CEM 232 rganic Chemistry I at Chicago Lecture 22 rganic Chemistry 1 Professor Duncan Wardrop April 1, 2010 1
Self Test Question Which starting material could not be used to prepare the tribromide below? A B? C D E University of Illinois at Chicago CEM 232, Spring 2010 Slide 2 2
Classification of Dienes isolated diene two p-systems are not adjacent no overlap between 2 p- systems conjugated diene two p-systems are adjacent overlap exists between 2 p- systems C cummulated diene (allene) at Chicago CEM 232, Spring 2010 C central carbon atom is part of two p-systems central carbon = sp p orbitals for each p-system are perpendicular Slide 3 3
Nomenclature isolated diene (2E, 5E)-2,5-heptadiene conjugated diene (2E, 4E)-2,4-heptadiene C cummulated diene (allene) at Chicago CEM 232, Spring 2010 3,4-heptadiene Slide 4 4
Relative Stabilities of Dienes (E)-1,3-Pentadiene is 26 kj/mol more stable than 1,4- pentadiene. Two reasons: more subst. & conjugation. 252-226 = 26 at Chicago CEM 232, Spring 2010 Slide 5 5
Substitution Energy (E)-2-Pentene is disubstituted and is 11 kj/mol more stable than 1-pentene, which is monosubstituted. 126-115 = 11 at Chicago CEM 232, Spring 2010 Slide 6 6
Delocalization Energy Upon hydrogenation of the terminal alkene, the conjugated double bond releases 15 kj/mol less than non-conjugated 126-111 = 15 at Chicago CEM 232, Spring 2010 Slide 7 7
Delocalization Energy Extra stability due to conjugation = conjugation energy or resonance energy or delocalization energy 126-111 = 15 at Chicago CEM 232, Spring 2010 Slide 8 8
Self Test Question List the molecules below in order of increasing heat of hydrogenation. int: some are trienes. A. a,b,c,d,e B. d,e,c,b,a C. a,c,b,e,d D. a,b,c,e,d E. e,d,b,c,a a b c d e University of Illinois at Chicago CEM 232, Spring 2010 Slide 9 9
Bonding in Conjugated Dienes single bonds between p-systems are stronger & shorter: increased s-character = increased attraction of electrons toward carbon nucleus = stronger C-C bonds = shorter bond lengths at Chicago CEM 232, Spring 2010 Slide 10 10
Conformational Analysis of Dienes All groups are at 90º angles. Lowest NRG? You might expect these conformations to be higher in energy since their are more eclipsing relationships. at Chicago CEM 232, Spring 2010 Slide 11 11
Conformations of Conjugated Dienes Prefered conformations are s-cis and s-trans s = single bond conformation between two alkenes at Chicago CEM 232, Spring 2010 Slide 12 12
Conformations of Conjugated Dienes Prefered conformations are s-cis and s-trans When two p-systems are co-planar they overlap and stabilize that conformation (delocalization energy) at Chicago CEM 232, Spring 2010 Slide 13 13
Conformations of Conjugated Dienes Prefered conformations are s-cis and s-trans When two p-systems are co-planar they overlap and stabilize that conformation (delocalization energy) at Chicago CEM 232, Spring 2010 Slide 14 14
s-trans More Stable than s-cis increased steric interactions in s-cis at Chicago CEM 232, Spring 2010 Slide 15 15
Bonding in Allenes central carbon atom is sp hybridized most s-character = stronger C-C bonds than even conjugated alkenes at Chicago CEM 232, Spring 2010 Slide 16 16
Bonding in Allenes planes defined by (C-1) and (C-3) are perpendicular p-orbital of C-1 and p-orbital of C-2 overlap to form -bond p-orbital of C-2 and p-orbital of C-3 overlap to form 2nd -bond allene is nonplanar; two -bonds are perpendicular to each other at Chicago CEM 232, Spring 2010 Slide 17 17
Bonding in Allenes Q: Are allenes stabilized by delocalization energy? A: No. -systems are orthogonal/perpendicular. No overlap, and thus delocalization, is possible. at Chicago CEM 232, Spring 2010 Slide 18 18
Allenes Can Be Chiral no plane or point of symmetry These two allenes are enantiomers: non superimposible mirror images at Chicago CEM 232, Spring 2010 Slide 19 19
Self Test Question A. 2-methyl-2,3-butadiene 3 C C C C 3 C B. 2-methyl-1,4-pentadiene 2 C C 3 C 2 Which of the following dienes is chiral? C. 2,3-heptadiene D. 2,4-dimethyl-2,4-hexadiene E. 1,4-dichloro-1,3-pentadiene C C C 3 C 3 C C 3 Cl C 3 C 3 C 3 Cl University of Illinois at Chicago CEM 232, Spring 2010 C 3 Slide 20 20
