Question

Haloalkanes react with KCN to form alkyl cyanides as main product while AgCN forms isocyanides as the chief product. Explain.

Answer 1

Hint: Cyanide ion is an ambidentate ligand (nucleophile or lewis base). It means that donation of a pair of electrons can be through either carbon or nitrogen, so it has two nucleophilic centres.
KCN is predominantly ionic and AgCN is predominantly covalent.

Complete step by step answer:
Haloalkanes or alkyl halides – general form is (R-X)
R = alkane, X = halide (X = Cl, Br,I)
According to Fajan’s rule, for cations with same charge and size, the transition element cation with $(n-1){{d}^{n}}n{{s}^{0}}$configuration show more covalent character than the cation with $n{{s}^{2}}n{{p}^{6}}$configuration, commonly found in alkali and alkaline earth metals.
${{K}^{+}}$has $n{{s}^{2}}n{{p}^{6}}$configuration, so KCN is ionic. So, there is heterolytic fission. The fission is as follows:
$KCN\text{ }\to \text{ }{{K}^{+}}\text{ }+\text{ }C{{N}^{-}}$
The attack takes place through the carbon atom and not through nitrogen atom because the C-C bond is more stable than the C-N bond. So, the main product formed is alkyl cyanide.
The reaction of KCN with R-X is as follows:
$\text{ }RX\text{ + }KCN\text{ }\to \text{ }R-CN+KX$
$A{{g}^{+}}$has$(n-1){{d}^{n}}n{{s}^{0}}$configuration, so AgCN is predominantly covalent. There is an equal sharing of electrons between two atoms that is homolytic fission. The fission is as follows:
$AgCN\to Ag\bullet \text{ }+\text{ }CN\bullet$
The nucleophilic attack takes place from the nitrogen side instead of carbon because carbon is covalently bonded to silver due to very less electronegativity difference and nitrogen is free to donate its electron pair. So, the main product formed is alkyl isocyanide.
The reaction of AgCN with R-X is as follows:
$RX\text{ }+\text{ }AgCN\text{ }\to \text{ }R-NC+AgX$

Note: Heterolytic fission is when a covalent bond is broken, the shared pair of electrons is taken by one of the atoms due to electronegativity difference.
Homolytic fission is when the pair of electrons is split equally between the separated atoms.