Transcript Organic Chemistry Fifth Edition
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
C C C+
allylic carbocation
C C C •
allylic radical
C C C C
conjugated diene
H
10.1
The Allyl Group
H H C C C H H
Vinylic versus Allylic
vinylic carbons C C C allylic carbon
Vinylic versus Allylic
H C C H C H Vinylic hydrogens are attached to vinylic carbons.
Vinylic versus Allylic
C C H C H H Allylic hydrogens are attached to allylic carbons.
Vinylic versus Allylic
X C C X C X Vinylic substituents are attached to vinylic carbons.
Vinylic versus Allylic
C C X C X X Allylic substituents are attached to allylic carbons.
10.2
Allylic Carbocations
C C C +
Allylic Carbocations
The fact that a tertiary allylic halide undergoes solvolysis (S N 1) faster than a simple tertiary alkyl halide… CH 3 CH 3 H 2 C CH C Cl CH 3 C Cl CH 123 3 CH 3 1 relative rates: (ethanolysis, 45 ° C)
Allylic Carbocations
provides good evidence that allylic carbocations are more stable than other carbocations. H 2 C CH CH 3 C + CH 3 CH 3 CH 3 C + CH 3 H 2 C=CH — stabilizes C+ better than does CH 3 —
Stabilization of Allylic Carbocations
Delocalization of electrons in the double bond stabilizes the carbocation.
resonance model orbital overlap model
Resonance Model
H 2 C CH CH 3 C + CH 3 H 2 + C + H 2 C CH CH 3 C + CH 3 CH C CH CH 3 3
Orbital Overlap Model
+ +
Orbital Overlap Model
Orbital Overlap Model
Orbital Overlap Model
10.3
S N 1 Reactions of Allylic Halides
H 2 C CH (85%)
Hydrolysis of an Allylic Halide
H 2 C H 2 CH O CH 3 C Cl CH 3 Na 2 CO 3 CH C CH 3 3 O H CH 3 + H O CH 2 CH C (15%) CH 3
H 2 C CH (85%)
Corollary Experiment
CH 3 Cl CH 2 H 2 CH O C CH 3 Na 2 CO 3 CH C CH 3 3 O H CH 3 + H O CH 2 CH C (15%) CH 3
H 2 C CH CH 3 C Cl and Cl CH 2 CH 3 CH Give the same products because they form the same carbocation.
C CH CH 3 3 H 2 C CH C + CH 3 CH 3 + H 2 C CH C CH 3 CH 3
H 2 C CH CH 3 C O H + H O CH 2 CH C CH CH 3 (15%) (85%) More positive charge on tertiary carbon; therefore more tertiary alcohol in product.
CH 3 3 H 2 C CH C + CH 3 CH 3 + H 2 C CH C CH 3 CH 3
10.4
S N 2 Reactions of Allylic Halides
Allylic S N 2 Reactions
Allylic halides also undergo S N 2 reactions faster than simple primary alkyl halides.
H 2 C CH CH 2 Cl 80 H 3 C CH 2 relative rates: (I , acetone) 1 CH 2 Cl
Allylic S N 2 reactions
Two factors: Steric Trigonal carbon smaller than tetrahedral carbon.
H 2 C CH CH 2 Cl H 3 C CH 2 80 relative rates: (I , acetone) 1 CH 2 Cl
Allylic S N 2 reactions
Two factors: Electronic Electron delocalization lowers LUMO energy which means lower activation energy.
H 2 C CH CH 2 Cl H 3 C CH 2 80 relative rates: (I , acetone) 1 CH 2 Cl
10.5
Allylic Free Radicals
C C C •
Allylic Free Radicals are Stabilized by Electron Delocalization
C C C • • C C C
Allylic Free Radicals are Stabilized by Electron Delocalization
Spin density is a measure of the unpaired electron distribution in a molecule.
The picture on the next slide shows 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
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• H 2 C CHCH 2 — H 368 kJ/mol H 2 C • CHCH 2 + H• C —H bond is weaker in propene because resulting radical (allyl) is more stable than radical (propyl) from propane.