CEM 232 rganic Chemistry I at Chicago Chapter 10 Preparation & Reactions of Dienes Sections: 10.12-10.15 21
Preparation of Dienes Dehydrogenation 3 C C 3 590-675 ºC catalyst 2 C C 2 + 2 This is an industrial process that works only when one possible diene can be obtained. Generally, it should not be used in the laboratory (synthesis questions). at Chicago CEM 232, Spring 2010 Slide 22 22
Preparation of Dienes Dehydration KS 4, heat (major) Dehydration is regioselective. The transition state leading to conjugated alkenes is lowest = fastest process. at Chicago CEM 232, Spring 2010 Slide 23 23
Preparation of Dienes Dehydrohalogenation Cl KC(C 3 ) 3 (major) DMS Dehydrohalogenation is also regioselective. The transition state leading to conjugated alkenes is lowest = fastest process. at Chicago CEM 232, Spring 2010 Slide 24 24
Addition of ydrogen alides to Conjugated Dienes 4 3 2 1 1,2-Addition Product (direct addition) + 1,4-Addition Product (conjugate addition) Kinetic product: major product at low temperature Thermodynamic product: major product at high temp. at Chicago CEM 232, Spring 2010 Slide 25 25
Control of X Addition to Conjugated Dienes TS δ δ + TS δ The reaction must be irreversible in order to be under kinetic control. δ + Kinetic Control: low temperatures = irreversible reaction (requirement) fastest forward process predominates lowest TS NRG = fastest reaction = major product here, lowest NRG TS = + on most substituted carbon at Chicago CEM 232, Spring 2010 kinetic product thermodynamic product Slide 26 26
Control of X Addition to Conjugated Dienes The reaction must be reversible in order to be under kinetic control. Thermodynamic Control: high temperatures = reversible reaction (requirement) most stable product predominates reverse process for most stable product = largest Ea here, most stable product = most substituted alkene at Chicago CEM 232, Spring 2010 kinetic product thermodynamic product Slide 27 27
Conditions of Kinetic and Thermodynamic Control 25 ºC 1,4-addition (thermodynamic product) -78 ºC 1,2-addition (kinetic product) at Chicago CEM 232, Spring 2010 Slide 28 28
Addition of X to Conjugated Dienes Addition of X to conjugated dienes is only useful in synthesis when there is only one carbocation intermediate; each resonance form does not count separately 1,2-addition (kinetic product) 1,4-addition (thermod. product) at Chicago CEM 232, Spring 2010 Slide 29 29
Self Test Question What is the kinetic product for addition to the conjugated diene below? C 3 A B C C 3 C 3 C 3 C 3 C 3 C 3 C 3-78 ºC D C 3 C 3 C 3 C 3 University of C 3 C 3 Illinois at Chicago CEM 232, Spring 2010 E C 3 C 3 Slide 30 30
Diels-Alder Reaction Terminology cycloaddition: forming a six-membered ring dienophile: typically an isolated alkene; lover of dienes (conjugated) pericyclic reaction: concerted rxn where transition state is cyclic (ring) Diels-Alder adduct: potentially any molecule with a cyclohexene ring at Chicago CEM 232, Spring 2010 Slide 31 31
Diels-Alder Reaction Diels-Alder reaction is stereospecific (the stereoisomeric product formed depends on the stereoisomer of the reactant). + C 3 C 3 diene dienophile Diels-Alder adduct Substituents that are trans on the dienophile are also trans in the Diels-Alder adduct. at Chicago CEM 232, Spring 2010 Slide 32 32
Diels-Alder Reaction Diels-Alder reaction is stereospecific (the stereoisomeric product formed depends on the stereoisomer of the reactant). + C N C 3 C N C 3 diene dienophile Diels-Alder adduct Substituents that are cis on the dienophile are also cis in the Diels-Alder adduct. at Chicago CEM 232, Spring 2010 Slide 33 33
Diels-Alder Reaction C N C N x C N C 3 C 3 C 3 to form new bonds, p-orbitals of diene overlap with p- orbitals of dienophile both bonds are being formed at the same time cannot rotate around dienophile bond = relationships of groups don t change at Chicago CEM 232, Spring 2010 Slide 34 34
