Preparation of Cyanides and Isocyanides

Both cyanides and isocyanides are made from the identical starting material -- a

haloalkane -- and the identical pseudohalide ion, $\text{CN}^-$; the ONLY variable that

decides which isomer forms is which METAL SALT supplies that cyanide ion, because

$\text{CN}^-$ is an ambident nucleophile capable of attacking through either its

carbon end or its nitrogen end.

With potassium cyanide, $\text{KCN}$ (predominantly IONIC in character, so the free

$\text{CN}^-$ ion reacts largely under thermodynamic/HSAB-type control): the more

electronegative, harder, more stable end of the anion -- carbon, which bears more of the

negative charge in the resonance structures of cyanide -- is the site that attacks the

electrophilic carbon of the haloalkane, giving the alkyl CYANIDE:

With silver cyanide, $\text{AgCN}$ (predominantly COVALENT in character, because

silver is a soft, highly polarisable cation that binds strongly and covalently to the

carbon end of cyanide, effectively "using up" that end and leaving nitrogen as the more

available, more nucleophilic terminus): the reaction instead proceeds through the

NITROGEN end, giving the alkyl ISOCYANIDE:

This is exactly the same "ionic vs. covalent salt decides which end of an ambident ion

reacts" logic already met for nitro compounds vs. nitrite esters (Section 25.3, …