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week 1 - 5 - anno - Chemistry S-20ab Chemistry E-2a Lecture...

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Test Yourself Now! Can you write a curved-arrow mechanism and predict the product of the following reaction? + CH 3 OH H + cat. Chemistry S-20ab Week 1 62
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Stereochemistry of Alkene Additions Consider adding HBr to a cyclic alkene, like the one shown below. The two new groups that are added (H and Br) could be added in two different ways: syn (both added to the same face of the alkene) or anti (added to opposite faces of the alkene): HBr + H Br H Br syn addition (H and Br add to the same side) anti addition (H and Br add to opposite sides) (Recall that we used the term stereoisomers before to refer to cis and trans alkenes.) Work through the mechanism of HBr addition. Which is preferred: syn or anti ? The addition of H 2 to the same cyclic alkene gives only one product. What type of addition is this, and why is only one product observed? H 2 , Pt H H Reading : Section 7.9 Chemistry S-20ab Week 1 63
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Chemistry S-20ab Week 1 64 Addition of Br 2 or Cl 2 to Alkenes First, let’s predict what we expect to happen when an alkene reacts with Br 2 . How do we make such a prediction? + Br Br What would your proposed mechanism predict about the stereochemistry of the outcome? The actual outcome of the reaction is shown below. What type of addition is this? Is this stereochemical outcome consistent with your proposed mechanism? + Br Br Br Br Reading : Section 5.1
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Chemistry S-20ab Week 1 65 The Mechanism of Br 2 or Cl 2 Addition Is there a mechanism of Br 2 addition that is consistent with the observed stereochemistry? + Br Br Br Br Consider the following alkene additions. Rearrangement is observed in one case but not in the other. Can you explain why? + H Br + Br Br Br Br Br H Reading : Section 5.1
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Addition of X 2 with Water or Alcohols If Br 2 or Cl 2 is added to an alkene in the presence of water (HOH) or an alcohol (ROH), a different product is formed. Can you propose a mechanism for this reaction?
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