Alkene Classification Tests

Two common types of unsaturated compounds are alkenes and alkynes, characterized by the carbon-carbon double and triple bond, respectively, as the functional group. There are no simple direct ways to prepare solid derivatives of unsaturated aliphatic compounds having no other functional groups. Two common qualitative tests for unsaturation are the reaction of the compounds with bromine in carbon tetrachloride and with potassium permanganate. In both cases a positive test is denoted by decolorization of the reagent.

A. Bromine in Carbon Tetrachloride Bromine will add to the carbon-carbon double bond of alkenes to produce dibromoalkanes and with alkynes to produce tetrabromoalkanes. When this reaction occurs, molecular bromine is consumed, and its characteristic dark redbrown color disappears if bromine is not added in excess. The rapid disappearance of the bromine color is a positive test for unsaturation.

The test is not unequivocal, however, because some alkenes do not react with bromine, and some react very slowly. In the case of a negative test, therefore, the potassium permanganate test should be performed.

EXPERIMENTAL

To three test tubes, add 20 drops of: a) cyclohexane

b) cyclohexene

c) your unknown

Add 0.1M Bromine in CCl4 dropwise to the cyclohexane, until a red-brown color persists for 30 seconds.

Add 0.1M Bromine in CCl4 dropwise to the cyclohexene, until a red-brown color persists,

OR until you have added ten (10) drops more than you added to the cyclohexane.

Add 0.1M Bromine in CCl4 dropwise to your product, until a red-brown color persists

OR until you have added ten (10) drops more than you added to the cyclohexane.

Dispose of the contents of the test tubes into the “Halogenated Organic Solvent” Bottle

Failure to do so will result in a 2 point deduction from your score

B. Potassium Permanganate (the Baeyer Test) A second qualitative test for unsaturation, the Baeyer test, depends on the ability of potassium permanganate to oxidize the carboncarbon double bond to give alkanediols or the carbon-carbon triple bond to give carboxylic acids

The permanganate is destroyed in the reaction, and a brown precipitate of MnO2 is produced. The disappearance of the characteristic color of the permanganate ion is a positive test for unsaturation. However, care must be taken, since compounds containing certain other types of functional groups (for example, aldehydes, containing the --CH=O group) also decolorize permanganate ion.

EXPERIMENTAL:

In a test tube, dissolve 1 or 2 drops of cyclohexane in 20 drops 95% ethanol, then add 0.1M

potassium permanganate solution drop-wise, swirling the test tube from time to time, and

observing the results. Count the number of drops added before the purple permanganate

color persists. (To test for permanganate color; with your stirring rod, place a drop of

solution from your test tube onto a piece of filter paper. The appearance of a purple ring

around the outer edge of your spot indicates the presence of permanganate.) Repeat this

process with cyclohexene, and your unknown. A significant difference in the number of

drops used in your cyclohexane test versus the number used in your cyclohexene and

unknown tests constitutes a positive test for un-saturation. You do not have to reach the permanganate color end point with cyclohexene or your product, just be sure that ten (10) more drops were added to these tubes than was added to the cyclohexane tube.

Dispose of the contents of the test tubes into the “Recovered Organic Solvents” bottle.

Failure to do so will result in a 2 point deduction from your score

Che 301 Su’2003

Lab Report

Title: Alkene Tests

Name:

Date:

Test: Bromine in CCl4

Equation:

What indicates a positive test:

Known Compounds / Results / Observations
Cyclohexane
Cyclohexene
Products / Results / Observations
Cyclohexene

Conclusions:

Your Cyclohexene behaved similarly to which known(s) ?

Test: 0.1M KMnO4

Equation:

What indicates a positive test:

Known Compounds / Results / Observations
Cyclohexane
Cyclohexene
Products / Results / Observations
Cyclohexene

Conclusions:

Your Cyclohexene behaved similarly to which known(s) ?