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Experiment 22

Title:
Reactions of aldehydes and ketones

Aim:

The purpose of this experiment is to compare some reactions of ethanal and propanone.

Introduction:
We have chosen ethanal and propanone as relatively safe examples of aldehydes and ketones to illustrate their reactions in simple test-tube experiments. The reactions or properties to be investigated are as follows:

A. Condensation (addition-elimination)
B. Oxidation
C. Triiodomethane (iodoform) reaction

Requirements:

Safety spectacles and gloves
6 test-tubes
ethanal, CH3CHO
propanone, CH3COCH3
2,4-dinitrophenylhydrazine solution, C6H3(NO2) 2NHNH2
sodium hydroxide solution, 2 M NaOH
Bunsen burner, tripod, gauze and bench mat
Beaker, 250 cm3
Potassium dichromate(VI) solution, 0.1 M K2Cr2O7
Sulphuric acid, dilute, 1 M H2SO4
Fehling’s solutions 1 and 2
Silver nitrate solution, 0.05 M AgNO3
Ammonia solution, 2 M NH3
Iodine solution, 10% (in KI (aq))

Procedure:
Part A

Condensation reaction with 2,4-dinitrophenylhydrazine

1. Put 1-2 drops of ethanal in a test-tube and add about 2 cm3 of 2,4-dinitrophenylhydrazine solution. Note your observations. 2. Repeat for propanone.

Part B

Oxidation reactions

A.With acidified potassium dichromate (VI)
3. Into a test-tube, put 5 drops of ethanal, 2 drops of potassium dichromate (VI) solution and 10 drops of dilute sulphuric acid. 4. Shake the tube gently and warm in a beaker of hot water. Note your observations. 5. Repeat for propanone.

B.With Fehling’s solution
6. Into a test-tube, put about 1 cm3 of Fehling’s solution 1 and then add Fehling’s solution 2 dropwise until the precipitate just dissolves. 7. Add about 7 drops of ethanal. Shake the tube gently and place in a beaker of boiling water for 5-10 minuters – until no further colour change occurs. Note your observations. 8. Repeat for propanone.

C. With Tollens reagent

9. Put about 1 cm3 of 0.05 M AgNO3 into a very clean test-tube and add 3-4...
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