Question

Hello I have a question about organic chemistry 1 lab.

When doing chemical tests the Br2 in CCl4. Shows a positive result for alkenes and alkynes? And negative test results mean a alkene and/or arene? I’m told in lab that we do not have potassium permanganate so we cannot use it. The second test that were told to preform if nothing else is shown on the IR is a halide test.

I hope this makes sense. I’m just confused about what the Br2 in CCl4 test show. Because despite the IR showing the functional groups we have to do a chemical tests. If the test shows the possibility for two functional groups, as in a alkene or alkynes, how do I know the difference between those two.

Thank you!

Answer 1

First of all reaction of alkene or alkynes with Br2/CCl4 is electrohilic addition. Decolorization of Br2 indicates that the compound has unsaturation like doubkd bonds and triple bonds.

And we know that benzene undergo Electrohilic substitution rather than addition.

If we trrat Br2 with benzene it gives 1,2-dibromocyclohexadiene. Benzene ring lost aromaticity

Hence this reaction is not feasible eventhough benzene has double bonds.

And by IR we can differentiate alkyne and alkene by observing the peaks at 2150 for carbon triple bond and at 1625 for Carbon double bond stretches respectively.

While for benzene , peak is at 1600 for C=C