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Chem 102 1st Edition Lecture 9Outline of Last Lecture I. Chapter 13: Section 13.1: Lessons 2.1-2.4Outline of Current Lecture II. Lesson 2.5 - Constitutional Isomerism in AlkenesIII.Lesson 2.6 - Cis-Trans Isomerism in Alkenes (Stereoisomerism)IV. Lesson 2.7 - Naturally Occurring AlkenesV. Lesson 2.8 - Physical Properties of Alkenes and CycloalkenesVI. Lesson 2.9 - Chemical Reactions of AlkenesCurrent Lecture2.5 Constitutional Isomerism in AlkenesIsomers – same molecular formula but different structural formula • Constitutional isomerism - Same molecular formula but different structural formulas a.) Positional isomers - same carbon chain arrangement but the position of the C=C is different 1-butene and 2-butene a.) Skeletal isomers - different carbon chain arrangements 1-butene and 2-methylpropene(1-Butene) C = C – C – C /\/ |(2-Mathylpropene) C – C = C //\ | C2.6 Cis-Trans Isomerism in Alkenes (Stereoisomerism)• Stereoisomerism: same connectivity but different in spatial arrangements of groups • Cis–Trans isomerism in alkenes results from the structural rigidity associated with C=C double bonds. • Rotation about C=C (or CΞC) is not possible unlike C-C.Cis-Trans Isomers – Conditions: Each carbon atom in the double bond contain two different groups attached to it. The two carbon atoms of the double bond must have one common group – Cis isomer: Two similar groups on same side of double bond – Trans isomer: Similar groups on opposite sides of the double bond- cis (identical or analogous substituents on same side)- trans (identical or analogous substituents on opposite sides)1-pentene and 3-pentene are examples of? a.) Positional isomers b.) Positional skeletal isomers c.) Skeletal isomers d.) Both (a) and (b) 2.7 Naturally Occurring AlkenesPheromones • A pheromone is a compound used by insects (and some animals) to transmit a message to other members of the same species. • Pheromones are often alkenes or alkene derivatives. • The biological activity of pheromones is C=C bonds present in a cis or a trans configuration. • Example: The sexual attractant of the female silkworm is a 16-carbon alkene derivative containing an -OH group.Terpenes • Organic compounds whose carbon skeleton is composed of two or more 5-carbon isoprene structural units • Isoprene (2-methyl-1,3-butadiene) is a five-carbon diene • Terpenes are widely distributed in nature• >22,000 Terpenes are found in biological systems and are responsible for the characteristic odors of many trees and plant fragrances2.8 Physical Properties of Alkenes and Cycloalkenes• Physical properties are similar to alkanes • The boiling and melting point of an alkene is usually lower than that of an alkane with same number of carbon atoms • Alkenes with 2 to 4 carbon atoms are gases at room temperature • Unsubstituted alkenes with 5 to 17 carbon atoms are liquids • Alkenes with >17 C atoms are solids2.9 Chemical Reactions of AlkenesChemical Properties • Combustion: Alkenes are very flammable and the combustion products are carbon dioxide and water. • Addition Reactions: Atoms or group of atoms are added to each carbon atom of a multiple bond.Symmetric Addition Reactions of alkenes • Hydrogenation reaction: In alkene hydrogenation a hydrogen atom is added to each carbon atom of a double bond. – Catalyst (usually Ni or Pt) • Halogenation reaction: In alkene halogenation a halogen atom is added to each carbon atom of a double bond. – No catalyst is neededPracticeWhat three alkenes yield isobutane on catalytic hydrogenation?\ / \ \ // \ \ / \\ || | |(2-Methyl 1-Butene) – (2-Methyl 2-Butene) – (3-Methyl 1-Butene)\/\ |Unsymmetrical Addition Reactions of alkenes • Markovnikov’s Rule: When an unsymmetrical molecule HQ (e.g., H-Cl, H-OH, H-F) reacts with an unsymmetrical alkene, the hydrogen atom from the HQ becomes attached to the unsaturated carbon atom that already has the most hydrogen atoms • Two types of unsymmetrical addition reactions: – Hydration: Addition of Water • H2SO4 is used as a catalyst. – Hydrohalogenation • Addition of H-X (X = F, Cl, Br, I)HCl0® C ClCatalytic hydration (Follows Markovnikov's Rule)50% H2SO450% H2O OH2.10 Polymerization of Alkenes: Addition Polymers• Polymers are macromolecules in which small units (monomers) are repeated again and again. • Examples: Polyethylene, polypropylene, polyester etc. • Polymers are huge molecules so their molecular formulas are not written.• The simplest repeating unit in parenthesis with “n” as subscript to show the number of monomers.Alkene Polymerization• Alkenes and substituted alkenes form addition polymers. • An addition polymer is a polymer in which the monomers simply “add together” with no other products formed besides the polymer.• Addition polymerization is similar to the addition reactions described previously except that there is noreactant other than the alkene or substituted alkene. • Example: Formation of polyethyleneButadiene-Based Addition Polymers • Polymerization of 1,3-butadiene results in a butadiene polymer • These polymers contain double bonds (unsaturated polymers) • The polymers are flexibleAddition Copolymers • A copolymer is a polymer in which two different monomers are present. • Examples: – Saran Wrap: a polymer chloroethene (vinyl chloride) and 1,1-dichloroethene. – Styrene–butadiene rubber: Leading synthetic rubber - contains 1,3-butadiene and styrene in a 3:1 ratio(major ingredient in automobile


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