Addition of Hydrogen Halides to Nitriles and Isonitriles

The addition of hydrogen chloride and hydrogen bromide to nitriles yields iminium halides or imidoyl halides, depending upon the stoichiometry 

For the polar 2 1 adduct the term nitrilium salt is often used. Hantzsch in 1931 postulated the formation of nitrilium salts, but it was not until later that iminium halides were detected and isolated ( 87,208  

Iminium halides are intermediates in the hydrolysis of nitriles and in a variety of chemical reactions. For example, in the Ritter reaction addition of a nitrile or hydrogen cyanide to a carbonium ion occurs, leading to the intermediate formation of a nitrilium salt. Nitrilium salt intermediates have also been postulated in the Schmidt reaction and in the Beckmann rearrangement, provided the latter was performed in concentrated sulfuric acid. However, since we are predominantly concerned with imidoyl halides, these reactions are not discussed in this monograph. 

The generation of imidoyl chlorides from nitriles and hydrogen halides in the presence of a suitable Lewis acid accounts for the formation of ketones in the Houben-Hoesch reaction (see Section IVD). 

Schand in 1946 demonstrated that addition of hydrogen chloride to 2-chloronitriles yields the expected 1 1 adducts. For example, 2-chloro-propionitrile and hydrogen chloride combine easily to form j8-chloro-propionimidoyl chloride (XXXIX). 

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