chemistry 242-002 organic chemistry ii with professor virgil percec tue. and thu. 9:00 am-10:30 am
TRANSCRIPT
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Chemistry 242-002
Organic Chemistry II
with Professor Virgil Percec
Tue. And Thu. 9:00 AM-10:30 AM
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CalendarWeek Class Dates (Tues-Thurs) Class Content:
Solomons,Orgnic Chemistry, 9th Edition
1 Th, Jan 17 (Taught by B. Rosen) Chapters 13
2 T Jan 22(Taught by B. Rosen);Th Jan 24 Chapters 13, 14
3 T Jan 29; Th Jan 31 Chapters 14, 15
4 T Feb 5; Th Feb 7 Chapters 15
Exam 1 – Thursday – Feb 7 Chapters 13, 14, 15
5 T Feb 12; Th Feb 14 Chapters 16
6 T Feb 19; Th Feb 21 Chapters 16, 17
7 T Feb 26; Th Feb 28 Chapters 17, 18
8 T Mar 4; Th Mar 6 Chapters 18
Exam 2 – Tuesday – Mar 4 Chapters 16, 17, 18
Spring Break: March 10-14 no classes
9 T Mar 18; Th Mar 20 Chapters 19
10 T Mar 25; Th Mar 27 Chapters 19, 20
11 T Apr 1; Th Apr 3 Chapters 20, 21
12 T Apr 8; Th Apr 10 Chapters 21
Exam 3 – Tuesday – Apr 8 Chapters 19, 20, 21
13 T Apr 15; Th Apr 17 Chapters 22
14 T Apr 22; Th Apr 24 Chapters 22, 23
15 T Apr 29 Chapters 24, 25
Exam 4 – Tuesday – Apr 29 Chapters 22, 23, 24, 25
Review Session for Final Exam TBA
Final Exam – Tuesday, May 13, 9:00 am – 11:00 am (Schedule from Registrar) Location TBA
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Contact and Other Information
Professor Virgil Percec E-mail:
[email protected] Office: Vagelos Labs
Room 4003 Office hours: T-Th
10:30 am - 12:30 pm or by appointment
Brad Rosen E-mail:
[email protected] Office: Vagelos Labs
Room 4080 Office Hours Thu Jan
17th and Tue Jan 24th 10:30 am – 12:30 pm
Class info on Blackboard: https://courseweb.upenn.edu/Sign-up for Workshops: http://www.penntutoring.info/orgochem/
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Course Policy Text & Other Requirements: (packaged at considerable price
savings): Solomons, Organic Chemistry, 9th Edition (John Wiley & Sons); Solomons, Study Guide and Solutions Manual for Solomons 9th Edition (John Wiley & Sons); Hehre, Shusterman & Nelson; Stull, Science on the Internet; John Wiley & Sons Molecular Model Set for Organic Chemistry. Students must read the assigned chapters before and after lectures for complete understanding of the material. Problem solving is an essential part of the course, and you should always try to do the problems before looking up the answers. Always read questions carefully when solving problems, both in the homework and in the tests.
Recitations & Workshops: teaching assistants and specific rooms are assigned for recitations and workshops and you are encouraged to take advantage of as many of these sessions as you can.
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Some more Course Policy Exams, Grading & Regrading: there will be four exams and one cumulative
final. There are no re-exams and no exams are dropped. However, in case of illness, etc. with an appropriate excuse, given before rather than after the exam, a student may be allowed to miss and reschedule one exam. I expect that A- to A+ will be given for final scores of 80 or 85 to 100% and B- to B+ for final scores of 60 or 70 to 80 or 85%. A very good class is expected to obtain up to 65 or 70% A and B. Regrading must be done within two days from the time the exam is returned. Questions must be directed to the grader in writing. You must not write on your exam in any fashion until after it has been regraded. Mid-Term Exams are scheduled for 5:00 – 7:00 PM in the locations posted above.
Drops, Withdrawals, or Incompletes: the deadlines for dropping or withdrawing must be rigorously observed.
Final grades: Final exams are scheduled by the Registrar’s office. Students missing a final examination must obtain permission to take the make-up exam the following semester (also scheduled by the registrar’s office) from an advisor in the SAS Dean’s office. The organic faculty has adopted a policy of not posting grades. You must obtain your grade by requesting in writing via email.
