Strong Oxidation of Alkene - Worked Example

In this video we want to discuss an interesting question that I have received from some of my students.

Let Chemistry Guru, Singapore's top JC Chemistry tuition centre, guide you through this question first:

We know the reaction is strong oxidation of alkenes since the question already mentioned the presence of multiple C=C bonds and reagents and conditions given is hot acidified KMnO₄.

So what we need to do is to deduce the alkene fragments that will give these products on oxidation, and piece them together to determine the original hydrocarbon C.

Let's do a recap on oxidative cleavage of alkenes.

From this summary we can determine the 3 fragments that are part of hydrocarbon C.

Fragment 2 must be the terminal fragment as it contains only 1 alkene, while fragments 1 and 3 must be bridging fragments (in the middle of the carbon chain) as they contain 2 alkenes each.

So now we have 2 possible scenarios, but we run into the problem of a missing fragment.

The missing fragment is accounted for by a CH₂= group. As the CH₂= group is oxidised to CO₂ and H₂O, it will not be detected as an organic product.

Sometimes questions will leave this observation out to make things a little more challenging.

We have now accounted for the missing fragment and there are 2 possible outcomes.

So there will be a total of 2 possible answers, right?

Unfortunately it gets a little bit more complicated than that, as fragment 1 is not symmetrical.

This means for each arrangement there will be 2 possible permutations depending on how we flip fragment 1, either AB or BA arrangement.

Finally we have all the concepts accounted for and the 4 possible answers are as shown.

We can draw out these 4 possible answers for hydrocarbon C, but to answer this question we just need to draw the structure of any one of them.

For the detailed step-by-step discussion on how to determine hydrocarbon C, check out this video!

Topic: Alkenes, Organic Chemistry, A Level Chemistry, Singapore

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