HT3ans - 2004 - Answers to Hour Examination #3, Chemistry...

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Answers to Hour Examination #3, Chemistry 301-301A - 2004 1. (a) There are two possible regiochemistries, each of which can, in principle, give cis or trans products. H 3 C H Br 2 CH 3 OH H 3 C H OR Br H 3 C H Br OR 1 possible regiochemistries - no stereochemistry implied (yet) Here are the stereochemical possibities: H 3 C H RO Br H 3 C RO H Br H 3 C H OR Br H 3 C H Br OR H 3 C H Br RO H 3 C Br H RO = = (b) As the starting materials are achiral, there can be no optical activity in the product. (c) The first step is formation of a bromonium ion. Opening of that ion by methanol is an S N 2 reaction, thus assuring overall trans addition. H 3 C H Br 2 CH 3 OH H 3 C H Br 2 CH 3 OH ROH H 3 C H Br H 3 C H Br ROH ROH H Br RO H H H 3 C H H 3 C RO Br H H 3 C RO Br Br RO H H 3 C 1 1 trans addition trans addition
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What about the regiochemistry? In fact, the major product is the one in which the RO group winds up on the more substituted carbon of the former alkene. The ‘onium ion is unsymmetric in this case and most of the charge on carbon is on the tertiary carbon,
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HT3ans - 2004 - Answers to Hour Examination #3, Chemistry...

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