In chemistry, a chemical trap is a chemical compound that is used to detect unstable compounds. The method relies on efficiency of bimolecular reactions with reagents to produce a more easily characterize trapped product. In some cases, the trapping agent is used in large excess.

Case studies

Cyclobutadiene

A famous example is the detection of cyclobutadiene released upon oxidation of cyclobutadieneiron tricarbonyl. When this degradation is conducted in the presence of an alkyne, the cyclobutadiene is trapped as a bicyclohexadiene. The requirement for this trapping experiment is that the oxidant (ceric ammonium nitrate) and the trapping agent be mutually compatible.

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Diphosphorus

Diphosphorus is an old target of chemists since it is the heavy analogue of N<sub>2</sub>. Its fleeting existence is inferred by the controlled degradation of certain niobium complexes in the presence of trapping agents. Again, a Diels-Alder strategy is employed in the trapping:

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Silylene

Another classic but elusive family of targets are silylenes, analogues of carbenes. It was proposed that dechlorination of dimethyldichlorosilane generates dimethylsilylene:

:SiCl<sub>2</sub>(CH<sub>3</sub>)<sub>2</sub> + 2 K → Si(CH<sub>3</sub>)<sub>2</sub> + 2 KCl

This inference is supported by conducting the dechlorination in the presence of trimethylsilane, the trapped product being pentamethyldisilane:

:Si(CH<sub>3</sub>)<sub>2</sub> + HSi(CH<sub>3</sub>)<sub>3</sub> → (CH<sub>3</sub>)<sub>2</sub>Si(H)-Si(CH<sub>3</sub>)<sub>3</sub>

Not that the trapping agent does not react with dimethyldichlorosilane or potassium metal.

In some cases, chemical trap is used to detect or infer a compound when present at concentrations below its detection limit or is present in a mixture, where other components interfere with its detection. The trapping agent, for example a dye, reacts with the chemical to be detected, giving a product that is more easily detected.

References