Examples of Presumptive Color Tests

In the case of the Marquis reagent and morphine, a linked oxonium carbonium ion has been suggested as the source of the violet-colored product via an intermediate linked structure. The mechanism is analogous to the amphetamine-methamphetamine pathway an attack on formaldehyde by the amine, the linkage of two drug molecules, and formation of the colored carbonium under acidic conditions. 

The generic reaction of nitroprusside with bases such as the amines is  

For methamphetamine (a secondary amine), the inclusion of acetaldehyde in the reagent facilitates an addition-condensation reaction to form an enamine and an 

Transition Metal Complexes Related to the Simon test is a family of color-producing reactions based on transition metal complexes (coordination complexes) and tightly associated ion pairs. Coordination complexes arise from a Lewis acid-base interaction between a metal cation, such as cobalt, and an atom with unshared electrons, such as water or, in the case of drugs, basic nitrogen found in alkaloids and amines. Metals that have been used in these reagents include copper, vanadium, bismuth, and cobalt Cobalt, as part of two common reagents (cobalt thiocyanate and Dilli-Koppanyi) is perhaps the most versatile. Cobalt has an electron structure of 3d 4s, while ttie cation has a 3d (2 ) or 3d (3 ) structure. 

Bonding interaction Bonding interaction Nonbonding (no interaction) 

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