Question

Nitroprusside will not work with primary and secondary amines if the pH is acidic. Why? (chemical toxicology)

What would happen if the Lieberman reagent was added to a sample of cocain HCl? Why? give a chemical explanation. (chemical toxicology)

Answer 1

The chemical formula for the nitroprusside is [Fe(CN)₅NO]^2-. This complex compound is sold normally as the disodium salt (Na₂[Fe(CN)₅NO]²), which furnishes the above complex anion on dissolution in water or alcohol.

The nitroso group in the nitroprusside normally exists as NO⁺ ligand in which Nitrogen center behaves as the electrophile (attracts the lone pair of electrons of the nucleophile, the amine group here). The electrophilic attack of amines (through lone pair of primary or secondary amines) usually occurs at the electrophilic N-center of the NO⁺ ligand forming an N-N bond in the intermediate, which structure resembles the ammonium ion. The thermodynamic stability of this intermediate is very low. Therefore, this reaction needs to happen with the simultaneous release of the proton of the amine. Since the pKa of ammonium is about ~9, a base is normally required to facilitate this reaction into the forward direction.

At the acidic pH, the deprotonation of the amine of the ammonium intermediate (formed after the nucleophilic attack of the amine group on the nitroso group of nitroprusside) is not possible. Hence this reaction does not move in the forward direction and fails to give the nitroprusside test.