Chapter 13 Chapter 13: Chemical Reactions Chemical Reactions
Chapter 13
71
Created by Professor William Tam & Dr. Phillis Chang Ch. 13 - 1 Chapter 13 Conjugated Unsaturated Systems
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Chapter 13. Conjugated Unsaturated Systems. About The Authors. These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife, Dr. Phillis Chang. - PowerPoint PPT Presentation
Transcript of Chapter 13
Created byProfessor William Tam & Dr. Phillis
Chang Ch. 13 - 1
Chapter 13
Conjugated UnsaturatedSystems
Ch. 13- 2
About The AuthorsThese PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife, Dr. Phillis Chang.
Professor William Tam received his B.Sc. at the University of Hong Kong in 1990 and his Ph.D. at the University of Toronto (Canada) in 1995. He was an NSERC postdoctoral fellow at the Imperial College (UK) and at Harvard University (USA). He joined the Department of Chemistry at the University of Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and Associate Chair in the department. Professor Tam has received several awards in research and teaching, and according to Essential Science Indicators, he is currently ranked as the Top 1% most cited Chemists worldwide. He has published four books and over 80 scientific papers in top international journals such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem.
Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She lives in Guelph with her husband, William, and their son, Matthew.
Ch. 13 - 3
1. Introduction A conjugated system involves at
least one atom with a p orbital adjacent to at least one p bond● e.g.
O
conjugateddiene
allylicradical
allylic cation
allylicanion
enone enyne
Ch. 13 - 4
XH X
X2high temp
(and low conc.of X2)
+
2. Allylic Substitution and the Allyl Radical
vinylic carbons (sp2)
X
X
X2low temp
CCl4
allylic carbon (sp3)
Ch. 13 - 5
2A.Allylic Chlorination(High Temperature)
Cl H Cl+ Cl2 +400oCgas phase
Ch. 13 - 6
Mechanism●Chain initiation
Cl Cl 2 Cl
●Chain propagationH H Cl++ Cl
(allylic radical)
Ch. 13 - 7
Mechanism●Chain propagation
●Chain termination
Cl Cl Cl+ + Cl
Cl+ Cl
Ch. 13 - 8
+ HH
DHo = 369 kJmol-1
DHo = 465 kJmol-1
H + H
Ch. 13 - 9
+ HXH + XEact(low)
H +Eact(high) HX+X
Relative stabilityof radicals: allylic > 3o > 2o > 1o > vinylic
Ch. 13 - 10
Ch. 13 - 11
2B. Allylic Bromination with N-Bromo-succinimide (Low Concentration of Br2)
NBS is a solid and nearly insoluble in CCl4● Low concentration of Br•
H NBr
OO
Br NH
OO
h or ROORheat, CCl4
+
+
(NBS)
Ch. 13 - 12
ExamplesBr
ROOR, CCl4heat
NBS
BrROOR, CCl4heat
NBS
Ch. 13 - 13
3. The Stability of the Allyl Radical3A.Molecular Orbital Description of
the Allyl Radical
Ch. 13 - 14
Ch. 13 - 15
3B.Resonance Description of the Allyl Radical
12
3 12
3
1
23
12
3
Ch. 13 - 16
4. The Allyl Cation Relative order of Carbocation
stability
(3o allylic) (allylic)(3o)
(2o) (1o) (vinylic)
> >
>>>
Ch. 13 - 17
5. Resonance Theory Revisited5A. Rules for Writing Resonance Structures Resonance structures exist only on
paper. Although they have no real existence of their own, resonance structures are useful because they allow us to describe molecules, radicals, and ions for which a single Lewis structure is inadequate
We connect these structures by double-headed arrows (), and we say that the hybrid of all of them represents the real molecule, radical, or ion
Ch. 13 - 18
In writing resonance structures, we are only allowed to move electrons
HH
resonance structures
not resonance structures
Ch. 13 - 19
All of the structures must be proper Lewis structures
O O: : 10 electrons!Xnot a proper
Lewis structure
Ch. 13 - 20
All resonance structures must have the same number of unpaired electrons
X
Ch. 13 - 21
All atoms that are part of the delocalized p-electron system must lie in a plane or be nearly planar
