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s20 2011 week 3 day 5

s20 2011 week 3 day 5 - Chemistry S-20ab Week 3 Chemistry...

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Addition of Water to Alkynes: Enol Formation The following reaction produces an unstable intermediate known as an enol (alk ene + alcoh ol = enol) . Draw a complete curved-arrow mechanism for this reaction up to the formation of the enol. C C H Hg 2+ , H + , H 2 O OH enol (unstable) Reading : Section 14.5 Chemistry S-20ab Week 3 17 ±
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Addition of Water to Alkynes: Enol Hydrolysis The enol is not the final product of the reaction of an alkyne with water. The enol reacts rapidly with the aqueous acid to form a carbonyl compound: in this case, a ketone . Provide a complete curved-arrow mechanism for the following reaction: OH enol (unstable) H + , H 2 O O ketone Show the complete synthetic transformation from an alkyne to a ketone : Reading : Section 14.5 Chemistry S-20ab Week 3 17 ±
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Hydroboration/Oxidation of Alkynes: Enol Formation The following reaction produces an unstable intermediate known as an enol (alk ene + alcoh ol = enol) . Draw a complete curved-arrow mechanism for this reaction up to the formation of the enol. You should also explain why the alkene geometry is trans (that is, you should explain the stereochemistry !) C C H enol (unstable) 1. R 2 BH (R = bulky alkyl group) 2. H 2 O 2 , OH OH Reading : Section 14.5 Chemistry S-20ab Week 3 17 ±
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Hydroboration/Oxidation of Alkynes: Enol Hydrolysis The enol is not the final product of hydroboration/oxidation of an alkyne. The enol reacts rapidly with the aqueous base (in the oxidation step) to form a carbonyl compound: in this case, an aldehyde . Provide a complete curved-arrow mechanism for the following reaction: enol (unstable) OH , H 2 O aldehyde OH H O Show the complete synthetic transformation from an alkyne to an aldehyde : Reading : Section 14.5 Chemistry S-20ab Week 3 1 ± 8
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Hydrogenation of Alkynes: Catalytic Hydrogenation Ordinarily, the reaction of an alkyne with H 2 / Pd adds two equivalents of hydrogen and yields an alkane. Show the intermediate and final product in the following transformation. Be sure to consider the stereochemistry ! C C H 2 Pd an alkene an alkane H 2 Pd However, we can use a poisoned catalyst (for example, a Lindlar catalyst) to halt the reaction at the alkene stage. This is one of the best ways of making a cis -alkene: C C H 2 Lindlar a cis -alkene Reading : Section 14.6 Chemistry S-20ab Week 3 1 ±²
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Hydrogenation of Alkynes: Reduction with Na in NH 3 We can convert an alkyne into a trans -alkene by using a unique reducing agent: sodium metal in liquid ammonia (!). Ammonia is a gas at room temperature; it boils at –33°C.
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