How can you identify alkenes?

1 Answer
Jul 14, 2016

Permanganate Oxidation (Von Baeyer test) and the addition of bromine are useful test to identify unsaturation.


Permanganate Oxidation (Von Baeyer test)

Under neutral conditions

# 3"Alkene"+ 2 KMnO_4 + 4 H_2O -> 3 "Glycol" + 2 MnO_2darr+ 2 KOH #

Note that in the course of this reaction under alkaline conditions, the purple color of the aqueous potassium permanganate is changed first to green as a result of the formation of manganate(VI) ions . . . Then the color changes to brown due to the formation of a brown solid which is manganese (IV) oxide (#MnO_2#).

The color change is a positive test for the presence of a double bond (or triple bond). Although this test has limitations, it is a good test to differentiate alkanes and aromatics from alkenes.

Addition of Bromine

The bromine test is another qualitative test for the presence of unsaturation, like the Baeyer's test, it has some limitations. The disappearance of the deep brown coloration of bromine indicates a positive test.

Keep in mind that modern spectroscopic methods ( NMR and IR) are better methods for determining the structural features and identity of unknown compounds.