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BU CHEM 102 - Unsaturated Hydroarbons
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Chem 102 1st Edition Lecture 8 Outline of Last Lecture I. Chapter 13 Lessons 2.5 – 2.9 : Constitutional Isomerism in Alkenes, Cis-Trans Isomerism in Alkenes (Stereoisomerism), Naturally Occurring Alkenes, Physical Properties of Alkenes and Cycloalkenes, Chemical Reactions of AlkenesOutline of Current Lecture II. 2.10 - Polymerization of Alkenes: Addition PolymersIII. 2.11 - AlkynesIV. 2.12 - Aromatic HydrocarbonsV. 2.13 - Names of Aromatic HydrocarbonsVI. 2.14 - Aromatic Hydrocarbons : Physical properties and sourcesVII. 2.15 - Chemical Reactions of Aromatic HydrocarbonsVIII. 2.16 - Fused Ring Aromatic HydrocarbonsCurrent Lecture2.10 - 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 polyethylene Substituted-Ethene Addition Polymers Butadiene-Based Addition Polymers • Polymerization of 1,3-butadiene results in a butadiene polymer• These polymers contain double bonds (unsaturated polymers) • The polymers are flexible Addition 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 2.11 – Alkynes• Alkyne: Unsaturated hydrocarbon that contains at least one or more carbon–carbon triple bonds. • Alkyne functional group: CΞC group • Naming: Names end with a suffix –yne • General formula: CnH2n-2 • Simplest alkynes: C2H2, C3H4 IUPAC Nomenclature for Alkynes • The rules for naming alkynes are identical to those used to name alkenes except the ending -yne is used instead of -ene. Isomerism and Alkynes • cis–trans isomerism (stereoisemerism) not possible because of the linearity (180º angles) about an alkyne’s triple bond • Constitutional isomers are possible a.) skeletal isomers – the carbon chain arrangement is differentb.) positional isomers - position of the triple bond is different Physical Properties of Alkynes Similar to those of alkenes and alkanes; • Insoluble in water but soluble in organic solvents• Densities less than that of water • Boiling points increase with molecular mass • Low-molecular-mass alkynes are gases at room temperature Chemical properties of alkynes • Combustion of alkynes give cabon dioxide and water • undergo addition reaction a.) in limited reactant, alkene derivative is formed. b.) in excess reactant, alkane derivative is formed. • Markovnikovs’ rule applies in case of unsymmetrical addition.2.12 Aromatic HydrocarbonsContains benzene ring with alternate double and single bonds • Double bonds are different from those of alkenes • These are called delocalized (=) bonds: All C-C bonds are similar • Resonance structures (differ only in arrangement of electrons2.13 Names of Aromatic Hydrocarbons• Name the substituent followed by the name benzene. • When more than one substituent present, indicate the position by number followed by name of the substituent and the name benzene – No need to indicate position if there is only one substituent – Position 1, 2 are also known as Ortho positions – Positions 1,3 are also known as Meta positions – Positions 1,4 are also known as Para positions 2.14 Aromatic Hydrocarbons : Physical properties and sources• Insoluble in water, soluble in non-polar solvents like hydrocarbons eg. hexanes. • Less dense than water therefore floats on water • Benzene is colorless, flammable liquid, burns with sooty flame to form carbon dioxide and water. • Petroleum is the primary source of aromatic hydrocarbons– Gasoline has significant amount of benzene in it2.15 Chemical Reactions of Aromatic Hydrocarbons• Benzene undergoes substitution reactions • Two important types of substitution reactions for benzene and other aromatic hydrocarbons are alkylation and halogenation • Halogenation2.16 Fused Ring Aromatic Hydrocarbons• Fused-ring aromatic hydrocarbons: Aromatic hydrocarbon whose structure contains two or more rings fused together and two carbon rings share a pair of carbon


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