Preparation of alkenes

Transcription

1 Lecture 11 אלקנים הכנה ותגובות של אלקנים: הידרוגנציה, סיפוח הידרוהלוגנים )כלל מארקובניקוב(, סיפוח הלוגנים והסטראוכימיה של תוצרי הסיפוח, הידרובורציה, אפוקסידציה, אוזונוליזה. 1

2 Preparation of alkenes 1. Dehydrohalogenation of alkyl halides (E 1 and E 2 ). Good for 2⁰ and 3⁰ alkyl halides. For 1⁰ S N 2 is preferred. Example E 2 2. Dehydration of alcohols (E 1 ) (loss of water). Requires an acid catalyst (mostly sulfuric and phosphoric acid) and heat. Zaitsev s rule is valid. Acid protonates the most basic (electron-rich) atom in the molecule. Protonation converts the very poor leaving group (OH-) into a good leaving group (H 2 O). 2

3 REACTIONS OF ALKENES Electrophilic Addition C C single bond: 83 kcal/mol C = C double bond: 146 kcal/mol D = 63 kcal/mol Relatively loosely held p electrons Electrons enriched region of the carbon carbon double bond attracted to E + Reaction starts with the addition of an electrophile to one of the sp 2 carbons of the alkene and concludes with the addition of a nucleophile to the other sp 2 carbon 3

4 H 3 C C H 3 OH Alcohols Ethers O CH 3 H 3 C Br Alkyl halides alkenes Aldehydes and Ketones Alkanes H 3 C CH 3 C H 2 O 4

5 1. Addition of Hydrogen Halides ) אלקטרופילי )סיפוח - HF, HCl, HBr, HI 5

6 Regioselectivity of the Addition Symmetric alkenes Asymmetric alkenes? Major product 6

7 Mechanism of the reaction RDS 7

8 Asymmetric alkenes Is more stable carbocation also formed more rapidly? Hammond postulate - the transition state is more similar in structure to the species to which it is more similar in energy. More stable carbocation is formed faster 8

9 Regioselectivity of the Addition regioselective Regioselective reaction - reaction in which two or more constitutional isomers could be obtained as products, but one of them predominates. not regioselective Markovnikov s rule halogen attaches itself to the carbon atom bearing the lesser number of hydrogen atoms or hydrogen adds the sp 2 carbon that is bonded to the greater number of hydrogens or electrophile adds to the sp 2 carbon that is bonded to the greater number of hydrogens 9

10 Stereochemistry One asymmetric carbon Carbocation is planar and attacked by Nu- from both sides, leading to two stereoisomers 10

11 Stereochemistry Two asymmetric carbons The two transition states will not have the same stability, so different amounts of the two diastereomers will be formed. 11

12 2. Addition of Water and Alcohols Addition of Water - hydration K w = 1x10-14 Acid-catalyzed addition of water H 2 SO 4 pka = -2 HCl pka = -5 Catalyst - species that increases the rate of a reaction and is not consumed during the course of the reaction. Catalysts increase the reaction rate by decreasing the activation energy of the reaction. 12

13 Addition of water - Mechanism for the acid-catalyzed reaction Markovnikov addition reaction Rearrangement of Carbocations catalyst Markovnikov addition reaction 13

14 Addition of Water and Alcohols - reversible Equilibrium can be driven toward the alcohol: low reaction temperatures large excess of water 14

15 Addition of Water and Alcohols - reversible The reversibility of alkene protonation leads to alkene equilibration 15

16 Rearrangement of carbocations in electrophilic addition reactions 1,2-hydride shift 16

17 1,2-methyl shift Rearrangement of carbocations in electrophilic addition reactions A shift involves only the movement of a species from one carbon to an adjacent carbocation 17

18 Addition of Water and Alcohols - Stereochemistry OH H 2 O OH Carbocation is planar and attacked by Nu- from both sides, leading to two stereoisomers 18

19 3. Addition of Halogens bond between two halogen atoms is relatively weak 19

20 Bromination Mechanism

21 Bromination Mechanismc First step bromonium ion formation electron-deficient region (the blue area) encompasses the carbons All the atoms in the bromonium ion have complete octets No carbocation formed no rearrangements Second step - Br - attacks the carbon releases the strain in the three-membered ring 21

22 Anti addition Bromination takes place through anti addition 22

23 Chlorination Mechanism, anti addition H 3 C Cl + Cl- H 3 C No carbocation formed no rearrangements Chlorination takes place through anti addition 23

24 F 2 and I 2 not used! Fluorine reacts explosively with alkenes The addition of I 2 to an alkene is a thermodynamically unfavorable reaction: The diiodides are unstable at room temperature, decomposing back to the alkene and I 2 24

