Chapter 10 Conjugation in Alkadienes and Allylic Systems Conjugare is a Latin verb meaning "to link or yoke together."

The Double Bond as a Substituent Allylic carbocation C + C C Allylic radical C C C Conjugated diene C C C C

C C C HHH H H 10.1 The Allyl Group

Vinylic versus Allylic C C C Vinylic carbons Allyliccarbon

Vinylic hydrogens are attached to vinylic carbons. Vinylic versus Allylic C C C H H H

Allylic hydrogens are attached to allylic carbons. Vinylic versus Allylic C C CHH H

C C C X X X Vinylic substituents are attached to vinylic carbons.

Vinylic versus Allylic C C C X X X Allylic substituents are attached to allylic carbons.

10.2 Allylic Carbocations C C C +

Allylic carbocations are more stable than other carbocations. CH 3 Formed faster Allylic Carbocations CC H2CH2CH2CH2C CH ++ CH 3

Allylic carbocations are more stable than other carbocations. CH 3 H 2 C=CH— stabilizes C+ better than does CH 3 —. Allylic Carbocations CC CH 3 H2CH2CH2CH2C CH + +

Delocalization of electrons in the double bond stabilizes the carbocation. Resonance model Orbital overlap model Stabilization of Allylic Carbocations

Resonance Model CH 3 H2CH2CH2CH2C CH + C H2CH2CH2CH2C CH + C C H2CH2CH2CH2C CH ++++ ++++

Orbital Overlap Model ++++ ++++

10.3 & 10.4 Nucleophilic Substitution Reactions of Allylic Halides S N 1 (and S N 2) reactions are faster for allylic halides than corresponding nonallylic halides.

A tertiary allylic halide undergoes solvolysis (S N 1) faster than a simple tertiary alkyl halide. Cl CH 3 Relative rates (ethanolysis, 45°C) 1231 Allylic Carbocations C Cl CH 3 C H2CH2CH2CH2C CH

H2OH2OH2OH2O Na 2 CO 3 Hydrolysis of an Allylic Halide Cl CH 3 C H2CH2CH2CH2C CH (85%) OHOHOHOH C H2CH2CH2CH2C CH (15%) + HOCH 2 CH C CH 3

H2OH2OH2OH2O Na 2 CO 3 Corollary Experiment (85%) OHOHOHOH CH 3 C H2CH2CH2CH2C CH (15%) + HOCH 2 CH C CH 3 ClCH 2 CH C CH 3

ClCH 2 CH C CH 3 Cl C H2CH2CH2CH2C CH and Give the same products because they form the same carbocation. CH 3 H2CH2CH2CH2C CH + C H2CH2CH2CH2C CH + C

HOCH 2 CH C CH 3 OHOHOHOH C H2CH2CH2CH2C CH +(85%)(15%) More positive charge on tertiary carbon; therefore, more tertiary alcohol in product. CH 3 H2CH2CH2CH2C CH + C H2CH2CH2CH2C CH + C

Allylic halides also undergo S N 2 reactions faster than simple primary alkyl halides. Relative rates 80 1 Allylic S N 2 Reactions Cl CH 2 H2CH2CH2CH2C CH Cl H3CH3CH3CH3C Reason: steric and electronic effects.

10.5 Allylic Free Radicals C C C

Allylic Free Radicals Are Stabilized by Electron Delocalization C C C C C C

Spin density is a measure of the unpaired electron distribution in a molecule. The unpaired electron in allyl radical "divides its time" equally between C-1 and C-3. Allylic Free Radicals Are Stabilized by Electron Delocalization

Spin density in allyl radical Allylic Free Radicals Are Stabilized by Electron Delocalization

Free-Radical Stabilities Are Related to Bond-Dissociation Energies CH 3 CH 2 CH 2 —H 410 kJ/mol CH 3 CH 2 CH 2 + H 368 kJ/mol + H CHCH 2 —H H2CH2CH2CH2C CHCH 2 H2CH2CH2CH2C C—H bond is weaker in propene because resulting radical (allyl) is more stable than radical (propyl) from propane.