Please Direct all Questions Regarding Course Policy to Professor Percec
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Lecture 1: Conjugated Unsaturated Systems
Chapter 13 in Solomons 9/e
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What Are Conjugated Unsaturated Systems
Any system where there is a p-orbital adjacent to a double (or triple) bond
allyl radical
allyl cation
1,3-butadiene
Archetypal Conjugated Unsaturated Systems
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Motivation: Synthetic Target
One often desires to make or study Medicinal Natural Products Containing Conjugated Unsaturated Systems
O
OOHHOH3C
O
H3C
H
H
H
Cortisone
CH3
CH3
CH3 CH3 CH3
CH3 CH3
CH3
H3C
CH3
Beta-Carotene
O
OO
H3C
OCH3
OH
O
O
O
O
ON CH3
OHOHH3C
Neocarzinostatin Chromophore
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Motivation: Unique Reactivity
X H Allylic Functionalization
Nu E
E
Nu
Mixture of Enatiomers
+
OOO
O
1,2 Additon
1,4 (Michael) Addition
Diels-Alder 4+2 Cycloadditon
O
O
O
O
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Motivation: Biology Vision
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Allylic Substitution and the Allylic Radical
Recall from Chapter 8: Chlorination of Double Bond
Cl2light or heat
2Cl
Cl
Initiation
Chain Propogation 1
Chain Propogation 2
Cl
Cl Cl Cl Cl
Indeed at low temperature:
X2
CCl4
X
X Addition Reaction
But at high temperature or low X2 Concentration
X2X + HX Allylic Substitution
Cl + Cl H
H
H
H
HH
Allylic Hydrogens
Vinylic Protons
Brief Note on Nomenclature
H
H H
Acetylinic Proton
Propargyl Protons
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Allylic Substitution and the Allylic Radical
Mechanism for Allylic Chlorination
Cl2light or heat
2ClInitiation
Chain Propogation 1
Chain Propogation 2
HH
H
Cl
Allylic Radical
+ HCl
Cl Cl Cl+ Cl
Why Allyl versus Vinyl or Alkyl Substitution ?
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Allylic versus Vinylic Substitution
Allylic Proton is easier to homolyze by 96 kJ/mol
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Allylic Bromination
OO N
Br
N-bromo-succinimide
light or ROOR OO N+Br Initiation
H+ Br + HBr
Br BrBr +Br
Propogation 1
Propogation 2
OO N
Br
+ HBr OOHN
+ Br2Propogation 3
O
O
N BrH +
General Reaction:
Mechanism:
Br + OOHNlight or ROOR
CCl4
Note: N-chloro-succinimide and N-iodo-succinimide exist and react in a similar way
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Rules of Resonance
1) The most important rule of resonance is that resonance structures are not real. They are merely a tool for rationalizing chemical behavior. We will revisit this in terms of the allylic and other conjugated systems.
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Rules of Resonance
2) In resonance we move only electrons, not atoms. And when we do it is usually π electrons.
H3C CH CH CH2 H3C CH CH CH2 H2C CH2 CH CH2
O OH
O
H
H
B-
Processes which involve “resonance” of atoms such as keto-enoltautomerization (Chapter 17) are true chemical equilibria with where each Isomer truely exists in solution
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Rules of Resonance
3) All resonance structures must be true Lewis Structures (Chapter 1.5)
4) Resonance structures must have the same
number of unpaired electrons.
1 unpaired electron 3 unpaired electron
These are not in resonsance, as they are systems in different spin states.
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Rules of Resonance
5) Another very important rule is that systems in resonance need to be coplanar.
transoid 1,3 Butadiene coplanarand in resonance
However 2,3-di-tert-butyl-1,3-Butadiene is twisted out of planeand is not in resonance
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Rules of resonance
6) For reason which will be explained shortly, the energies of structures in resonance are always lower than those of their prototypical resonance forms.
7) Equivalent resonance structures make equivalent contributions to energies of the resonating compound
8) The more stable the resonance structure the larger its contribution
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Assessing Resonance Structure Stability
1) The more covalent bonds the better the structure.
2) The more complete valance shells the better the structure.
3) The less charge separation the better.
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Source of Allylic Radical Stability: Resonance
H
H
H
H
HH
H
H
H
H
H
H
H
H
H
1/2 1/2H
H
H
H
H
or
H
H
H
H
HResonance in the Allyl Radical 1
2
3
One explanation for the peculiar stability of the Allyl Radical is through implications of resonance.
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Allyl Cation also Stabilized via Resonance ?
