MWF_21 - 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 7 LECTURE 21 Today's Topics: Alkenes-reduction; Alkynes: intro Notice & Announcements: HW 6 posted 1 Organic Lecture Series Reactions of Alkenes Chapter 6 2 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. 3 Organic Lecture Series Oxidation with O3 The initial product is a molozonide which rearranges to an isomeric ozonide. 4 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. 5 Organic Lecture Series Reduction of Alkenes Mechanism of catalytic hydrogenation. 6 Organic Lecture Series Ho of Hydrogenation 7 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. 8 Organic Lecture Series Alkynes Chapter 7 9 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". 10 Organic Lecture Series Physical Properties Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton. 11 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. 12 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. 13 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. 14 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. 15 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: 16 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. 17 ...
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