In contrast to the metal hydride reagents, diborane is a relatively electrophilic reagent, as witnessed by its ability to reduce alkenes.
The organometallic reagent is a source of a nucleophilic alkyl or aryl group (colored blue), which bonds to the electrophilic carbon of the carbonyl group (colored magenta).
This deep blue reagent is reduced to cuprous oxide, which precipitates as a red to yellow solid.
Tollens' reagent is usually ammoniacal silver nitrate, but can also be other things, as long as there is an aqueous diamminesilver(I) complex.
Once it has been identified that there is a carbonyl group on the organic molecule using 2,4-dinitrophenylhydrazine (also known as Brady's reagent or 2,4-DNPH), Tollens' reagent can be used to ascertain whether the compound is a ketone or an aldehyde.
Tollen'sreagent is also a test for alkynes with a triple bond in the 1-position.
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