Distinction between pairs of compounds
Ethyl cyanide `(CH_(3)CH_(2)CN)` and ethyl isocyanide `(CH_(3)CH_(2)Noverset(=)rarrC)`

Explanation

Following test helps to distinguish these :
Hydrolysis test. Ethyl isocyanide on hydrolysis with aqueous mineral acid gives formic acid which reduces Tollen.s reagent to silver mirror. Ethyl cyanide gives propanoic acid which does not reduce Tollen.s reagent.
(a) `underset("Ethyl isocyanide")(CH_(3)CH_(2)Noverset(=)rarrC)+2H_(2)Ooverset(HCl)rarrCH_(3)CH_(2)NH_(2)+underset("Formic acid")(HCOOH)`
`rarrunderset("Silver mirror")(2Ag+CO_(2))+4NH_(3)+2H_(2)O`
(b) `underset("Ethyl cyanide")(CH_(3)CH_(2)CN)+2H_(2)Ooverset(HCl)rarrunderset("Propanoic acid")(CH_(3)CH_(2)COOH)+Noverset(+)H_(4)`