MWF_20 - 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 Today's Topics: Alkenes-addition Rxns LECTURE 20 Notice & Announcements: HW 5 posted 1 Organic Lecture Series Reactions of Alkenes Chapter 6 2 Organic Lecture Series Oxymercuration/Reduction Oxymercuration followed by reduction results in hydration of a carbon-carbon double bond. 3 Organic Lecture Series Oxymercuration/Reduction An important feature of oxymercuration/reduction is that it occurs without rearrangement. Oxymercuration occurs with anti stereoselectivity. 4 Organic Lecture Series Hydroboration/Oxidation Hydroboration: The addition of borane, BH3, to an alkene to form a trialkylborane. Borane dimerizes to diborane, B2H6. Not exam material 5 Organic Lecture Series Hydroboration/Oxidation Hydroboration is both regioselective (boron bonds to the less hindered carbon) and syn stereoselective. 6 Organic Lecture Series Hydroboration/Oxidation Concerted regioselective and syn stereoselective addition of B and H to the carbon-carbon double bond. Trialkylboranes are rarely isolated. Oxidation with alkaline hydrogen peroxide gives an alcohol and sodium borate. 7 Organic Lecture Series Oxidation/Reduction Oxidation: The loss of electrons. Alternatively, the loss of H, the gain of O, or both. Reduction: The gain of electrons. Alternatively, the gain of H, the loss of O, or both. 8 Organic Lecture Series Oxidation with OsO4 OsO4 oxidizes an alkene to a glycol, a compound with OH groups on adjacent carbons. Oxidation is syn stereoselective. 9 Organic Lecture Series Oxidation with OsO4 OsO4 is both expensive and highly toxic. It is used in catalytic amounts with another oxidizing agent to reoxidize its reduced forms and, thus, recycle OsO4. Two commonly used oxidizing agents are 10 Organic Lecture Series Oxidation with O3 Treatment of an alkene with ozone followed by a weak reducing agent cleaves the C=C and forms two carbonyl groups in its place. In the following example, the weak reducing agent is dimethylsulfide, (CH3)2S. 11 Organic Lecture Series Oxidation with O3 The initial product is a molozonide which rearranges to an isomeric ozonide. 12 Organic Lecture Series Reduction of Alkenes Most alkenes react with H2 in the presence of a transition metal catalyst to give alkanes. Commonly used catalysts are Pt, Pd, Ru, and Ni. The process is called catalytic reduction or, alternatively, catalytic hydrogenation. Addition occurs with syn stereoselectivity. 13 Organic Lecture Series Reduction of Alkenes Mechanism of catalytic hydrogenation. 14 Organic Lecture Series Ho of Hydrogenation 15 Organic Lecture Series Ho of Hydrogenation Reduction of an alkene to an alkane is exothermic. There is net conversion of one pi bond to two sigma bond. Ho depends on the degree of substitution of the carbon atoms of the double bond. The greater the substitution, the lower the value of Hi.e. greater substitution is more stable Ho for a trans alkene is lower than that of an isomeric cis alkene. A trans alkene is more stable than a cis alkene. 16 ...
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