Lecture 22 Organic Chemistry 1

Transcription

1 CEM 232 rganic Chemistry I at Chicago Lecture 22 rganic Chemistry 1 Professor Duncan Wardrop April 1,

2 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

3 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

4 Nomenclature isolated diene (2E, 5E)-2,5-heptadiene conjugated diene (2E, 4E)-2,4-heptadiene C cummulated diene (allene) at Chicago CEM 232, Spring ,4-heptadiene Slide 4 4

5 Relative Stabilities of Dienes (E)-1,3-Pentadiene is 26 kj/mol more stable than 1,4- pentadiene. Two reasons: more subst. & conjugation = 26 at Chicago CEM 232, Spring 2010 Slide 5 5

6 Substitution Energy (E)-2-Pentene is disubstituted and is 11 kj/mol more stable than 1-pentene, which is monosubstituted = 11 at Chicago CEM 232, Spring 2010 Slide 6 6

7 Delocalization Energy Upon hydrogenation of the terminal alkene, the conjugated double bond releases 15 kj/mol less than non-conjugated = 15 at Chicago CEM 232, Spring 2010 Slide 7 7

8 Delocalization Energy Extra stability due to conjugation = conjugation energy or resonance energy or delocalization energy = 15 at Chicago CEM 232, Spring 2010 Slide 8 8

9 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

10 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

11 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

12 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

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 13 13

14 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

15 s-trans More Stable than s-cis increased steric interactions in s-cis at Chicago CEM 232, Spring 2010 Slide 15 15

16 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

17 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

18 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

19 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

20 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

21 CEM 232 rganic Chemistry I at Chicago Chapter 10 Preparation & Reactions of Dienes Sections:

22 Preparation of Dienes Dehydrogenation 3 C C ºC catalyst 2 C C 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

23 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

24 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

25 Addition of ydrogen alides to Conjugated Dienes ,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

26 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

27 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

28 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

29 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

30 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

31 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

32 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

33 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

34 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

35 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

36 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

37 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

38 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

39 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

40 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

41 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

42 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

43 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

44 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

45 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

46 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

47 Diels Alder Reaction More complex examples... C 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

48 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

49 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

50 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

51 Self Test Question Predict the product. A 1. KC(C 3 ) Pd/C, 2 B C D University of Illinois at Chicago CEM 232, Spring 2010 Slide 51 51

52 CEM 232 rganic Chemistry I at Chicago Next Lecture... Chapter 10: Sections & Chapter 11: Sections Quiz Next Week... Chapter 10 & Synthesis Problems 52

53 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 me no later than Friday, Feb

54 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

55 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