Projection Experiment: Oxidation of Butanol Isomers

Demonstration of Microscale Projection Experiments - Chemistry en miniature

Oxidation of Butanol Isomers

Distinction between Primary, Secondary and Tertiary Alcohol

Peter Keusch

German version

Chemicals:
1-butanol (d = 0.8098)
2-butanol (d = 0.8080)
tert-butanol (d = 0.7887)
potassium dichromate
conc. sulfuric acid
Test reagent: 60 mL 0.4 N aqueous solution of K₂Cr₂O₇ (1.177 g / 60 mL), 5 mL conc. H₂SO₄

Hazards and safety precautions:

Tert-butanol is harmful if inhaled. Skin and respiratory irritant. Severe eye irritant.

Potassium dichromate: Hexavalent chromium compounds are generally more toxic than trivalent chromium compounds. May be fatal if absorbed through the skin, if swallowed or inhaled. Contains chromium (VI), a known cancer hazard. Allergen. Skin eye and respiratory irritant. May act as a sensitizer.

Conc. sulfuric acid is highly toxic. Causes severe burns. May be fatal if swallowed. May cause cancer through inhalation. Very destructive of mucous membranes.

Safety goggles and protective gloves must be worn. Adequate ventilation required. The reagent solution is prepared in a fume hood!

Experimental procedure:

The temperature of the water in the cuvette is approx. 50°C (water bath).

Three test tubes are each filled with 1 mL of the test reagent. A Pasteur pipette is placed in each of the three test tubes. The solutions layered with
1-butanol, 2-butanol and tert-butanol, respectively, are simultaneously mixed by gentle squeezing of the pipette bulbs.

Test tube 1 1 mL of test reagent    30.8 mL of 1-butanol
Test tube 2 1 mL of test reagent    30.8 mL of 2-butanol
Test tube 3 1 mL of test reagent    30.8 mL of tert-butanol

Results:

The orange color of the dichromate solution in T2 and T3 treated with primary and secondary butanol, respectively, turns olive green and finally green-blue. The orange color of the solution in test tube 3 remains unchanged.

Test tube 1 Test tube 2 Test tube 3
after 1 minute   Photo 1 tawny tawny orange
after 2 minutes   Photo 2 faint green-blue brownish orange
after 3 minutes   Photo 3 green-blue olive green orange

Photo 1
Photo 2

Photo 3

Video clip (Download RealPlayer .rm file )

Discussion:

The most important reaction of alcohols is their oxidation to carbonyl compounds. Primary alcohols yield either aldehydes or carboxylic acids, depending on the conditions used. Secondary alcohols yield ketones - and tertiary alcohols resist oxdation with all known oxidizing agents. Heating of tertiary butanol with strong oxidizers (peroxides) results in the breaking of CC bonds. Decomposition products such as isobutene may be formed.

· 1-butanol is oxidized rapidly, 2-butanol more slowly. Under the given reaction conditions tert-butanol proves to be stable.

· Primary and secondary alcohol reduce orange Cr₂O₇^2- ions to Cr (aq) ³⁺ ions, which appear green-blue.

Waste disposal: Also the dichromate solution in R3 is to be reduced to Cr³⁺. The Cr³⁺-solution is mixed with ammonium chloride. The precipitate - Cr(OH)₃ - is mixed with ammonia. Either filter or let stand for a week to let the precipitate settle out. The supernatent liquid can be washed down the drain with 50 times its volume of water. The precipitate should be washed with hot water, dried, packaged and labelled, and then taken to a toxic waste disposal.

References:
Demonstration Experiment on Video Oxidation of the Butanol Isomers
Redox Chemistry

General experimental instructions and index of experiments