H
H
H
H
HH
H
H
H
H
H
H
H
H
H
1/2 1/2H
H
H
H
H
or
H
H
H
H
HResonance in the Allyl Radical 1
2
3
Indeed according to suggested stability via resonance , the allyl cation is unusually stable
CC
CC
>C
C
C
C> C
CC
H
H>
CC
C
H>
CC
H
H>
H
H H
Substuted allylic > 3° > Allyl > 2° > 1° > Vinyl
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Resonance Structures are Just A Tool
Keep in mind that resonance structures do not really exist.
Resonance Structures allow a chemist to quickly ascertain stabilities and relativities of compounds from their line drawings.
More accurate energies and electron distributions require computational chemistry.
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Molecular Orbital Description of Ethene
E=α+β
E=α-β
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The Molecular Orbital (MO) Approach to the Allyl System
E=α
E=α-1.41β
E=α+1.41β
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Polyunsaturated Systems:Nomenclature and Classification
H2C C CH2
Conjugated Poly Unsaturated Systems
buta-1,3-diene
(E)-penta-1,3-diene
(2E,4E)-hexa-2,4-diene
(2Z,4E)-hexa-2,4-diene
(2E,4E,6E)-octa-2,4,6-triene
cyclohexa-1,3-diene
1 3
pent-1-en-4-yne
cyclohexa-1,4-diene
propa-1,2-diene
Non-conjugated Poly Unsaturated Systems
Cumulated Diene
aka allene
penta-1,4-diene
14
1
4
CH2 n
AKA isolated dienes
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Molecular Orbital Description of 1,3 Butadiene
E=α+β
E=α-β
E=α+1.62β
E=α+0.62β
E=α-0.62β
E=α-1.62β
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Conformations of 1,3 Butadiene
s-trans-1,3-butadienes-cis-1,3,butadiene
free rotation
lower energyrequired for certainreaction
1.34 Å1.47 Å
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Stability of Conjugated Dienes
Conjugative Stability
CompoundEquivalents of Hydrogen Added
ΔHº (kJ/mol) Conjugative Stability
1-Butene 1 -127
1-Pentene 1 -126
trans-2-Pentene 1 -115
1,3 Butadiene 2 -239 15
trans-1,3-Pentadiene 2 -226 15
1,4-Pentadiene 2 -254
1,5-Hexadiene 2 -253
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UV-Vis Spectroscopy
Beer’s LawA=ε x c x l = log (Io/I)ε=extinction coefficient/molar absorptivityc=concentrationl= path length
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UV-VIS of Extended trans π-systems
Compound
ethene
1,3-butadiene
1,3,5-hexatriene
1,3,5,7-octatetraene
1,3,5,7,9-decapentaene
1,3,5,7,9,11-dodecahexaene
165
217
256
290
334
364
15,000
21,000
50,000
85,000
125,000
138,000
max
Relationship Between Number of Conjugated Double Bonds and λ max
y = 39.429x + 133
150
200
250
300
350
400
1 2 3 4 5 6
Relationship of Number of Conjugated Double Bonds to Extinction Coefficient
10000
30000
50000
70000
90000
110000
130000
150000
1 2 3 4 5 6
All π π* transitions
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Electrophillic Attack on Conjugated Dienes
Cl
ClHCl
25 °C1,3 butadiene 3-Chloro-1-butene 1-Chloro-2-butene (78%) (22%)
HCl
25 °C
Cl
H (1,2 Addition)
HCl
25 °C ClH (1,4 Addition)
Expected Markovnikov Product
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Mechanism of Electrophillic Addition to Conjugated Dienes
HCl +
allyl cation
Step 1:
Step 2:
Cl
Cl+ b
a Cl
H (1,2 Addition)
ClH (1,4 Addition)
a
b
HCl +
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Comparison of 1,4 AdditionCl
ClHCl
25 °C1,3 butadiene 3-Chloro-1-butene 1-Chloro-2-butene (78%) (22%)
Br
BrHBr
40 °C1,3 butadiene 3-Bromo-1-butene 1-Bromo-2-butene (80%) (20%)
Br
BrBr2
-15 °C1,3 butadiene 1,2-dibromo-1-butene 1,4-dibromo-2-butene (54%) (46%)
Br Br
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Kinetic versus Thermodynamic Control
Kinetic Control
Thermodynamic Control
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Diels-Alder Reaction:The Basics
O
O
O
Maleic anhydide(dienophile)
s-cis-1,3-butadiene(diene)
O
O
O
Simplest Diels Alders Reaction
200 C
sealed tube(20%)
H3C
H3CO2C CH3
CH3
Cl CN
Works Better with Electron Rich Dienes and Electron Poor Olefins
H3C CH3ClCN
H3C
MeO2C130 °C
85%
S-cis diene required, s-trans does not work