25 4. Halohydrins formation - Trapping by other nucleophiles Water as a solvent A halohydrin (or more specifically, a bromohydrin or a chlorohydrin) is an organic molecule that contains both a halogen and an OH group. 25

26 Mechanism for halohydrin formation H 2 O is the solvent, its concentration far exceeds that of Br- Regioselectivity Breaking of the C-Br bond has occurred to a greater extent than has the formation of the C-O bond, there is a partial positive charge on the carbon that is attacked by the nucleophile Anti addition of Br and OH to the double bond The electrophile adds to the carbon that is bonded to the greater number of hydrogens 26

27 Trapping by other nucleophiles Act as Nu- 27

28 5. Oxymercuration and Alkoxymercuration Reduction carbocation intermediates are not formed, carbocation rearrangements do not occur acidic conditions (in acid catalyzed water addition) are harmful to many organic molecules Oxymercuration Alkoxymercuration 28

29 Mechanism for oxymercuration STEP 1. Dissociation STEP 2. Electrophilic attack STEP 3. Nucleophilic opening (Markovnikov regioselectivity) carbocation rearrangements do not occur Water attacks the more substituted carbon of the mercurinium ion, attacking at the more substituted carbon leads to the more stable transition state 29

30 Mechanism for reduction Reduction reaction - A reaction that increases the number of C-H bonds or decreases the number of C-O, C-N, or C-X bonds in a compound. The mechanism of the reduction reaction is not fully understood, the intermediate is a radical. 30

31 Regio- and Stereochemistry Oxymercuration is anti stereospecific and regioselective 31

32 6. Addition of Borane: Hydroboration Oxidation 32

33 Mechanism of Hydroboration Borane has only six shared electrons in its valence shell, serving as the electrophile. syn addition All three B H bonds are reactive in this way. Hydroboration is stereospecific (syn addition) and regioselective (boron binds to the less substituted carbon) 33

34 Mechanism of Hydroboration Why does boron add preferentially to the carbon bonded to the greater number of hydrogens? All three B H bonds are reactive The boron binds to the less hindered (less substituted) carbon carbocation-like transition state Most stable carbocation-like transition state Steric effects 34

35 Oxidation of alkylboranes X 3 rearrangement (alkyl group migrates with its electron pair) with retention of configuration 35

36 Addition of Borane: Hydroboration Oxidation anti-markovnikov addition Carbocation rearrangements do not occur Markovnikov addition 36

37 7. Radical addition reactions anti-markovnikov addition of HBr Markovnikov addition (ROOR) anti-markovnikov addition 37

38 anti-markovnikov addition of HBr - mechanism creating the relatively more stable radical 38

39 Radical formation 39

40 No anti-markovnikov addition of HCl and HI (!?) Only Markovnikov addition 40

41 No anti-markovnikov addition of HCl and HI (!?) In a radical reaction, the steps that propagate the chain reaction compete with the steps that terminate it. Termination steps are always exothermic. Only when both propagation steps are exothermic can propagation compete with termination With HCl or HI in the presence of a peroxide, any chain reaction that is initiated is terminated rather than propagated because propagation cannot compete with termination 41

42 REACTIONS OF ALKENES oxidation reactions 42

43 8. REACTIONS OF ALKENES Epoxidation The transfer of oxygen is stereospecifically syn Stereochemistry of the starting alkene being retained in the product 43

44 Stereochemistry meso compound pair of enantiomers cis alkene forms a cis epoxide trans alkene forms a trans epoxide 44

45 Cyclic transition state Epoxidation mechanism electrophilic oxygen is added to the p bond peroxycarboxylic acid proton is transferred to its own carbonyl group releasing a molecule of carboxylic acid 45

46 9. REACTIONS OF ALKENES Vicinal Syn Dihydroxylation The reactions occur because manganese and osmium are in a highly positive oxidation state and, therefore, attract electrons syn addition 46

47 10. REACTIONS OF ALKENES Ozonolysis 47

48 REACTIONS OF ALKENES Mechanism of Ozonolysis Explosive, not isolated molozonide indicates that one mole of ozone has added to the alkene, unstable because it has two O-O bonds 48

49 REACTIONS OF ALKENES Examples of Ozonolysis 49

50 11. REACTIONS OF ALKENES Catalytic Hydrogenation 50

51 Catalytic Hydrogenation mechanism The major function of the catalyst is the activation of hydrogen to generate metal-bound hydrogen on the catalyst surface syn addition 51

52 Electrophilic Additions Hydrohalogenation Hydration Halogenation Vicinal Haloalcohol and Haloether Synthesis Hydroboration Oxidation Oxymercuration Demercuration 52

53 Oxidation Epoxidation Radical Additions Radical Hydrobromination Vicinal Syn Dihydroxylation Reduction Catalytic Hydrogenation Ozonolysis 53