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Tests for Aldehydes and Ketones

2,4-DNP Test for Aldehydes and Ketones

Tollen's Test for Aldehydes

Jones (Chromic Acid) Oxidation Test for Aldehydes

Iodoform Test for Methyl Ketones


2,4-DNP Test for Aldehydes and Ketones

Aldehyde or Ketone

Standards
Cyclohexanone, Benzophenone, and Benzaldehyde

Procedure
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.
The 2,4-dinitrophenylhydrazine reagent will already be prepared for you.

Positive test
Formation of a precipitate is a positive test.

Complications

  • 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.

 

Tollen’s Test for Aldehydes

Aldehyde

Standards
Cyclohexanone and Benzaldehyde

Procedure
Add one drop or a few crystals of unknown to 1 mL of the freshly prepared Tollens reagent. Gentle heating can be employed if no reaction is immediately observed.
Tollens reagent: Into a test tube which has been cleaned with 3M sodium hydroxide, place 2 mL of 0.2 M silver nitrate solution, and add a drop of 3M sodium hydroxide. Add 2.8% ammonia solution, drop by drop, with constant shaking, until almost all of the precipitate of silver oxide dissolves. Don't use more than 3 mL of ammonia. Then dilute the entire solution to a final volume of 10 mL with water.

Positive Test
Formation of silver mirror or a black precipitate is a positive test.

Complications

  • The test tube must be clean and oil-free if a silver mirror is to be observed.
  • Easily oxidized compounds give a positive test. For example: aromatic amine and some phenols.

Cleaning up
Place all solutions used in this experiment in an appropriate waste container.

 

Jones (Chromic Acid) Oxidation Test for Aldehydes

Aldehydes

Standards
Cyclohexanone and Benzaldehyde

Procedure
Dissolve 10 mg or 2 drops of the unknown in 1 mL of pure acetone in a test tube and add to the solution 1 small drop of Jones reagent (chronic acid in sulfuric acid). A positive test is marked by the formation of a green color within 5 seconds upon addition of the orange-yellow reagent to a primary or secondary alcohol. Aldehydes also give a positive test, but tertiary alcohols do not.
The Jones reagent will already be prepared for you.

Positive Test
A positive test for aldehydes and primary or secondary alcohols consists in the production of an opaque suspension with a green to blue color. Tertiary alcohols give no visible reaction within 2 seconds, the solution remaining orange in color. Disregard any changes after 15 seconds.

Complications

  • Aldehydes are better characterized in other ways. The color usually develops in 5-15 seconds.

Cleaning up
Place the test solution in the appropriate waste container.

 

Iodoform Test for Methyl Ketones

Ketone

Standard
Acetone

Procedure
If the substance to be tested is water soluble, dissolve 4 drops of a liquid or an estimated 50 mg of a solid in 2 mL of water in a large test tube. Add 2 mL of 3 M sodium hydroxide and then slowly add 3 mL of the iodine solution. Stopper the test tube and shake vigorously. A positive test will result in the brown color of the reagent disappearing and the yellow iodoform solid precipitating out of solution. If the substance to be tested is insoluble in water, dissolve it in 2 mL of 1,2-dimethoxyethane, proceed as above, and at the end dilute with 10 mL of water.

Positive Test
Formation of solid iodoform (yellow) is a positive test. (Iodoform can be recognized by its odor and yellow color and, more securely, from the melting point 119o-123oC).

Complications
Test will not be positive if the R group is a di-ortho substituted aryl group

Cleaning Up
Place solutions in the appropriate waste container.