10.6 Allylic Halogenation

ClCH 2 CHCH 3 Cl Addition 500°C Substitution CHCH 3 H2CH2CH2CH2C + Cl 2 CHCH 2 Cl H2CH2CH2CH2C + HCl Chlorination of Propene

Selective for replacement of allylic hydrogen. Free radical mechanism. Allylic radical is intermediate. Allylic Halogenation

410 kJ/mol Hydrogen-Atom Abstraction Step C C CHH H H H H 368 kJ/mol Cl :..... Allylic C—H bond weaker than vinylic. Chlorine atom abstracts allylic H in propagation step.

410 kJ/mol Hydrogen-Atom Abstraction Step C C C H H H H H 368 kJ/mol H Cl : :....

Reagent used (instead of Br 2 ) for allylic bromination. N-Bromosuccinimide OO NBr CCl 4 Br+ heat (82-87%) + O O NHNHNHNH

All of the allylic hydrogens are equivalent and the resonance forms of allylic radical are equivalent. Limited Scope Allylic halogenation is only used when:

Example Cyclohexene satisfies both requirements. Both resonance forms are equivalent. H H H H H H All allylic hydrogens are equivalent. HHHH

Example 2-Butene CH 3 CH CHCH 3 Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides. But CH 3 CH CH CH 2 CH 3 CH CH CH 2

10.8 Classes of Dienes

Isolated diene Conjugated diene Cumulated diene C Classification of Dienes

(2E,5E)-2,5-Heptadiene (2E,4E)-2,4-Heptadiene 3,4-Heptadiene C Nomenclature

10.9 Relative Stabilities of Dienes

252 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

252 kJ/mol 226 kJ/mol 126 kJ/mol 115 kJ/mol 126 kJ/mol 111 kJ/mol Heats of Hydrogenation

126 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.

126 kJ/mol 111 kJ/mol Heats of Hydrogenation This extra 15 kJ/mol is known by several terms: Stabilization energy Delocalization energy Resonance energy

Cumulated double bonds have relatively high heats of hydrogenation. Heats of Hydrogenation H2CH2CH2CH2C CH 2 C + 2H 2 CH 3 CH 2 CH 3  H° = -295 kJ/mol  H° = -125 kJ/mol H2CH2CH2CH2C CH 2 CH 3 + H2H2H2H2 CH 3 CH 2 CH 3

Conjugated diene = most stable Isolated diene Cumulated diene = least stable C Stabilities of Dienes

10.10 Bonding in Conjugated Dienes

Isolated diene Conjugated diene 1,4-Pentadiene 1,3-Pentadiene

Isolated diene Conjugated diene  bonds are independent of each other. 1,3-Pentadiene

Isolated diene Conjugated diene  bonds are independent of each other. p orbitals overlap to give extended  bond encompassing four carbons.

Isolated diene Conjugated diene Less electron delocalization; less stable. More electron delocalization; more stable.

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

s-trans s-cis Conformations of Dienes Both conformations allow electron delocalization via overlap of p orbitals to give extended  system.

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.

16 kJ/mol 12 kJ/mol

10.12 Preparation of Dienes

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. 1,3-Butadiene H2CH2CH2CH2C CHCH CH 2 + 2H 2

KHSO 4 heat Dehydration of Alcohols OHOHOHOH Major product; 88% yield

KOH heat Br Dehydrohalogenation of Alkyl Halides Major product; 78% yield

10.13 Addition of Hydrogen Halides to Conjugated 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. Reactions of Dienes

Proton adds to end of diene system. Carbocation formed is allylic. Electrophilic Addition to Conjugated Dienes H X H +

