Hoffman's Isocyanide test: How does addition of concentrated HCl degrade the isocyanide formed?

This question is regarding a laboratory procedure followed for [Hoffman's Isocyanide test](https://en.wikipedia.org/wiki/Carbylamine_reaction) (also known as the "Carbylamine test")


I've been taught that the isocyanide formed in this reaction is highly toxic and must therefore be destroyed *in situ*.


I'm told that one method of carrying this out is to cool the test tube and carefully add an excess of concentrated $HCl$.


Though it wasn't mentioned, I'd like to know how exactly the addition of "excess concentrated $HCl$" results in the degradation of the isocyanide formed.


I've provided the reaction-mechanism for a primary amine subjected to the Hoffman Isocyanide test below;


[![I am providing mechanism here](https://i.stack.imgur.com/E1zq1.jpg)](https://i.stack.imgur.com/E1zq1.jpg)




Isocyanide is not as toxic as its isomer cyanide and can be tolerated. The purpose of its removal is due to its disagreeable odor which is not at all tolerable.



> 
> While some isocyanides (e.g., cyclohexyl isocyanide) are toxic, others
>  "**exhibit no appreciable toxicity for mammals**". Referring to ethyl
>  isocyanide, toxicological studies in the 1960s at Bayer showed that
>  "oral and subcutaneous doses of 500-5000 mg/kg **can be tolerated by
>  mice".**([Wikipedia](https://en.wikipedia.org/wiki/Isocyanide))
> 
> 
> 




---


Isocyanide is removed on the basis of its property of acidic hydrolysis.



> 
> Isocyanides are stable to strong base (they are often made under
>  strongly basic conditions), but they are sensitive to acid. In the
>  presence of aqueous acid, isocyanides hydrolyse to the corresponding
>  formamides:
> 
> 
> $$\ce{RNC + H2O → RN(H)C(O)H}$$
> 
> 
> **This reaction is used to destroy odorous isocyanide mixtures**. Some
>  isocyanides can polymerize in the presence of Lewis and Bronsted
>  acids.
> 
> 
> 


But the story does not here. The formamides are still pretty unstable and further degrades to form amines and formic acid in the presence of acidic medium.([Here](http://www.questtutorials.com/media/static/xii/chemistry/15_Organic%20Compounds%20with%20Functional%20Groups%20Containing%20Nitrogen/02_Cyanides%20and%20Isocyanides.htm))



> 
> Isocyanides are hydrolyzed by dilute mineral acids to give primary
>  amine and formic acid (Isocyanides are not hydrolyzed by bases).
>  Isocyanides are hydrolysed only by acids and not alkalies because the
>  negative charge present on carbon atom in isocyanides initially
>  attracts electrophiles ($\ce{H+}$) but repels nucleophiles ($\ce{OH–}$). When a
>  proton gets attracted to negatively charged carbon atom, the tendency
>  of this carbon atom to attract a nucleophile increases due to presence
>  of positive charge on the N atom and hydrolysis is facilitated as
>  shown below:[![enter image description here](https://i.stack.imgur.com/mSh8w.png)](https://i.stack.imgur.com/mSh8w.png)
> 
> 
> **Mechanism:**
> 
> 
> [![enter image description here](https://i.stack.imgur.com/HrXxd.png)](https://i.stack.imgur.com/HrXxd.png)
> 
> 
> 


Here is [a research paper](http://www.sciencedirect.com/science/article/pii/S0040403901008632) for a reference.


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