10.8 Classes of Dienes

10.8 Classes of Dienes Dr. Wolf's CHM 201 & 202

C Classification of Dienes isolated diene conjugated diene cumulated diene Dr. Wolf's CHM 201 & 202

C Nomenclature (2E,5E)-2,5-heptadiene (2E,4E)-2,4-heptadiene 3,4-heptadiene Dr. Wolf's CHM 201 & 202

10.9Relative Stabilitiesof Dienes Dr. Wolf's CHM 201 & 202

Heats of Hydrogenation 1,3-pentadiene is 26 kJ/mol more stable than 1,4-pentadiene, but some of this stabilization is because it also contains a more highly substituted double bond 252 kJ/mol 226 kJ/mol Dr. Wolf's CHM 201 & 202

Heats of Hydrogenation 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol Dr. Wolf's CHM 201 & 202

Heats of Hydrogenation 126 kJ/mol 111 kJ/mol 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol Dr. Wolf's CHM 201 & 202

Heats of Hydrogenation when terminal double bond is conjugated with other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated 126 kJ/mol 111 kJ/mol Dr. Wolf's CHM 201 & 202

Heats of Hydrogenation this extra 15 kJ/mol is known by several termsstabilization energydelocalization energyresonance energy 126 kJ/mol 111 kJ/mol Dr. Wolf's CHM 201 & 202

H2C CH2CH3 Heats of Hydrogenation Cumulated double bonds have relatively high heats of hydrogenation + CH3CH2CH3 C 2H2 H2C CH2 DH° = -295 kJ + CH3CH2CH3 H2 DH° = -125 kJ Dr. Wolf's CHM 201 & 202

10.10Bondingin Conjugated Dienes Dr. Wolf's CHM 201 & 202

Isolated diene 1,4-pentadiene 1,3-pentadiene Conjugated diene Dr. Wolf's CHM 201 & 202

Isolated diene p bonds are independent of each other 1,3-pentadiene Conjugated diene Dr. Wolf's CHM 201 & 202

Isolated diene p bonds are independent of each other p orbitals overlap to give extended p bond encompassing four carbons Conjugated diene Dr. Wolf's CHM 201 & 202

Isolated diene less electron delocalization; less stable more electron delocalization; more stable Conjugated diene Dr. Wolf's CHM 201 & 202

H H H H H H H H H H H H Conformations of Dienes s prefix designates conformation around single bond s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case) s-trans s-cis Dr. Wolf's CHM 201 & 202

Conformations of Dienes Both conformations allow electron delocalization via overlap of p orbitals to give extended p system s-trans s-cis Dr. Wolf's CHM 201 & 202

s-trans is more stable than s-cis Interconversion of conformations requires two p bonds to be at right angles to each other and prevents conjugation 12 kJ/mol Dr. Wolf's CHM 201 & 202

Dr. Wolf's CHM 201 & 202

16 kJ/mol 12 kJ/mol Dr. Wolf's CHM 201 & 202

10.11Bonding in Allenes Dr. Wolf's CHM 201 & 202

C C C Cumulated Dienes cumulated dienes are less stable thanisolated and conjugated dienes (see Problem 10.7 on p 375) Dr. Wolf's CHM 201 & 202

Structure of Allene linear arrangement of carbons nonplanar geometry 118.4° 131 pm Dr. Wolf's CHM 201 & 202

Bonding in Allene sp sp 2 sp 2 Dr. Wolf's CHM 201 & 202

Bonding in Allene Dr. Wolf's CHM 201 & 202

C C C Chiral Allenes Allenes of the type shown are chiral X A Y B A ¹B; X ¹Y Have a stereogenic axis Dr. Wolf's CHM 201 & 202

Stereogenic Axis analogous to difference between: a screw with a right-hand thread and one with a left-hand thread a right-handed helix and a left-handed helix Dr. Wolf's CHM 201 & 202

10.12Preparation of Dienes Dr. Wolf's CHM 201 & 202

CH2 H2C CHCH 1,3-Butadiene 590-675°C CH3CH2CH2CH3 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) chromia- alumina + 2H2 Dr. Wolf's CHM 201 & 202

OH Dehydration of Alcohols KHSO4 heat Dr. Wolf's CHM 201 & 202

OH Dehydration of Alcohols major product; 88% yield KHSO4 heat Dr. Wolf's CHM 201 & 202

Br Dehydrohalogenation of Alkyl Halides KOH heat Dr. Wolf's CHM 201 & 202

Br Dehydrohalogenation of Alkyl Halides major product; 78% yield KOH heat Dr. Wolf's CHM 201 & 202

Reactions of Dienes 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 Dr. Wolf's CHM 201 & 202

10.13Addition of Hydrogen HalidestoConjugated Dienes Dr. Wolf's CHM 201 & 202

Electrophilic Addition to Conjugated Dienes Proton adds to end of diene system Carbocation formed is allylic + H X H Dr. Wolf's CHM 201 & 202

H H H H H H H H H H Cl H H Cl H H H H H H H H Example: HCl ? ? Dr. Wolf's CHM 201 & 202

H H H H H H H H Cl H H H H H Example: HCl Dr. Wolf's CHM 201 & 202

H H H H + H H H H H H H H H H H + H H H H H via: H X Dr. Wolf's CHM 201 & 202

H H H H Cl + H H H H H H H H H H H H + H H H H H and: Cl– 3-Chlorocyclopentene H H Cl H H H H H Dr. Wolf's CHM 201 & 202

Y X 1,2-Addition versus 1,4-Addition 1,2-addition of XY Dr. Wolf's CHM 201 & 202

Y Y X X 1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY Dr. Wolf's CHM 201 & 202

Y Y X X + X 1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY via Dr. Wolf's CHM 201 & 202

CH2 H2C CHCH CH3CHCH CH3CH CH2 CHCH2Br Br HBr Addition to 1,3-Butadiene electrophilic addition 1,2 and 1,4-addition both observed product ratio depends on temperature HBr + Dr. Wolf's CHM 201 & 202

CH3CHCH CH3CH CH2 CHCH2Br Br CH3CHCH CH3CH CH2 CHCH2 Rationale 3-Bromo-1-butene is formed faster than1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. + via: + + Dr. Wolf's CHM 201 & 202

CH3CHCH CH3CH CH2 CHCH2Br Br Rationale 3-Bromo-1-butene is formed faster than1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. + formed faster Dr. Wolf's CHM 201 & 202

10.8 Classes of Dienes

10.8 Classes of Dienes

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