Chapter16 Chem 241b
-
Upload
logan-richardson -
Category
Documents
-
view
229 -
download
0
Transcript of Chapter16 Chem 241b
-
7/30/2019 Chapter16 Chem 241b
1/45
"You cannot reason a personout of a position he did not
reason himself into in the first
place." Jonathan Swift
-
7/30/2019 Chapter16 Chem 241b
2/45
Chapter 16
Conjugated double bonds
Lycopene (red dye in tomatoes, berries)
NH
HN
O
O
Indigo (Blue in Blue Jeans)
NN
NN
OO
MeO
H3C
H3C CH3
CH3H3C
OO
R
Mg
Chlorophyll
NN
NN
H3C
CH3
CH3
Fe
H3C
CO2H
HO2C
Heme
2,6,10,14,19,23,27,31-Octamethyl-dotriaconta-2,6,8,10,12,14,16,18,20,22,24,26,30-tridecaene
NH
HN
O
O
Tyrian Purple (Mollusks)
Br
Br
-
7/30/2019 Chapter16 Chem 241b
3/45
3
Conjugated Alkenes
Conjugated
Not Conjugated
Red Shift in electronic spectra More stable than corresponding non-conjugated mole
sp2-sp2s bonds are shorter and stronger than sp3-sp2
CO2H
H
H
HH
Linolenic Acid
CO2H
5.5 kJ/mole more stable than linolenic acid (belo
-
7/30/2019 Chapter16 Chem 241b
4/45
4
Identifying Conjugated Organics
O
NN
1,3-cyclohexadiene
1,4-cyclohexadienebenzene
cyclopentadiene1,3-butadiene1,4-pentadiene 1,3-pentadiene
1,2-diphenylethane 1,2-diphenylethyne trans-1,2-diphenylethene trans-azobenzene
allylbenzeneor 3-phenylpropeneor 3-propenyl benzene
1-propenylbenzenecyclohexa-2,5-dienone3-methylenecyclohexene
-
7/30/2019 Chapter16 Chem 241b
5/45
Conjugated C=C Bonds and Electronic Spectra
-
7/30/2019 Chapter16 Chem 241b
6/45
Conjugated Alkenes
Crocetin (from Saffron; 422 nm)
HO2CCO2H
Lycopene (505 nm)NH
HN
O
O
Indigo Blue (605 nm)
430 nm 500 nm
560 nm
580 nm
400 nm
420 nm
465nm
530 nm
600 nm620 nm
640 nm
680 nm
720 nm
NN
NN
OO
MeO
H3C
H3C CH3
CH3H3C
OO
R
Mg
Chlorophyll a (675 nm)
-
7/30/2019 Chapter16 Chem 241b
7/45
Structure Determination in Conjugated
Systems: UV Spectroscopy The electrons in the highest occupied molecular orbital (HOMO) undergo a transition to
the lowest unoccupied molecular orbital (LUMO)
-
7/30/2019 Chapter16 Chem 241b
8/45
Conjugated Alkenes
430 nm 500 nm
560 nm
580 nm
400 nm
420 nm
465nm
530 nm
600 nm
620 nm
640 nm680 nm
720 nm
# Double Bonds max
1
2
3
4
5
6
11
177 nm
224 nm
275 nm
310 nm
342 nm
380 nm
455 nm
-
7/30/2019 Chapter16 Chem 241b
9/45
Butadiene: Molecular Orbitals
4 p-orbitals
HHH
H
H
HH
H
H
H
H
30 kJ/mole
1
2
3
4
1,3-Butadiene Ethylene
185 nm215 nm
-
7/30/2019 Chapter16 Chem 241b
10/45
Conjugated C=C Bonds and Electronic Spectra
-
7/30/2019 Chapter16 Chem 241b
11/45
430 nm 500 nm
560 nm
580 nm
400 nm
420 nm
465nm
530 nm
600 nm620 nm
640 nm
680 nm
720 nm
-
7/30/2019 Chapter16 Chem 241b
12/45
Conjugation, Color and the Chemistry
of Vision b-Carotene is converted to Vitamin A, which is converted to
11-cis-retinal:
-
7/30/2019 Chapter16 Chem 241b
13/45
Conjugation, Color and the Chemistry
of Vision 11-cis-retinal is converted to rhodopsin in the rod cells of the
retina.
