10-1 Dr. Wolf's CHM 201 & Classes of Dienes

10-2 Dr. Wolf's CHM 201 & 202 isolated diene conjugated diene cumulated diene C Classification of Dienes

10-3 Dr. Wolf's CHM 201 & 202 (2E,5E)-2,5-heptadiene (2E,4E)-2,4-heptadiene 3,4-heptadiene C NomenclatureNomenclature

10-4 Dr. Wolf's CHM 201 & Relative Stabilities of Dienes

10-5 Dr. Wolf's CHM 201 & kJ/mol 226 kJ/mol 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 Heats of Hydrogenation

10-6 Dr. Wolf's CHM 201 & kJ/mol 226 kJ/mol 126 kJ/mol 115 kJ/mol Heats of Hydrogenation

10-7 Dr. Wolf's CHM 201 & kJ/mol 226 kJ/mol 126 kJ/mol 115 kJ/mol 126 kJ/mol 111 kJ/mol Heats of Hydrogenation

10-8 Dr. Wolf's CHM 201 & kJ/mol 111 kJ/mol 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

10-9 Dr. Wolf's CHM 201 & kJ/mol 111 kJ/mol Heats of Hydrogenation this extra 15 kJ/mol is known by several terms stabilization energy delocalization energy resonance energy

10-10 Dr. Wolf's CHM 201 & 202 Cumulated double bonds have relatively high heats of hydrogenation  H° = -295 kJ Heats of Hydrogenation H2CH2CH2CH2C CH 2 C + 2H 2 CH 3 CH 2 CH 3  H° = -125 kJ H2CH2CH2CH2C CH 2 CH 3 + H2H2H2H2 CH 3 CH 2 CH 3

10-11 Dr. Wolf's CHM 201 & Bonding in Conjugated Dienes

10-12 Dr. Wolf's CHM 201 & 202 Isolated diene Conjugated diene 1,4-pentadiene 1,3-pentadiene

10-13 Dr. Wolf's CHM 201 & 202 Isolated diene Conjugated diene  bonds are independent of each other 1,3-pentadiene

10-14 Dr. Wolf's CHM 201 & 202 Isolated diene Conjugated diene  bonds are independent of each other p orbitals overlap to give extended  bond encompassing four carbons

10-15 Dr. Wolf's CHM 201 & 202 Isolated diene Conjugated diene less electron delocalization; less stable more electron delocalization; more stable

10-16 Dr. Wolf's CHM 201 & 202 s-trans s-cis 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) H HH HHHHH HHHH

10-17 Dr. Wolf's CHM 201 & 202 s-trans s-cis 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) H HH HHHHH HHHH

10-18 Dr. Wolf's CHM 201 & 202 s-trans s-cis Conformations of Dienes Both conformations allow electron delocalization via overlap of p orbitals to give extended  system

10-19 Dr. Wolf's CHM 201 & 202 s-trans is more stable than s-cis 12 kJ/mol Interconversion of conformations requires two  bonds to be at right angles to each other and prevents conjugation

10-20 Dr. Wolf's CHM 201 & 202

10-21 Dr. Wolf's CHM 201 & kJ/mol 12 kJ/mol

10-22 Dr. Wolf's CHM 201 & Bonding in Allenes

10-23 Dr. Wolf's CHM 201 & 202 cumulated dienes are less stable than isolated and conjugated dienes (see Problem 10.7 on p 375) Cumulated Dienes CC C

10-24 Dr. Wolf's CHM 201 & pm Structure of Allene 118.4° linear arrangement of carbons nonplanar geometry

10-25 Dr. Wolf's CHM 201 & 202 Structure of Allene 131 pm 118.4° linear arrangement of carbons nonplanar geometry

10-26 Dr. Wolf's CHM 201 & 202 sp 2 sp Bonding in Allene sp 2

10-27 Dr. Wolf's CHM 201 & 202 Bonding in Allene

10-28 Dr. Wolf's CHM 201 & 202 Bonding in Allene

10-29 Dr. Wolf's CHM 201 & 202 Bonding in Allene

10-30 Dr. Wolf's CHM 201 & 202 Allenes of the type shown are chiral A B X Y A  B; X  Y Have a stereogenic axis Chiral Allenes CC C

10-31 Dr. Wolf's CHM 201 & 202 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 Stereogenic Axis

10-32 Dr. Wolf's CHM 201 & Preparation of Dienes

10-33 Dr. Wolf's CHM 201 & 202 CH 3 CH 2 CH 2 CH °C 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-Butadiene1,3-Butadiene H2CH2CH2CH2C CHCH CH 2 + 2H 2

