My book mentions addition reactions of isocyanides with halogens, ozone, sulphur, etc. but it doesn't mention any such reaction for cyanides. Instead, it bluntly states: "Cyanides do not undergo addition reactions". My question is: why is this so? I understand the structures of the ions, but am unable to relate the exact correct logic in this situation.

Here are the addition reactions of isocyanides I am talking about:

$$ \begin{align} \ce{R-NC + O3 &->RNCO + O2}\text{(methyl isocyanate)}\\ \ce{R-NC + 2HgO &-> RNCO + Hg2O}\text{ (methyl isocyanate)}\\ \ce{R-NC + S &-> RNCS} \text{ (methyl isothiocyante)}\\ \ce{R-NC + Cl2 &-> RNC\cdot Cl2} \text{ (methyl imino carbonyl chloride)}\\ \end{align} $$

  • $\begingroup$ I would think that the nitrogen in cyanide is much less nucleophilic than the carbon in isocyanide, from the look at their resonance structures and how much they might contribute to the real structure. Maybe some reason along those lines? $\endgroup$ – ralk912 Mar 15 '18 at 3:41
  • $\begingroup$ @ralk912 Well I wish I knew the reason myself :P if you think you're in the right direction, please consider posting an answer. Thanks! $\endgroup$ – Gaurang Tandon Mar 15 '18 at 3:56
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    $\begingroup$ Well, with ozone is likely to be through a 1,3-dipolarcycloaddition, but I can't figure out how the ozonolide would collapse. I can't really find many references either, so I'm not confident on the answer. Interesting question for sure. $\endgroup$ – ralk912 Mar 15 '18 at 3:58

Isocyanide can undergo addition reaction due to the fact that it is capable to attach both the electrophile and nucleophile in the same carbon atom while it is not achieved by cyanide. The negative charge on carbon atom accepts the electrophile to form an intermediate species in which the positive charge on nitrogen atom gets neutralised by the addition of the nucleophile to the carbon atoma and the nucleophile gets attached in the same carbon atom. Cyanide is incapable of undergoing such mechanism because of less nucleophilicity of nitrogen atom of cyanide and thus don't undergo addition reaction. See the mechanism of addition reaction below(taken from here):

enter image description here

Also see this.

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