Ch_322b_16.14 - Chapter 16.14 lecture note

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Analyses of Aldehydes and Ketones Chemical Analyses The earliest methods for characterizing aldehydes and ketones were based on well-defined chemical reactions and their products that typically were solids. Before the availability of spectroscopic methods of analysis in the latter half of the 20th century, an unknown aldehyde or ketone was identified by the melting points of several derivatives . A widely used derivatizing compound was 2,4-dinitrophenylhydrazine (2,4-DNP) that usually gives products with sharp melting points. C R R' O = often a colorless liquid + NHNH 2 NO 2 O 2 N 2,4-DNP a red solid H + ethanol C R R' =NN- H NO 2 NO 2 a 2,4-dinitrophenylhydrazone yellow to red solids
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Aldehydes After an unknown compound is identified as an aldehyde or ketone by its reaction with 2,4-DNP (a "positive test" ), it could be characterized further by the Tollens' reagent , which oxidizes aldehydes to carboxylates. This reagent is a solution of silver nitrate (Ag + is an oxidizing agent) in aqueous ammonium hydroxide. If a few drops of the unknown
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