3), 405-411 (DOI: https://doi.org/10.1139/v68-06).
  5. R. U. Lemieux, T. L. Nagabhushan, and I. K. O'Neill, "A new approach to the synthesis of 2-amino-2-deoxyglycosides," Tetrahedron Lett. 1965, 5(29), 1909-1916 (DOI: https://doi.org/10.1016/S0040-4039(01)89472-7).
  6. Philip T. Manolopoulos, Morton. Mednick, and Norman N. Lichtin, "Oxymercuration of D-Glucal and Its 3,4,6-Triacetate," J. Am. Chem. Soc. 1962, 84(11), 2203–2210 (DOI: https://doi.org/10.1021/ja00870a036).

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