10.6
Allylic Halogenation
H 2 C CHCH 3
Chlorination of Propene
+ Cl 2 addition ClCH 2 CHCH 3 Cl 500 ° C H 2 C CHCH + HCl 2 Cl substitution
Allylic Halogenation
Selective for replacement of allylic hydrogen Free radical mechanism Allylic radical is intermediate
Hydrogen-atom Abstraction Step
H H H C C H
.
..
Cl
:
C H 410 kJ/mol 368 kJ/mol H Allylic C —H bond is weaker than vinylic.
Chlorine atom abstracts allylic H in propagation step.
Hydrogen-atom Abstraction Step
H H 410 kJ/mol C C H C • H 368 kJ/mol
..
H Cl
:
H
N-Bromosuccinimide
Reagent used (instead of Br 2 ) for allylic bromination. O Br O N Br + heat CCl 4 + N H O O (82-87%)
Limited Scope
Allylic halogenation is only used when: all of the allylic hydrogens are equivalent and the resonance forms of allylic radical are equivalent.
Example
H H Cyclohexene satisfies both requirements.
All allylic hydrogens are equivalent.
H H H H
•
H H
•
H Both resonance forms are equivalent.
H
Example
2-Butene CH 3 CH CHCH 3 All allylic hydrogens are equivalent.
But CH 3 CH CH
•
CH 2
•
CH 3 CH CH CH 2 Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides.
Example
2-Butene CH 3 CH CHCH 3 All allylic hydrogens are equivalent.
CH 3 CH CH forms Br CH 2 and Br CH 3 CH CH CH 2 Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides.
10.7
Allylic Anions
C C C -
H 3 C CH CH 2 pKa ~ 43 H 2 C CH CH 2
Acidity of Propene
H 3 C CH 2 CH 3 pKa ~ 62 H 2 C CH 2 CH 3 Propene is significantly more acidic than propane.
Resonance Model
H 2 C CH CH 2 H 2 C CH CH 2 H 2 C CH CH 2 Charge is delocalized to both terminal carbons, stabilizing the conjugate base.
10.8 Classes of Dienes
C
Classification of Dienes
Isolated diene Conjugated diene Cumulated diene
C
Nomenclature
(2
E
,5
E
)-2,5-heptadiene (2
E
,4
E
)-2,4-heptadiene 3,4-heptadiene
10.9
Relative Stabilities of Dienes
252 kJ/mol
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.
226 kJ/mol
Heats of Hydrogenation
126 kJ/mol 111 kJ/mol 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol
Heats of Hydrogenation
126 kJ/mol 111 kJ/mol When terminal double bond is conjugated with other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated.
Heats of Hydrogenation
126 kJ/mol 111 kJ/mol This extra 15 kJ/mol is known by several terms: conjugation energy delocalization energy resonance energy
Heats of Hydrogenation
Cumulated double bonds have relatively high heats of hydrogenation.
H 2 C C CH 2 + 2H 2
H
° CH = -295 kJ 3 CH 2 CH 3 H 2 C CH 2 CH 3 + H 2 CH 3 CH 2 CH 3
H
° = -125 kJ
10.10
Bonding in Conjugated Dienes
Isolated diene 1,4-pentadiene 1,3-pentadiene Conjugated diene
Isolated diene bonds are independent of each other.
1,3-pentadiene Conjugated diene
Isolated diene Conjugated diene bonds are independent of each other.
p
orbitals overlap to give extended bond encompassing four carbons.
Isolated diene less electron delocalization; less stable more electron delocalization; more stable Conjugated diene
Conformations of Dienes
H H H H H H H H H H H H
s-
trans
s-
cis
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).
Conformations of Dienes s-
trans
s-
cis Both conformations allow electron delocalization via overlap of
p
orbitals to give extended system.
s-trans is More Stable Than s-cis
Interconversion of conformations requires two bonds to be at right angles to each other and prevents conjugation.