10-34 Dr. Wolf's CHM 201 & 202 KHSO 4 heat Dehydration of Alcohols OH

10-35 Dr. Wolf's CHM 201 & 202 KHSO 4 heat Dehydration of Alcohols OH major product; 88% yield

10-36 Dr. Wolf's CHM 201 & 202 KOH heat Dehydrohalogenation of Alkyl Halides Br

10-37 Dr. Wolf's CHM 201 & 202 KOH heat Br major product; 78% yield Dehydrohalogenation of Alkyl Halides

10-38 Dr. Wolf's CHM 201 & 202 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

10-39 Dr. Wolf's CHM 201 & Addition of Hydrogen Halides to Conjugated Dienes

10-40 Dr. Wolf's CHM 201 & 202 Proton adds to end of diene system Carbocation formed is allylic Electrophilic Addition to Conjugated Dienes H X H +

10-41 Dr. Wolf's CHM 201 & 202 HCl Example:Example:HH HHH H ClHH HHH H H H HHHHCl H H ??

10-42 Dr. Wolf's CHM 201 & 202 HCl Example:Example:HH HHH H ClHH HHH H H

10-43 Dr. Wolf's CHM 201 & 202 via:HH HHH H H + HH HHH H H XHH HHH H H +

10-44 Dr. Wolf's CHM 201 & 202 and:HH HHH H H + HH HHH H H + Cl – ClHH HHH H H H H H H H H H Cl 3-Chlorocyclopentene

10-45 Dr. Wolf's CHM 201 & 202 1,2-Addition versus 1,4-Addition 1,2-addition of XY XY

10-46 Dr. Wolf's CHM 201 & 202 1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY XY XY

10-47 Dr. Wolf's CHM 201 & 202 via 1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY XY XY X +

10-48 Dr. Wolf's CHM 201 & 202 electrophilic addition 1,2 and 1,4-addition both observed product ratio depends on temperature HBr Addition to 1,3-Butadiene H2CH2CH2CH2C CHCH CH 2 HBr Br CH 3 CHCH CHCH 2 Br CH 3 CH +

10-49 Dr. Wolf's CHM 201 & 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. RationaleRationale Br CH 2 CH 3 CHCH CHCH 2 Br CH 3 CH + CH 2 CH 3 CHCH CHCH 2 CH 3 CH via: ++

10-50 Dr. Wolf's CHM 201 & 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. RationaleRationale Br CH 2 CH 3 CHCH CHCH 2 Br CH 3 CH + formed faster

10-51 Dr. Wolf's CHM 201 & 202 more stable RationaleRationale Br CH 2 CH 3 CHCH CHCH 2 Br CH 3 CH + 1-Bromo-2-butene is more stable than 3-bromo-1-butene because it has a more highly substituted double bond.

10-52 Dr. Wolf's CHM 201 & 202 major product at -80°C RationaleRationale major product at 25°C The two products equilibrate at 25°C. Once equilibrium is established, the more stable isomer predominates. Br CH 2 CH 3 CHCH CHCH 2 Br CH 3 CH (formed faster) (more stable)

10-53 Dr. Wolf's CHM 201 & 202 Kinetic Control versus Thermodynamic Control Kinetic control: major product is the one formed at the fastest rateKinetic control: major product is the one formed at the fastest rate Thermodynamic control: major product is the one that is the most stableThermodynamic control: major product is the one that is the most stable

10-54 Dr. Wolf's CHM 201 & 202 H2CH2C CHCH CH 2 HBr CH 2 CH 3 CHCH CHCH 2 CH 3 CH ++

10-55 Dr. Wolf's CHM 201 & 202 CH 2 CH 3 CHCH + CHCH 2 CH 3 CH + Br CH 2 CH 3 CHCH CHCH 2 Br CH 3 CH higher activation energy formed more slowly

10-56 Dr. Wolf's CHM 201 & 202 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? + HCl?

10-57 Dr. Wolf's CHM 201 & 202 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

10-58 Dr. Wolf's CHM 201 & 202 Think mechanistically H Cl + + one resonance form is tertiary carbocation; other is primary

10-59 Dr. Wolf's CHM 201 & 202 Think mechanistically H Cl + + one resonance form is secondary carbocation; other is primary one resonance form is tertiary carbocation; other is primary Cl H + +

10-60 Dr. Wolf's CHM 201 & 202 Think mechanistically H Cl + + 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

10-61 Dr. Wolf's CHM 201 & 202 Think mechanistically H Cl + + one resonance form is tertiary carbocation; other is primary Cl Cl – major product