Self Test Question Predict the product. A B 1. NaC 2 C 3 2. (Z)-2-pentene C 1. D + 2. E University of Illinois at Chicago CEM 232, Spring 2010 Slide 35 35
Reactivity of Diels Alder + Ethylene and 1,3-butadiene are unreactive. In other words, they react very slowly. at Chicago CEM 232, Spring 2010 Slide 36 36
Reactivity of Diels Alder EWG + Electron withdrawing groups (EWG) increase the reactivity of the dienophile = faster reaction at Chicago CEM 232, Spring 2010 Slide 37 37
Reactivity of Diels Alder + carbonyl groups Electron withdrawing groups (EWG) increase the reactivity of the dienophile = faster reaction at Chicago CEM 232, Spring 2010 Slide 38 38
Reactivity of Diels Alder 2 N + nitro group Electron withdrawing groups (EWG) increase the reactivity of the dienophile = faster reaction at Chicago CEM 232, Spring 2010 Slide 39 39
Reactivity of Diels Alder N C + nitrile (cyano) group Electron withdrawing groups (EWG) increase the reactivity of the dienophile = faster reaction at Chicago CEM 232, Spring 2010 Slide 40 40
Reactivity of Diels Alder EDG EDG + Electron donating groups (EDG) increase the reactivity of the diene = faster reaction at Chicago CEM 232, Spring 2010 Slide 41 41
Reactivity of Diels Alder C 3 C 3 + methyl group Electron donating groups (EDG) increase the reactivity of the diene = faster reaction at Chicago CEM 232, Spring 2010 Slide 42 42
Reactivity of Diels Alder C 3 C 3 + methoxy group Electron donating groups (EDG) increase the reactivity of the diene = faster reaction at Chicago CEM 232, Spring 2010 Slide 43 43
Reactivity of Diels Alder N 2 N 2 + amino group Electron donating groups (EDG) increase the reactivity of the diene = faster reaction at Chicago CEM 232, Spring 2010 Slide 44 44
Reactivity of Diels Alder + C 3 C 3 Fastest reaction have both an EWG on the dienophile and an EDG on the diene. We ll discover why next week. at Chicago CEM 232, Spring 2010 Slide 45 45
Self Test Question Which of the following dienes or dienophiles would be the least reactive in a Diels-Alder reaction? A B C 3 EWG + EDG EWG EDG C D 2 N CN C 3 E C 3 University of Illinois at Chicago CEM 232, Spring 2010 Slide 46 46
Diels Alder Reaction More complex examples... C 3 + 3 C 3 C C 3 Alkynes can also participate as a dienophile as long as the y are activated with EWGs at Chicago CEM 232, Spring 2010 Slide 47 47
Diels Alder Reaction More complex examples... + C 3 C 3 C 3 C 3 C 3 C 3 Cyclic dienes gives bridged bicylco Diels Alder adducts at Chicago CEM 232, Spring 2010 Slide 48 48
Diels Alder in Synthesis + diene dienophile Molecules with cyclohexene rings can often be made through Diels-Alder reaction at Chicago CEM 232, Spring 2010 Slide 49 49
Synthesis Example C 3 C 3? CN CN C 3 CN C 3 CN + C 3 CN C 3 CN C 3 Na, N 3 C 3 1,3-butadiene C 3 Pd/C, 2 C 3 CN CN CN CN at Chicago CEM 232, Spring 2010 Slide 50 50
Self Test Question Predict the product. A 1. KC(C 3 ) 3 2. 3. Pd/C, 2 B C 1. 2. 3. D University of Illinois at Chicago CEM 232, Spring 2010 Slide 51 51
CEM 232 rganic Chemistry I at Chicago Next Lecture... Chapter 10: Sections 10.16 & 10.17 Chapter 11: Sections 11.1-11.9 Quiz Next Week... Chapter 10 & Synthesis Problems 52
CEM 232 rganic Chemistry I at Chicago Exam Two Monday, April 5 6:00-7:15 p.m. 250 SES Chapters 6-10 (everything!) Makeup Exam: Monday, April 12th, time t.b.a. Makeup policy: There are no makeup exams without prior approval. nly students showing proof of a class conflict will have the option to take a makeup exam. To be added to the makeup list, you must email me no later than Friday, Feb. 12. 53
CEM 232 rganic Chemistry I at Chicago Exam ne Grade Distribution Q1. Ranking (50 points) Q2. Predict the Products (50 points) Q3. Arrow-Pushing Mechanism (50 points) Q4. Nomenclature (20 points) Q5. Drawing & Conformational Analysis (50 points) Q6. Functional Groups (30 points) 54
CEM 232 rganic Chemistry I at Chicago Exam ne Policies Non-scientific calculators allowed only No cell phones, ipods or others electronic devices No molecular models Periodic table will be provided Seating will be assigned ing Your I.D. 55