12 kJ/mol
16 kJ/mol 12 kJ/mol
10.11
Bonding in Allenes
Cumulated Dienes
C C C Cumulated dienes are less stable than isolated and conjugated dienes.
(see Problem 10.10)
Structure of Allene
118.4
° 131 pm Linear arrangement of carbons Nonplanar geometry
Bonding in Allene sp 2 sp sp 2
Bonding in Allene
Bonding in Allene
Bonding in Allene
Chiral Allenes
Allenes of the type shown are chiral A X C C C B Y A B; X Y Have a chirality axis (Section 7.9)
Chirality 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.
10.12
Preparation of Dienes
1,3-Butadiene
CH 3 CH 2 CH 2 CH 3 590-675 ° C chromia alumina H 2 C CHCH CH 2 + 2H 2 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, p. 406).
Dehydration of Alcohols
KHSO 4 heat O H major product; 88% yield
Dehydrohalogenation of Alkyl Halides
Br major product; 78% yield KOH heat
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.
10.13
Addition of Hydrogen Halides to Conjugated Dienes
Electrophilic Addition to Conjugated Dienes
+ H X Proton adds to end of diene system.
Carbocation formed is allylic.
H
Example:
H H H H H H H Cl H H H H H H ?
H Cl ?
H H H H H H H Cl
Example:
H H H H H H H Cl H H H H H H Cl H
via:
H H H H H H H X Protonation of the end of the diene unit gives an allylic carbocation.
H H H H + H H H H H H H + H H H
and:
H H H H + H H H Cl – H H H H + H H H H H H H H Cl H H 3-Chlorocyclopentene H H Cl H H H H H
1,2-Addition versus 1,4-Addition
1,2-addition of XY Y X X + via 1,4-addition of XY Y X
HBr Addition to 1,3-Butadiene
H 2 C CHCH HBr CH 2 CH 3 CHCH CH 2 + CH 3 CH CHCH 2 Br Br Electrophilic addition 1,2 and 1,4-addition both observed Product ratio depends on temperature
Rationale
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. CH 3 CHCH CH 2 Br (formed faster) + CH 3 CH CHCH 2 Br via: + CH 3 CHCH CH 2 CH 3 CH + CHCH 2
Rationale
1-Bromo-2-butene is more stable than 3-bromo-1-butene because it has a more highly substituted double bond.
CH 3 CHCH CH 2 Br (formed faster) + CH 3 CH CHCH 2 Br (more stable)
Rationale
The two products equilibrate at 25 ° C.
Once equilibrium is established, the more stable isomer predominates.
CH 3 CHCH CH 2 Br major product at -80 ° C (formed faster) 1,2 addition product CH 3 CH CHCH 2 Br major product at 25 ° C (more stable) 1,4 addition product
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.
+ CH 3 CHCH CH 2 CH 3 CH + CHCH 2 HBr H 2 C CHCH CH 2
CH 3 + CHCH CH 2 higher activation energy CH 3 CH + CHCH 2 CH 3 CHCH CH 2 Br formed more slowly CH 3 CH CHCH 2 Br
Example Problem
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 HCl Kinetically controlled product corresponds to attack by chloride ion at carbon that has the greatest share of positive charge in the carbocation.
Example Problem
Think mechanistically H Cl Cl H + + One resonance form is tertiary carbocation; other is primary.
+ One resonance form is secondary carbocation; other is primary.
+
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.
Example Problem
Think mechanistically H Cl + + One resonance form is tertiary carbocation; other is primary.
Cl – major product Cl
10.14
Halogen Addition to Dienes
Gives mixtures of 1,2 and 1,4-addition products
Example
Br CH 2 CHCH Br (37%) H 2 C CHCH CH 2 Br 2 CH 2 + Br CH 2 CH CHCH 2 Br (63%)
10.15 The Diels-Alder Reaction
Synthetic method for preparing compounds containing a cyclohexene ring
In General...
+ conjugated diene alkene (dienophile) cyclohexene
via
transition state
Mechanistic Features
Concerted mechanism Cycloaddition Pericyclic reaction A concerted reaction that proceeds through a cyclic transition state.