no delocalizationof p-electrons
delocalizationof p-electrons
Ch. 13 - 22
The energy of the actual molecule is lower than the energy that might be estimated for any contributing structure
Equivalent resonance structures make equal contributions to the hybrid, and a system described by them has a large resonance stabilization
Ch. 13 - 23
The more stable a structure is (when taken by itself), the greater is its contribution to the hybrid
(3o allylic cation)greater contribution
(2o allylic cation)
Ch. 13 - 24
5B.Estimating the Relative Stability of Resonance Structures
The more covalent bonds a structure has, the more stable it is
(more stable) (less stable)
O O
(more stable) (less stable)
Ch. 13 - 25
Structures in which all of the atoms have a complete valence shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid
O O
this carbon has6 electrons
this carbon has 8 electrons
Ch. 13 - 26
Charge separation decreases stability
(more stable) (less stable)
OMe OMe
Ch. 13 - 27
6. Alkadienes and Polyunsaturated Hydrocarbons
1,3-Butadiene
(2E,4E)-2,4-Hexadiene1,3-Cyclohexadiene
12
3
4
12
3
4
5
6
1
2 3
4
56
Alkadienes (“Dienes”)
Ch. 13 - 28
Alkatrienes (“Trienes”)
1
2
3
4
5
6
7
8
(2E,4E,6E)-Octa-2,4,6-triene
Ch. 13 - 29
Alkadiynes (“Diynes”)1 2 3 4 5 6
2,4-Hexadiynes
1
23
456 1
2
3
4
5 6 7 8
Hex-1-en-5-yne (2E)-Oct-2-en-6-yne
Alkenynes (“Enynes”)
Ch. 13 - 30
Cumulenes
(Allene)(a 1,2-diene)
C C CH
HH
HC C C
H
HH
H
enantiomers
Ch. 13 - 31
Conjugated dienes
Isolated double bonds
Ch. 13 - 32
7. 1,3-Butadiene: Electron Delocalization
1
2
3
4
7A.Bond Lengths of 1,3-Butadiene
1.34 Å
1.47 Å
1.54 Å 1.50 Å 1.46 Å
sp3 sp3spsp3sp2
Ch. 13 - 33
7B.Conformations of 1,3-Butadiene
(s-cis) (s-trans)
H H
(less stable)
cis
transsinglebond
singlebond
Ch. 13 - 34
7C. Molecular Orbitals of 1,3-Butadiene
Ch. 13 - 35
8. The Stability of Conjugated Dienes
Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes
2 + 2 H2 2 2 x (-127)=-254
H o (kJmol-1)
=-239
Difference 15
+ 2 H2
Ch. 13 - 36
Ch. 13 - 37
9. Ultraviolet–Visible Spectroscopy
The absorption of UV–Vis radiation is caused by transfer of energy from the radiation beam to electrons that can be excited to higher energy orbitals
Ch. 13 - 38
9A.The Electromagnetic Spectrum
Ch. 13 - 39
9B.UV–Vis Spectrophotometers
Ch. 13 - 40
Ch. 13 - 41
Beer’s law A = absorbancee = molar absorptivityc = concentrationℓ = path length
A = e x c x ℓ A
c x ℓor e =
●e.g. 2,5-Dimethyl-2,4-hexadienelmax(methanol) 242.5 nm(e = 13,100)
Ch. 13 - 42
9C. Absorption Maxima for Nonconjugatedand Conjugated Dienes
Ch. 13 - 43
O OAcetone
Ground state
n plmax = 280 nmemax = 15
p* Excited state
O
n p
p plmax = 324 nm, emax = 24
lmax = 219 nm, emax = 3600
Ch. 13 - 44
9D. Analytical Uses of UV–Vis Spectroscopy UV–Vis spectroscopy can be used in
the structure elucidation of organic molecules to indicate whether conjugation is present in a given sample
A more widespread use of UV–Vis spectroscopy, however, has to do with determining the concentration of an unknown sample
Quantitative analysis using UV–Vis spectroscopy is routinely used in biochemical studies to measure the rates of enzymatic reactions
Ch. 13 - 45
10. Electrophilic Attack on ConjugatedDienes: 1,4 Addition
ClH
ClH
1
2
3
4 H Cl25oC
+
(78%)(1,2-Addition)
(22%)(1,4-Addition)
Ch. 13 - 46
(a)
ClH
Mechanism
Cl H + H(a)
H(b)
HX
H+ +
Cl(b)
ClH
(a)
(b)
Ch. 13 - 47
10A. Kinetic Control versus Thermodynamic Control of a Chemical Reaction
+HBr
Br
Br+
(80%)
-80oC
(20%)
(80%)40oC
Br
Br+
(20%)
Ch. 13 - 48
Br
Br
40oC, HBr
1,2-Additionproduct
1,4-Additionproduct
Ch. 13 - 49
Ch. 13 - 50
11.The Diels–Alder Reaction: A 1,4-Cycloaddition Reaction of Dienes
[4p+2p]+
(diene) (dienophile) (adduct)
Ch. 13 - 51
O
O
O
O
O
O1,3-Butadiene
(diene)Maleic
anhydride(dienophile)
Adduct(100%)
+ benzene100oC
e.g.