Visual pigments are responsible for absorbing light in eye and
triggering nerves to send signal to brain
-
7/30/2019 Chapter16 Chem 241b
14/45
O
OH
H2N
4-Aminobenzoic acid
PABA
HO
NN
N
HO NN
N
2,2_-methanediylbis[6-(2H-benzotriazol-2-yl)-4-(2,4,4-trimethylpentan-2-yl)phenol]
Bisoctrizole
O
O
N
2-ethylhexyl 2-cyano-3,3-diphenyl-2-propenoate
Octocrylene
O O
O
1-(4-Methoxyphenyl)-3-(4-tert-butylphenyl)propane-1,3-dione
Avobenzone
SUNSCREENS
-
7/30/2019 Chapter16 Chem 241b
15/45
Special Properties of Conjugated
Dienes
Length of the central single bond is shorter than non-
conjugated similar molecule
Comparison of 1,3-Butadiene and Butane with C-13
NMR
H2C=CH-CH=CH2 CH3-CH2-CH2-CH3
148 pm 153 pm
1,3-Butadiene Butane
Shorter
Bond
-
7/30/2019 Chapter16 Chem 241b
16/45
Why more stable (thermodynamics) than non-
conjugated?
p-p overlap is better than hyperconjugation
1
2
3
4
Conjugated
LUMO
HOMO
LUMO
HOMO
Isolated
-
7/30/2019 Chapter16 Chem 241b
17/45
1
2
3
4
Conjugated
LUMO
HOMO
LUMO
HOMO
Isolated
More reactive (kinetics) than non-conjugated
LUMO is lower
moreelectrophilic
HOMO is
highermorenucleophilic
-
7/30/2019 Chapter16 Chem 241b
18/45
Br
H
Br
H
HBr
H
Br
Br
H
Ea
Ea
HBr
H
Br
H
Br
HBr
Faster reactions, less G for reactions (start lower)
-
7/30/2019 Chapter16 Chem 241b
19/45
1
2
3
4
Conjugated
LUMO
HOMO
LUMO
HOMO
Isolated
30 kJ/mole barrier to rotation
Adds Double bond character to C2-C3
Substracts Double bond character to C2-C3
-
7/30/2019 Chapter16 Chem 241b
20/45
Diene Preparation
H H
Cyclohexene 3-
Bromocyclohexene
1,3-
Cyclohexadiene
NBS
CCl4
Br+K -OC(CH3)3
HOC(CH3)3
Based Induced Elimination of HX
-
7/30/2019 Chapter16 Chem 241b
21/45
Electrophilic Additions to
Conjugated Dienes: Allylic
Carbocations
Electrophilic addition to 1,3-Butadiene yields a
mixture of two products: 1,2 addition
1,4 addition
-
7/30/2019 Chapter16 Chem 241b
22/45
Non-conjugated alkene addition
reactions
CH3
C+
CH3
CH3
HCl
EtherCH3
C
CH3
CH2
CH3
C
CH3
CH3
Cl
2-
Methylpropene
Tertiary
Carbocation2-Chloro-2-
methylpropane
CH2
CH
CH
2
CH
CH2
HCl
EtherCH
3CH
CH
2
CH
CH3
Cl Cl
-
7/30/2019 Chapter16 Chem 241b
23/45
At room temperature, electrophilic addition
to a conjugated diene leads to a product
mixture where the 1,2 adduct predominates
over the 1,4 adduct.
At high temperatures, the product ratio
changes and the 1,4 adduct predominates
CH2
CH
CH
CH2 + HBr CH2 C
HCH
CH3
Br
CH3
CH
CH
CH
2
Br+
1,2 adduct 1,4 adduct
At 0oC: 71% 29%
At 40oC: 15% 85%
-
7/30/2019 Chapter16 Chem 241b
24/45
Reacting Butadiene with
ElectrophilesH Cl
HH
H
HH
HCl-
Cl Cl
1
2
3
Non-Bonding
Anti-Bonding
Bonding
1/2 1/2or
-
7/30/2019 Chapter16 Chem 241b
25/45
Allyl Cations
-
7/30/2019 Chapter16 Chem 241b
26/45
Allylic and Benzylic Cations
HCl
Cl
Ether
H
H
H
HBr
Br
Br
majorminor
H
H
H H
-
7/30/2019 Chapter16 Chem 241b
27/45
Diene polymerizations
-
7/30/2019 Chapter16 Chem 241b
28/45
-
7/30/2019 Chapter16 Chem 241b
29/45
Vulcanization:
-
7/30/2019 Chapter16 Chem 241b
30/45
Diels-Alder Reaction
O
O
O
O
O
O
Electron pooralkene
Electron richdiene
Net: 2 Pi bonds converted to sigma bondsin cycloaddition reaction yielding a six-membered ring
RT
-
7/30/2019 Chapter16 Chem 241b
31/45
Diels-Alder Reaction
Dienes
DienophilesN
O
O
R
O
OMe
O
OMe
O
Me
O
Me
CN CN
NCO
OMe
O
MeO
CO2Me
O
