Add a solution of 1 or 2 drops or 30 mg of unknown in 2 mL of 95% ethanol to 3 mL of 2,4-dinitrophenylhydrazine reagent. Shake vigorously, and, if no precipitate forms immediately, allow the solution to stand for 15 minutes.
2,4-Dinitrophenylhydrazine reagent: Dissolve 3 g of 2,4-Dinitrophenylhydrazine in 15 mL of conc. sulfuric acid. This solution is then added with stirring to 20 mL of water and 70 mL of 95% ethanol. This solution is mixed thoroughly and filtered.
Formation of a precipitate is a positive test.
Some ketones give oils which will not solidify.
Some allylic alcohols are oxidized by the reagent to aldehydes and give a positive test.
Some alcohols, if not purified, may contain aldehyde or ketone impurities.
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