TTh_13 - Organic Lecture Series CH 310/318 M Textbook...

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Unformatted text preview: Organic Lecture Series CH 310/318 M Textbook Assignment: Chapter 6 LECTURE 13 Today's Topics: Alkene- addition reactions-cont'd Notice & Announcements: HW 5 posted 1 Organic Lecture Series Reactions of Alkenes Chapter 6 2 Organic Lecture Series Addition of H2O Addition of water is called hydration. Acid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the less substituted carbon of the double bond (to the carbon bearing the greater number of hydrogens). HOH adds in accordance with Markovnikov's rule. 3 Organic Lecture Series Addition of H2O Step 1: Proton transfer from H3O+ to the alkene. Step 2: Reaction of the carbocation (an electrophile) with water (a nucleophile) gives an oxonium ion. Step 3: Proton transfer to water gives the alcohol. 4 Organic Lecture Series Additions of Cl2 and Br2 Carried out with either the pure reagents or in an inert solvent such as CH2Cl2. Addition of bromine or chlorine to a cycloalkene gives a trans-dihalocycloalkane as a racemic mixture. Addition occurs with anti stereoselectivity; halogen atoms add from the opposite face of the double bond 5 Organic Lecture Series Additions of Cl2 and Br2 Step 1: Formation of a bridged bromonium ion intermediate. 6 Organic Lecture Series Additions of Cl2 and Br2 Step 2: Attack of halide ion (a nucleophile) from the opposite side of the bromonium ion (an electrophile) opens the three-membered ring to give the product. 7 Organic Lecture Series Additions of HOCl and HOBr Treatment of an alkene with Br2 or Cl2 in water forms a halohydrin. Halohydrin: A compound containing -OH and -X on adjacent carbons. 8 Organic Lecture Series Additions of HOCl and HOBr Reaction is both regioselective (OH adds to the more substituted carbon) and anti stereoselective. Both selectivities are illustrated by the addition of HOBr to 1-methylcyclopentene. To account for the regioselectivity and the anti stereoselectivity, chemists propose a three-step mechanism. 9 Organic Lecture Series Additions of HOCl and HOBr Step 1: Formation of a bridged halonium ion intermediate 10 Organic Lecture Series Additions of HOCl and HOBr Step 2: Attack of H2O on the more substituted carbon opens the three-membered ring. 11 Organic Lecture Series Additions of HOCl and HOBr Step 3: Proton transfer to H2O completes the reaction 12 Organic Lecture Series Organic Lecture Series Oxymercuration/Reduction Oxymercuration followed by reduction results in hydration of a carbon-carbon double bond. Reduction 13 14 Oxidation Organic Lecture Series Oxymercuration/Reduction An important feature of oxymercuration/reduction is that it occurs without rearrangement. Oxymercuration occurs with anti stereoselectivity. 15 ...
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This note was uploaded on 03/18/2012 for the course CH 310 M taught by Professor Iverson during the Spring '09 term at University of Texas at Austin.

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