Example:HH HHH H HCl Cl H H H H H H H ? H HHHHCl H H ? 1,3-Cyclopentadiene

Example:HH HHH H ClHH HHH H H 3-Chlorocyclopentene

via:HH HHH H H + HH HHH H H X H H H H H H H + Protonation of the end of the diene unit gives an allylic carbocation.

and:HH HHH H H + H H H H H H H + Cl – ClHH HHH H H H H H H H H H Cl 3-Chlorocyclopentene

1,2-Addition of XY XY 1,4-Addition of XY XY 1,2-Addition versus 1,4-Addition via: X + X + X

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 2 CH 3 CHCH CHCH 2 Br CH 3 CH +

3-Bromo-1-butene (left) is formed faster than 1-bromo-2-butene (right) because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. Rationale Br CH 2 CH 3 CHCH CHCH 2 Br CH 3 CH + CH 2 CH 3 CHCH CHCH 2 CH 3 CH via: ++ Formed faster

More stable Rationale 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. Formed faster

Major product at -80°C Rationale 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) Major product at 25°C (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

H2CH2C CHCH CH 2 HBr CH 2 CH 3 CHCH CHCH 2 CH 3 CH ++

+ 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

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? Example Problem

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 Example Problem

H Cl + + One resonance form is secondary carbocation; other is primary. One resonance form is tertiary carbocation; other is primary. Cl H + + Example Problem 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. Think mechanistically Example Problem

H Cl + + One resonance form is tertiary carbocation; other is primary. Cl Cl – Major product Think mechanistically Example Problem

Gives mixtures of 1,2- and 1,4-addition products Halogen Addition to Dienes

Example H2CH2CH2CH2C CHCH CH 2 Br 2 Br CH 2 BrCH 2 CHCH C C + (37%) (63%) CHCl 3, room temperature + 1,3-Butadiene 3,4-Dibromo-1-butene (E)-1,4-Dibromo-2-butene BrCH 2 H H CH 2 Br

10.15 The Diels-Alder Reaction Synthetic method for preparing compounds containing a cyclohexene ring.

conjugated diene alkene (dienophile) cyclohexene + General Reaction

Transition state via

Concerted mechanism Cycloaddition Pericyclic reaction A concerted reaction that proceeds through a cyclic transition state. Mechanistic Features

Conjugated diene Alkene (dienophile) Cyclohexene + Recall the General Reaction... The equation as written is somewhat misleading because ethylene is a relatively unreactive dienophile.

What Makes a Dienophile Reactive? The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond. Typical EWGs C O CN C CEWG

+ benzene 100°C H2CH2CH2CH2C CHCH CH 2 H2CH2CH2CH2C CH CHO (100%) CHOExample CH O via:

+ benzene 100°COO O Example H2CH2CH2CH2C CHC CH 3 (100%) H3CH3CH3CH3COO O H3CH3CH3CH3C via: O O O

+ benzene 100°C H2CH2CH2CH2C CHCH CH 2 Acetylenic Dienophile O CCOCH 2 CH 3 CH 3 CH 2 OCC O (98%) COCH 2 CH 3 OO

Diels-Alder: syn addition to alkene. cis-trans relationship of substituents on alkene retained in cyclohexene product. Diels-Alder Reaction Is Stereospecific* *A stereospecific reaction is one in which stereoisomeric starting materials give stereoisomeric products; characterized by terms like syn addition, anti elimination, inversion of configuration, etc.

+ H2CH2CH2CH2C CHCH CH 2 Example C C C6H5C6H5C6H5C6H5 COH HH O cis product H C6H5C6H5C6H5C6H5 H COH O cis dienophile

+ H2CH2CH2CH2C CHCH CH 2 Example C C C6H5C6H5C6H5C6H5 COH H HO trans product H C6H5C6H5C6H5C6H5H COH O trans dienophile

Cyclic Dienes Yield Bridged Bicyclic Diels-Alder Adducts

+ C C COCH 3 H HO CH 3 OC O H H COCH 3 O O

is the same as H H COCH 3 O O H H O O