Chem: distinguishing test for alkenes

Audi SG

Is there any DEFINITE test for allkenes besides the Bromine (aq) method which we learnt in O level?

I was thinking of oxidation by cold dilute KMnO4 which forms a diol, is that ok?

UltimaOnline SG

Is there any DEFINITE test for allkenes besides the Bromine (aq) method which we learnt in O level?

I was thinking of oxidation by cold dilute KMnO4 which forms a diol, is that ok?

No, there isn't. You'll have to use a series of tests to distinguish and single out a particular compound from the others.

For instance, Br(aq) will also react (ie. observable colour change) with activated benzene rings, eg. phenol and phenylamine. And cold KMnO4 will also react (ie. observable colour change) with primary and secondary alcohols.

So no, there isn't any single definitive test for alkenes. Moreover, you'll likely encounter (in the exam) molecules with multiple functional groups, more than one of which, may react with any single reagent, eg. KMnO4.

SBS n SMRT SG

actually can, add sodium metal, no effervenscence of H2 gas, add 2,4 DNPH, no orange ppt, add dilute acidified potassium dichromate solution, decolourises with no white ppt.

UltimaOnline SG

actually can, add sodium metal, no effervenscence of H2 gas, add 2,4 DNPH, no orange ppt, add dilute acidified potassium dichromate solution, decolourises with no white ppt.

As I said, no SINGLE definitive test.

Also, K2Cr2O7 does not react with alkenes, and even if it did, you wouldn't get a colourless solution (which is what "decolourizes" implies).

SBS n SMRT SG

oops, should be hot acidified KMnO4.