10.9 Preparation of Dienes,CH3CH2CH2CH3,590-675C,chromia- alumina,More than 4 billion pounds of 1,3-butadiene prepared by this method in U.S. each year used to prepare synthetic rubber (See “Diene Polymers“ box),1,3-Butadiene,+,2H2,KHSO4,heat,Dehydration of Alcohols,OH,KHSO4,heat,Dehydration of Alcohols,OH,major product; 88% yield,KOH,heat,Dehydrohalogenation of Alkyl Halides,Br,KOH,heat,Br,major product; 78% yield,Dehydrohalogenation of Alkyl Halides,isolated dienes: double bonds react independently of one another cumulated dienes: specialized topic conjugated dienes: reactivity pattern requires us to think of conjugated diene system as a functional group of its own,Reactions of Dienes,10.10 Addition of Hydrogen Halides to Conjugated Dienes,Proton adds to end of diene system Carbocation formed is allylic,Electrophilic Addition to Conjugated Dienes,H,X,H,+,HCl,Example:,?,?,HCl,Example:,via:,H,X,and:,Cl,H,H,H,H,H,H,H,Cl,3-Chlorocyclopentene,1,2-Addition versus 1,4-Addition,1,2-addition of XY,1,2-addition of XY,1,4-addition of XY,1,2-Addition versus 1,4-Addition,via,1,2-addition of XY,1,4-addition of XY,1,2-Addition versus 1,4-Addition,electrophilic addition 1,2 and 1,4-addition both observed product ratio depends on temperature,HBr Addition to 1,3-Butadiene,HBr,+,3-Bromo-1-butene is formed faster than 1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge.,Rationale,+,via:,+,+,3-Bromo-1-butene is formed faster than 1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge.,Rationale,+,formed faster,more stable,Rationale,+,1-Bromo-2-butene is more stable than 3-bromo-1-butene because it has a more highly substituted double bond.,major product at -80C,Rationale,major product at 25C,The two products equilibrate at 25C. Once equilibrium is established, the more stable isomer predominates.,(formed faster),(more stable),Kinetic Control versus Thermodynamic Control,Kinetic control: major product is the one formed at the fastest rate Thermodynamic control: major product is the one that is the most stable,HBr,CH2,CH3CHCH,+,CHCH2,CH3CH,+,higher activation energy,formed more slowly,Addition of hydrogen chloride to 2-methyl-1,3-butadiene is a kinetically controlled reaction and gives one product in much greater amounts than any isomers. What is this product?,Problem 10.10 (page 382),+,HCl,?,Think mechanistically. Protonation occurs: at end of diene system in direction that gives most stable carbocation Kinetically controlled product corresponds to attack by chloride ion at carbon that has the greatest share of positive charge in the carbocation,+,HCl,Problem 10.10 (page 382),Think mechanistically,+,+,one resonance form is tertiary carbocation; other is primary,Problem 10.10 (page 382),+,+,one resonance form is secondary carbocation; other is primary,one resonance form is tertiary carbocation; other is primary,Cl,H,+,+,Problem 10.10 (page 382),Think mechanistically,+,+,one resonance form is tertiary carbocation; other is primary,More stable carbocation Is attacked by chloride ion at carbon that bears greater share of positive charge,Problem 10.10 (page 382),Think mechanistically,+,+,one resonance form is tertiary carbocation; other is primary,Cl,Cl,major product,Problem 10.10 (page 382),Think mechanistically,gives mixtures of 1,2 and 1,4-addition products,10.11 Halogen Addition to Dienes,Example,Br2,+,(37%),(63%),