TTh_14 - 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 14 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 Organic Lecture Series Oxymercuration/Reduction Oxymercuration followed by reduction results in hydration of a carbon-carbon double bond. Reduction 3 4 Oxidation Organic Lecture Series Oxymercuration/Reduction An important feature of oxymercuration/reduction is that it occurs without rearrangement. Oxymercuration occurs with anti stereoselectivity. 5 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 6 Organic Lecture Series Hydroboration/Oxidation Hydroboration is both regioselective (boron bonds to the less hindered carbon) and syn stereoselective. 7 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. 8 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. 9 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. 10 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 11 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. 12 Organic Lecture Series Oxidation with O3 The initial product is a molozonide which rearranges to an isomeric ozonide. 13 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. 14 Organic Lecture Series Reduction of Alkenes "Mechanism" of catalytic hydrogenation. 15 Organic Lecture Series Ho of Hydrogenation 16 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. 17 Organic Lecture Series Alkynes Chapter 7 18 Organic Lecture Series Nomenclature IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond. Common names: prefix the substituents on the triple bond to the word "acetylene". 19 Organic Lecture Series Physical Properties Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton. 20 Organic Lecture Series Acidity The pKa of acetylene and terminal alkynes is approximately 25, which makes them stronger acids than ammonia but weaker acids than alcohols (Section 4.1). Terminal alkynes react with sodium amide to form alkyne anions. 21 Organic Lecture Series Acidity Terminal alkynes can also be converted to alkyne anions by reaction with sodium hydride or lithium diisopropylamide (LDA). Because water is a stronger acid than terminal alkynes, hydroxide ion is not a strong enough base to convert a terminal alkyne to an alkyne anion. 22 Organic Lecture Series Alkylation of Alkyne Anions Alkyne anions are both strong bases and good nucleophiles. They participate in nucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylation. Because alkyne anions are also strong bases, alkylation is practical only with methyl and 1 halides With 2& 3 halides, elimination is the major reaction. 23 Organic Lecture Series Alkylation of Alkyne Anions Alkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynes. Alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne. 24 Organic Lecture Series Alkyne Anions as Strong Bases Because alkyne anions are also strong bases, alkylation is practical only with methyl and 1halides, however- With 2 & 3 halides, elimination is the major reaction: 25 Organic Lecture Series Preparation from Alkenes Treatment of a vicinal dibromoalkane with two moles of base, most commonly sodium amide, results in two successive dehydrohalogenation reactions (removal of H and X from adjacent carbons) and formation of an alkyne. 26 Organic Lecture Series Preparation from Alkenes For a terminal alkene to a terminal alkyne, 3 moles of base are required: 27 Organic Lecture Series Addition of X2 Alkynes add one mole of bromine to give a dibromoalkene. Addition shows anti stereoselectivity. 28 ...
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