A ketone is a carbonyl with two alkyl (or aryl) groups attached. An aldehyde has the same structure as a ketone, except one of the side groups is a hydrogen.
Since the carbonyl carbon is an electrophile, the typical reaction undergone by an aldehyde or ketone is a nucleophilic addition.
The DNPH test is used to indicate the presence of an aldehyde or ketone. Additionally, the DNPH test can distinguish aldehydes and ketones from alcohols or esters, as the latter compounds will not react with DNPH.
Under basic conditions, a methyl ketone reacts with excess halogen. The three methyl hydrogens are substituted by the halogen, which then leave the molecule and form a precipitate. The test gives a negative result for any other ketone or aldehyde, as it requires three hydrogens in the neighboring carbon to proceed.
The Tollens’ test distinguishes between aldehydes and ketones. Tollens’ reagent oxidizes an aldehyde to a carboxylic acid. The silver ion in the Tollens’ reagent is then reduced to solid silver, which coats the walls of the test tube. Ketones do not react with Tollens’ reagent.
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