AROMATIC ALDEHYDES AND KETONES

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					2005


AROMATIC ALDEHYDES AND KETONES

(A)       Reactions of benzaldehyde

      1. The Silver Mirror Test
         To 1cm3 of a solution of silver nitrate, add 1 drop of dilute sodium hydroxide
         solution to form a precipitate of silver oxide. Add 2M ammonia solution, drop by
         drop, until the brown precipitate just redissolves. [This solution is known as
         Tollen’s reagent]. Pour the ammoniacal solution into a clean test-tube. [The test-
         tube should have been thoroughly washed with water, then with propanone, and
         then dried]. Add 3 drops of benzaldehyde and warm the test-tube in a beaker of
         boiling water.

      2. Reaction with Fehling’s Solution
         Fehlings’s solution is freshly prepared by adding solution B to 1cm3 of solution A
         until the blue precipitate just redissolves to give a deep blue solution. Add 3 drops
         of benzaldehyde to the solution and boil the mixture for a few minutes.

      3. Schiff’s Reagent
         Place about 3 drops of benzaldehyde in a test-tube and add 2 drops of Schiff’s
         reagent.

      4. To 1cm3 of M sulphuric acid, add 2 drops of dilute potassium manganate(VIII)
         solution. Add 2 or 3 drops of benzaldehyde. Warm gently. Note the colour
         changes.

      5. Repeat experiment 4 using acidified potassium dichromate(VI) solution in place
         of potassium manganate(VII) solution. Again note any colour changes.

      6. To 5 drops of 2, 4-dinitrophenylhydrazine solution (Brady’s Reagent) in a test
         tube, add 3 drops of benzaldehyde.

      7. To 3 drops of benzaldeyde in a test tube, add (CARE) concentrated sodium
         hydroxide solution drop by drop. Explain why benzaldehyde undergoes the
         Cannizzaro Reaction rather than the Aldol Reaction.

      8. Explain why benzaldehyde will not undergo the iodoform/haloform reaction.

(B)       Reactions of phenylethanone (acetophenone)

      Repeat experiments A1 to A6, using phenylethanone in place of benzaldehyde.

      7. To 5 drops of phenylethanone in a test tube, add 5 drops of iodine (in potassium
        iodide solution) and then 2M sodium hydroxide solution dropwise until the brown
        colour is just discharged. Warm if necessary. Note your observations. Comment on
        the smell.



HB Chem Reactions of benzaldehyde