O
O
R3SiO
OMe
O
-
7/30/2019 Chapter16 Chem 241b
32/45
Diels-Alder Reaction:Diene Reactivity
s-cis"cis-like" arrangement about
the middle carbon-carbon singlebond
s-trans"trans-like" arrangement about
the middle carbon-carbon singlebond
NO Diels-AlderDiels-Alder Possible
Generally more reacti
than acyclic dienes
H
H
H
H
H
H
HH
s-cis
H
H
HH
s-trans
No Reaction
Sterically driven to s-transconformation
Locked in s-trans conformation
-
7/30/2019 Chapter16 Chem 241b
33/45
Diels-Alder Reaction
H
O
O
O OO
O
H
H
H
H
O
H
H O
O
ONLY SYN ADDITION
O
O
OH
O
H
H O
O
O
O
OH
H
TWO MODES OF APPROACH
No way of telling which approach is used with this syst
-
7/30/2019 Chapter16 Chem 241b
34/45
Diels-Alder Reaction
H
O
O
O OO
O
H
H
H
H
O
H
H O
O
ONLY SYN ADDITIONBUT ONLY THE ENDO (Bottom) DIASTEREOMER FORMSAT ROOM TEMP (KINETIC PRODUCT)
O
O
OH
O
H
H O
O
O
O
OH
H
TWO MODES OF APPROACH
O
H
H O
O
O
H
H O
O(R)
(S)(R)
(S)(R)
(S)
(R)
(S)
(R)
(S)
(R)
(S)
(R)
(S)
(R)
(S)
Meso
Meso
ENDO
EXO
RoomTemperature
200 C
-
7/30/2019 Chapter16 Chem 241b
35/45
Diels-Alder Reaction
O
O
O
Ea
GO
H
H O
O
O
O
OHH
EXO
ENDO
O
O
O H
H
O
H
H O
O
O
O
O
Ea
G
Stability of exo product is greater
Higher temperature is required
to reach product for exo path
O
H
H O
O
O
H
H O
O
ENDO EXO
200 C
Running reaction at higher temp
gives thermodynamic product (e
Running reaction at room temp
gives kinetic product (endo)
-
7/30/2019 Chapter16 Chem 241b
36/45
Diels-Alder Reactions
Me3SiO
OMeMe
O
Me OMeO
Me3SiOH
Me O
OH
1) Heat
2) Aqueousacid
O
OMe
H
H
O
OMe
H
H
CO2MeH
CN CN
NC
CNCN
CN
-
7/30/2019 Chapter16 Chem 241b
37/45
Diels Alder in Synthesis
Look for cyclohexene = product of DA
H
H
O
O
OH
H
O
O
O
S id S h i
-
7/30/2019 Chapter16 Chem 241b
38/45
38
The key DielsAlder reactions used to prepare the C ring of
estrone and the B ring of cortisone are as follows:
Steroid Synthesis
-
7/30/2019 Chapter16 Chem 241b
39/45
Diels-Alder Reactions
190 C
RT
CO2Me
Me
Me MeMe
Me
Me
CO2Me
CO2MeCO2Me
Me
Me
MeO2C
Me
Me
CO2Me
CO2Me CO2MeMe
Me MeO2C
Me
Me
CO2Me
-
7/30/2019 Chapter16 Chem 241b
40/45
Diels-Alder ReactionsO O
H
H
H
CN
NC
O
O CN
CN
H
N
O
O
OSi
Me Me
O
O O
OSi
O
MeMe
N
O
O
Room Temperature
> 90 CN
O
O
OSi O
MeMe
N
O
O
O
OSi
Me Me O
O
Glassy Polymer
Viscous Liquid
-
7/30/2019 Chapter16 Chem 241b
41/45
Diels-Alder Reactions
N
NN
O
O
R
N
N N
O
O
R
H
H
-
7/30/2019 Chapter16 Chem 241b
42/45
Diels-Alder Reactions
O
Benzene
155 C O
H
H
O
O
O
O
-CO
Diels-AlderPolymer
-
7/30/2019 Chapter16 Chem 241b
43/45
NMR of Dienes
01234567
PPM
2.15
2.15
5.80
5.90
5.90
5.80
020406080100120
PPM
22.1
22.1
124.7
125.8
125.8
124.7
-
7/30/2019 Chapter16 Chem 241b
44/45
NMR of Dienes
01234567
PPM
2.05
2.05
6.03
6.035.61
5.61
H
HH
H
020406080100120140
PPM
126.9
130.3
130.3
126.9
19.3
19.3
-
7/30/2019 Chapter16 Chem 241b
45/45
Need to know
Diene nomenclature
Recognize conjugated versus non-conjugated systems
Know reaction and mechanism:
-addition of Br2 or Cl2 to dienes
-addition of HCl or HBr to dienes-Diels Alder cycloaddition (exo and endo)
Understand kinetic versus thermodynamics products for
diene reactions and Diels Alder cycloaddition reactions
Be able to synthesize dienes from alkanes throughbromination and elimination reactions.
Extra special: predict regiochemistry of Diels Alder
cycloadditions
45