Ch. 13 - 52
11A. Factors Favoring the Diels–AlderReaction
EDGEWG
EDGEWG
+
Type A
● Type A and Type B are normal Diels-Alder reactions
+Type B
EDG
EWG EWG
EDG
Ch. 13 - 53
EWGEDG
EWGEDG
+
Type C
● Type C and Type D are Inverse Demand Diels-Alder reactions
+Type D
EWG
EDG EDG
EWG
Ch. 13 - 54
Relative rate
Diene D.A. cycloadduct+ 30oCO
O
OOMe
> >Diene
t1/2 20 min. 70 min. 4 h.
Ch. 13 - 55
Relative rate
Dienophile D.A. cycloadduct+ 20oC
> >Dienophile
t1/2 0.002 sec. 20 min. 28 h.
NC CN
NC CN
CN
CN
CN
Ch. 13 - 56
Steric effects
> >Dienophile:
Relative rate: 1 0.14 0.007
COOEt COOEt COOEt
Ch. 13 - 57
11B. Stereochemistry of the Diels–Alder Reaction
O
O
OMeOMe
H
H
OMe
O
OMe
OH
H
+
Dimethyl maleate(a cis-dienophile)
Dimethyl cyclohex-4-ene-cis-1,2-dicarboxylate
1. The Diels–Alder reaction is stereospecific: The reaction is a syn addition, and the configuration of the dienophile is retained in the product
Ch. 13 - 58
O
OMeH
OMe
O
OMe
OH
H
+
Dimethyl fumarate(a trans -dienophile)
Dimethyl cyclohex-4-ene-trans -1,2-
dicarboxylate
HMeO
O
Ch. 13 - 59
2. The diene, of necessity, reacts in the s-cis rather than in the s-trans conformation
s-cis Configuration s-trans Configuration
R
O+
O
R
Highly strained
X
Ch. 13 - 60
e.g.COOMe COOMe
heat+
(diene lockedin s-cis
conformation)COOMe
+ No Reaction
(diene lockedin s-trans
conformation)
heat
Ch. 13 - 61
Cyclic dienes in which the double bonds are held in the s-cis conformation are usually highly reactive in the Diels–Alder reaction
Relative rate
Diene D.A. cycloadduct+ 30oCO
O
O
> >Diene
t1/2 11 sec. 130 sec. 4 h.
Ch. 13 - 62
3. The Diels–Alder reaction occurs primarily in an endo rather than an exo fashion when the reaction is kinetically controlled
H H
H H
R
H
H
Rlongest bridge R is exo
R is endo
Ch. 13 - 63
Alder-Endo Rule●If a dienophile contains
activating groups with p bonds they will prefer an ENDO orientation in the transition state
XX
XX
HH
Ch. 13 - 64
e.g.OO O
O
O
O
HH
+
100% endo
Ch. 13 - 65
Stereospecific reactionX
X
X
X+
X X
X+
X
(i)
Ch. 13 - 66
Stereospecific reaction
+
+
(ii) Y
Y
Y
YY
Y
Y
Y
Ch. 13 - 67
Examples
CN
CN+
MeNC
NC
CNCN
CN
CNMe(A)
D.A.
CN+
NC
Me
MeNC
CN
CNCN
CN
CN
MeMe(B)D.A.
Ch. 13 - 68
Diene A reacts 103 times faster than diene B even though diene B has two electron-donating methyl groupsMe
MeH
Me
Me
(s-cis) (s-trans)
Ch. 13 - 69
Examples
+
(C)
O
O
O
O
H
H
O
O
D.A.
+
(D)
O
O
O
O
H
H
O
O
D.A.
Ch. 13 - 70
Examples
+
(E)
O
O
O
D.A. No Reaction
● Rate of Diene C > Diene D (27 times), but Diene D >> Diene E
● In Diene C, tBu group electron donating group increase rate
● In Diene E, 2 tBu group steric effect, cannot adopt s-cis conformation
Ch. 13 - 71
END OF CHAPTER 13
