Todd:Chem3x11 Example Answer

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An Example 3x11 Worked Answer

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This question was on the 2011 paper:

Question from the 3x11 Exam Paper
Question from the 3x11 Exam Paper

You are required to "Predict the product(s) formed ... and explain fully the reaction mechanism (where stereochemical issues are involved you will need to indicate the precise stereochemical outcome)" for 10 marks.

Remember in exam questions to state the obvious, before giving any complex explanations. Therefore the first thing to do is actually draw the 2D outcome.

Draw the Arrows and Reaction Outcome
Draw the Arrows and Reaction Outcome

Notice there there are stereochemical things we have to address here, but get the product connectivity and arrows established first. We have spent some time describing the orbital interactions behind pericyclic reactions, so let's deal with that.

Draw the Orbital Interactions
Draw the Orbital Interactions

Once we've shown how the orbitals interact, we ought to explain the stereochemical outcome. We should show that the product formed is the endo product. We need to point out that the H's are cis in such a product. If we are careful in drawing our figure we can show that the methyls on the ends of the diene end up also cis, but anti to the H's - as shown. (The product is racemic - the stereochemistry shown is relative, not absolute). We didn't cover this fully in lectures, but it's another very useful feature of the DA reaction. You could say that though the endo product is formed, that is a kinetic outcome, and given the reversibility of everything we should eventually see the thermodynamic exo product if we push the reaction enough.

Draw the Effect of the Orbital Interactions in the Stereochemistry of the Product
Draw the Effect of the Orbital Interactions in the Stereochemistry of the Product

10 marks. Should take you 10 minutes.

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