Recall the General Reaction...
+ conjugated diene alkene (dienophile) cyclohexene The equation as written is somewhat misleading because ethylene is a relatively unreactive dienophile.
What Makes a Reactive Dienophile?
The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond.
Typical EWGs C C EWG C O C N
H 2 C via:
Example
CHCH CH 2 + H 2 C O benzene 100 ° C O CH CH CH O CH (100%)
H 3 via:
Example
H 2 C O CHC CH 2 + O CH 3 benzene 100 ° C O O O C H 3 C O O O (100%) O
H 2 C
Acetylenic Dienophile
CHCH O CH 2 + CH 3 CH 2 OCC benzene 100 ° C O CCOCH 2 CH 3 O COCH 2 CH 3 (98%) COCH 2 CH 3 O
Diels-Alder Reaction is Stereospecific* *A stereospecific reaction is one in which stereoisomeric starting materials yield products that are stereoisomers of each other; characterized by terms like syn addition, anti elimination, inversion of configuration, etc.
Diels-Alder: syn addition to alkene cis-trans relationship of substituents on alkene retained in cyclohexene product
Example
H 2 C CHCH CH 2 + C 6 H 5 C C O COH H H only product H C 6 H 5 COH H O + enantiomer
Example
H 2 C CHCH CH 2 + H C 6 H 5 C C O COH H only product C 6 H 5 H COH H O + enantiomer
Cyclic Dienes Yield Bridged Bicyclic Diels-Alder Adducts
+ CH 3 H C C O COCH 3 OC O H + enantiomer H O COCH 3 H COCH 3 O
H O COCH 3 H COCH 3 O is the same as O H COCH 3 H COCH 3 O
10.16 The
Molecular Orbitals of Ethylene and 1,3-Butadiene
Orbitals and Chemical Reactions
A deeper understanding of chemical reactivity can be gained by focusing on the
frontier orbitals
of the reactants.
Electrons flow from the highest occupied molecular orbital (HOMO) of one reactant to the lowest unoccupied molecular orbital (LUMO) of the other.
Orbitals and Chemical Reactions
We can illustrate HOMO-LUMO interactions by way of the Diels-Alder reaction between ethylene and 1,3-butadiene.
We need only consider only the electrons of ethylene and 1,3-butadiene. We can ignore the framework of bonds in each molecule.
The
MOs of Ethylene
Red and blue colors distinguish sign of wave function.
Bonding MO is antisymmetric with respect to plane of molecule.
Bonding orbital of ethylene; two electrons in this orbital.
The
MOs of Ethylene
Antibonding orbital of ethylene; no electrons in this orbital.
LUMO HOMO Bonding orbital of ethylene; two electrons in this orbital.
The
MOs of 1,3-Butadiene
Four
p
orbitals contribute to the system of 1,3 butadiene; therefore, there are four molecular orbitals.
Two of these orbitals are bonding; two are antibonding.
The Two Bonding
MOs of 1,3-Butadiene
HOMO 4 electrons; 2 in each orbital.
Lowest energy orbital
The Two Antibonding
MOs of 1,3-Butadiene
Highest energy orbital LUMO Both antibonding orbitals are vacant.
10.17
A
Molecular Orbital Analysis of the Diels-Alder Reaction
MO Analysis of Diels-Alder Reaction
Since electron-withdrawing groups increase the reactivity of a dienophile, we assume electrons flow from the HOMO of the diene to the LUMO of the dienophile.
MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene HOMO of 1,3-butadiene and LUMO of ethylene are in phase with one another.
Allows bond formation between the alkene and the diene.
LUMO of ethylene (dienophile)
MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene LUMO of ethylene (dienophile)
A “Forbidden" Reaction
H 2 C + CH 2 H 2 C CH 2 The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not?
A “Forbidden" Reaction
H 2 C + CH 2 H 2 C CH 2 HOMO-LUMO mismatch of two ethylene molecules precludes single-step formation of two new bonds.
HOMO of one ethylene molecule.
LUMO of other ethylene molecule.