Tests for Unsaturation

Bromine Test for Multiple Bonds

Baeyer Test for Multiple Bonds

Ignition Test for High Degrees of Unsaturation

Bromine (in Methylene Chloride) Test for Multiple Bonds

Alkene

Alkyne

Standards
Cyclohexane, Cyclohexene, and Bromobenzene

Procedure
In a hood, 0.02 g or 1 drop of the unknown is added to 0.5 mL of methylene chloride. Add a dilute solution of bromine in methylene chloride dropwise, with shaking, until the bromine color persists. The bromine solution must be fresh.

Positive Test
Discharging of the bromine color without the evolution of hydrogen bromide gas is a positive test.

Complications

• Should be employed in conjunction with Baeyer test (dilute KMnO₄).
• Electron-withdrawing groups in the vinylic position can slow down bromine addition to the point that a negative test is erroneously produced.
• Tertiary amines (like pyridine) form perbromides upon treatment with bromine and lead to false positive tests.
• Aliphatic and aromatic amines can discharge the bromine color without the evolution of HBr gas.

Cleaning up
The solution should be placed in the appropriate waste container.

Baeyer Test for Multiple Bonds (Potassium Permanganate Solution)

Alkene

Alkyne

Standards
Cyclohexane, Cyclohexene and Bromobenzene.

Procedure
Dissolve 1 drop or 0.02 grams of the unknown in 0.5 mL reagent grade acetone. Add a 1% aqueous solution of potassium permanganate dropwise with shaking. If more than one drop of reagent is required to give a purple color to the solution, unsaturation or an easily oxidized functional group is present. Run parallel tests on pure acetone and, as usual, the standards listed above.

Positive Test
The disappearance of the KMnO₄'s purple color and the appearance of a brown suspension of MnO₂ is a positive test.

Complications

• Water insoluble compounds should be dissolved in ethanol, methanol, or acetone.
• Often, the brown precipitate fails to form and the solution turns reddish-brown.
• Easily oxidized compounds give a positive test: a) Most aldehydes give a positive test. b) Formic acid and its esters give a positive test.
• Alcohols with trace impurities give a positive test.
• Phenols and aryl amines give a positive test.
• Carbonyl compounds which decolorize bromine/methylene chloride usually give a negative test.

Cleaning up
The solution should be diluted with water and flushed down the drain. Any solutions containing organic compound should be placed in the appropriate waste container.

Ignition Test for High Degrees of Unsaturation

Standards
Benzophenone, cyclohexane, and hexanes.

Procedure
Heat a small sample on a spatula. First, hold the sample near the side of a bunsen burner to see if it melts normally and then burns. Heat it in the flame. Aromatic compounds often burn with a smoky flame.

Positive Test
A sooty yellow flame is an indication of an aromatic ring or other centers of unsaturation.