ANALYSIS OF CARBONYL COMPOUNDS (ALDEHYDES AND KETONES)

• The characteristic reaction of aldehydes and ketones is nucleophilic addition at the partial positive carbonyl carbon. Typical nucleophilic agents are phenylhydrazine, sodium bisulfite, alcohols, and others.
• 2,4-DNPH test

  The carbonyl groups of aldehydes and ketones react with nucleophiles that contain nitrogen such as 2,4-dinitrophenylhydrazine (DNPH). This reaction gives a precipitate of 2,4-dinitrophenylhydrazone which may either be yellow or red-orange depending on the structure of the carbonyl compound.
• Bisulfite test

  Sodium bisulfite adds to aldehydes and methyl ketones to form addition products. The reaction is carried out by mixing the aldehyde or ketone with a concentrated aqueous solution of sodium bisulfite, the product separates as a crystalline solid.
• Aldehydes are more easily oxidized than ketones. Examples of oxidizing agents employed in this experiment are Tollen’s reagent and Fehling’s reagent and Schiff's reagent.
• Tollen’s test

  Tollen’s reagent is an alkaline solution of silver ammonia complex ion. When Tollen’s reagent is added to a test tube that contains an aldehyde, the aldehyde is oxidized and the metallic silver is deposited as mirror on the wall of the test tube. Ketones, except for a-hydroxy ketones, are not oxidized by Tollen's reagent.
• Fehling's test

  Fehling’s solution is a basic solution of cupric ion, Cu+2, complexed with tartrate ion. It oxidizes aldehydes to carboxylic acids as the cupric ion reduced to cuprous oxide. Fehling's solution has the characteristic blue color of Cu+2 which fades as the red precipitate of cuprous oxide forms.
• Another reagent that reacts more readily with aldehydes is Schiff's reagent. Schiff's reagent (leucosulfonic acid) is a colorless solution made by mixing the fuschia colored fuchsin dye and sulfurous acid. When treated with aldehyde, Schiff's reagent is converted to a magenta colored quinoid dye.
• A test that can distinguish methyl ketones from other ketones is the iodoform. The methyl ketone is treated with iodine in a basic solution to give a water insoluble yellow colored iodoform. The test is also positive for compounds that are easily oxidized to methyl ketones, such as methyl carbinols and ethanol.
• APPARATUS

  Test tubes, droppers, burner, acetaldehyde, 450 mL, beaker, thermometer, 25 mL graduated cylinder
• 2,4-DNPH


  Can cause genetic mutations, harmful if swallowed, inhaled or absorbed through the skin
• If you come in contact with 2,4-DNPH solution

  Immediately wash with soap and copious amounts of water for at least 15 minutes
• Tollen's reagent


  Avoid spilling on skin, in case of contact immediately wash affected area with large amounts of water
• Acetone and ethanol


  Highly volatile and flammable solvents, do not have any flames in the vicinity when they are being used
• Iodine


  Corrosive and toxic, do not allow it to come in contact with skin and avoid inhaling its vapor
• SAFETY PRECAUTION
• 1. 2,4-DNPH test

  a.Place 5 drops each of acetaldehyde, benzaldehyde, and acetone separate test tubes.

  b. Add 5 drops of 2,4-DNPH in solution to each. Heat in a water bath for 5 minutes. Record your observations.
• 2. Bisulfite test

  a.In a test tube, place 20 drops of saturated NaHSO3 solution and 10 drops of acetaldehyde.

  b. Shake the contents thoroughly and observe the result.

  c. Repeat procedures (a) benzaldehyde and acetone. and (b) using the following compounds:
• 3. Tollen's Test

  a.In three separate test tubes, place 5 drops each of acetaldehyde, benzaldehyde, and acetone.

  b. Add 5 drops of freshly prepared Tollen’s reagent into each test tube.

  c. Mix well and allow to stand for 5 minutes. If no change is observed, place the tubes in a warm water bath for 5 minutes. Record your observations.
• 4. Fehling’s test

  a.Place 5 drops each of the following substances in separate test tubes: acetaldehyde, acetone, benzaldehyde

  b. To each test tube, add 10 drops each of Fehling’s A and B reagents.

  c. Mix well and place the test tubes in a boiling water bath.

  d. Record the time required for any color change to occur.
• 5. Schiff’s test

  a.Place 5 drops each of acetaldehyde, benzaldehyde, and acetone In separate test tubes.

  b. Add 10 drops of Schiff's reagent to each. Record your observations.
• 6. lodoform test for Methyl Ketones

  a.Place 5 drops each of acetaldehyde, acetone, and ethanol in separate test tubes.

  b. Add 1 mL of 10% NaOH to each tube. Then add I2 in Kl solution drop by drop until the brown color of iodine persists. Heat the test tubes in a water bath at 60°C for 2 minutes.

  c. If the NaOH brown color of iodine still persists after heating, add more 10% NaOH dropwise until the brown color disappears and a yellow color remains.

  d. Add 2 mL of water to each tube and shake each tube vigorously. Allow the tubes to stand at room temperature. Record the results.
• CLEANING UP

  1. Remove any precipitated cuprous oxide in the Fehling's test by filtration and discard it in the solid waste disposal container. Flush filtrate down the drain with excess water.
  2. Remove any precipitated iodoform by filtration and place it in the solid waste disposal container. Place the residual aqueous solutions the container for halogenated liquids.
  3. Remove the Ag mirror from the test tube with nitric acid and the resulting solution can be disposed of by washing down the sink with water.
• REAGENTS

  2,4-DNPH, acetaldehyde, benzaldehyde, acetone, saturated NaHSOs,

  1% AgNO3, 10% NaOH, 6M NH4OH, Fehling's reagent, Schiff's reagent, I2 in Kl (Lugol’s solution), ethanol