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's take a look at this question first:

strong oxidation of alkenes worked example question

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.

strong oxidation of alkenes worked example summary

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

strong oxidation of alkenes worked example deduce fragments of alkenes from products

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.

strong oxidation of alkenes worked example 2 possible scenarios

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.

strong oxidation of alkenes worked example deducing missing fragment CH2

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.

strong oxidation of alkenes worked example putting missing piece into puzzle

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.

strong oxidation of alkenes worked example consequence of asymmetrical fragment 1

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.

strong oxidation of alkenes worked example 4 possible answers schematic

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.

strong oxidation of alkenes worked example 4 possible answers final

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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