Part C- section 1 p-bonds as nucleophiles

Transcription

1 Part C- section 1 p-bonds as nucleophiles

2 Chemistry of Alkenes (Ch 8, 9, 10) - the double bond prevents free rotation - isomerism cis and trans - nomenclature E and Z (3 or 4 different substituents around the double bond) F Cl 2

3 Nomenclature 1. Select the longest chain and replace the suffix ane with ene 2. Start numbering the chain at the end nearer to the double bond 3. Designate the location of the double bond C 2 =CC 2 C 3 C 3 C=CC 2 C 2 C 3 1-butene 2-hexene (not 3-butene) (not 4-hexene) 4. Indicate the location of substituents by the number of the C atoms C 3 C 3 C 3 C 3 C 3 C 3 C=CC 3 C 3 C=CCC 3 C 3 C=CC 2 C-C methyl-2-butene C 3 2,4-dimethyl-2-pentene 2,5,5-trimethyl-2-hexene 3

4 Nomenclature 5. Number susbtituted cycloalkenes in the way that C atoms at the double bonds have 1 and 2 positions and that substituents have lower # at the first point of difference methylcyclopentene (not 2-methylcyclopentene ,5-dimethylcyclohexene not 4,6-dimethylcyclohexene) 6. If an alcohol group is present give the lowest possible # to it. C 3 C 3 C=CCC 3 4-methyl-3-penten-2-ol 4

5 7. Decide about cis and trans or E and Z Two «common» names: C 2 =C- vinyl C 2 =CC 2 - allyl 5

6 E and Z Cl F Z = heavier groups on the same side (Z)-2-bromo-1-chloro-1-fluoroethene E = heavier groups on the two opposite sides Cl F (E)-2-bromo-1-chloro-1-fluoroethene 6

7 (E)-1-bromo-1-choro-1-pentene Cl C 2 C 2 C 3 Cl (E)-2-bromo-1-chloro-1-iodo-1-butene I C 2 C 3 3 C C 2 C(C 3 ) 2 (Z)-3,5-dimethyl-2-hexene C 3 7

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10 Evaluating alkene relative stability 10

11 trans more stable than cis (release of steric hindrance) By using either heat of combustion or heat of hydrogenation It is possible to demonstrate that : The greater # of groups to the double bond the greater is the alkene stability > > > > > > Electron donating ability of alkyl groups to satisfy the increased electronegativity of sp 2 C atoms 11

12 Cycloalkenes Always cis till cycloheptene cis-cyclooctene trans-cyclooctene 12

13 Additions to Alkenes -X X -S 3 S 3 - X-X X X 13

14 Additions to Alkenes + X-Y p-bond s-bond two s-bonds X Y bonds broken eactions usually hexothermic bonds formed Electron-rich region Susceptible of electrophilic attack 14 Electron-rich region

15 X- X X Electrophiles are: Lewis Acids 15

16 Markovnikov s rule bservation: C atom with the greater # of 16

17 Markovnikov s rule Explanation: 3 0 carbocation - t-butyl bromide!! 17

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19 General formulation: In the ionic addition of an asymmetrical reagent to a double bond, the positive portion of the reagent bonds in a way which generates the most stable carbocation. +d -d I-Cl I 3 0 carbocation Cl I Cl These reactions are called: regioselective 19

20 Stereochemistry of the addition reaction X Et-C = C 2 -X Et C 2 - X Et (S) C 3 C3 Et () X Addition of 2 S 4 to form the alkyl hydrogen sulphate X = S 4 Addition of 2 to form the corresponding alcohol (it needs acid as a catalyst. Carbocation transpositions are possible with 1 0 and 2 0 carbocations) X = A valuable reaction 20

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22 Synthesis of alcohols 1. acid-catalyzed hydration of alkenes (follows Markovnikov s rule) Et-C = C 2 -X Et C 2-2 Et C 3 Markovnikov rule: the bulkier group goes on the most crowded C atom 22

23 Carbocation rearrangements 3 C 3 C C 3 C C 3 3 C 3 C d + d+ C 3 C C 3 2 C C 3 C 3 C C 3 23

24 Synthesis of alcohols 2. xymercuration-demercuration (also follows Markovnikov s rule but no skeletal rearrangement) (1) g(ac) 2, 2 (2) NaB 4 24

25 chanism 1 st step: 2 nd step: g(ac) 2 g(ac) + + Ac - +d g(ac) +d g(ac) 3 rd step: +d 2 2 g(ac) g(ac) g(ac) +d 4 th step: NaB 4 g(ac) 25

26 The reaction can also be used for the synthesis of ethers just using instead of 2 +d g(ac) g(ac) g(ac) +d NaB 4 g(ac) 26

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29 3. Addition of water to the double bond (anti-markovnikov) 1 st step: ydroboration Less crowded C atom 3 C 3 C 3 C B B syn attack!! B B 3 B 2 29

30 3. Addition of water to the double bond anti-markovnikov 2 nd step: oxidation / hydrolysis 1) Na 3 B Na 3 B 3 2) acid 3 C 3 C B -- B C configuration retained!! + B 30

31 Summary of Stereochemistry of hydroboration 1. Anti-markovnikov addition of 2 to the olefin 2. Syn addition C 3 1. hydroboration C 3 2. oxidation/hydrolysis + enantiomer 31

32 The basic principle of asymmetric synthesis C % (S) B 2 B* C 3 () + B* (S) C 3 100% (S) 100% (S) S, S,S diastereomers!!! (different stability and rate of formation) Enantiomeric enrichment 32

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35 Provide the structure of the major organic product of the reaction below. Answer:! 35

36 adical Alkene Additions--Anti-Markovnikov Addition of Before 1933, the addition of to alkenes gave confusing results. In contrast to Cl, which always gave the Markovnikov addition product, the reaction sometimes gave the anti-markovnikov product. Cl + Cl isopropyl chloride (Markovnikov Product) + sometimes propyl bromide (Anti-Markovnikov Product)

37 The ole of rganic Peroxides In 1933, Morris Kharasch and Frank Mayo (U of Chicago) reported that the erratic behavior of was due to trace amounts of organic peroxides, sometimes present as a contaminant in the alkenes, that initiated a competing free radical chain addition. : : : : --- organic peroxide : : : : --- organic hydroperoxide They showed the anti-markovnikov addition product could be suppressed by adding small amounts of free radical "traps" that inhibited the propagation of the free radical chain reaction.

38 Peroxides : : : : : heat or radiation 2 -. : : alkoxyl radical Azo Compounds :heat or :N=N: radiation : 2. alkyl radicals + :N N: alogens : : X X : : : : : heat or radiation 2 : X. : : halogen radical

39 eactivity of Free adicals Most free radicals are highly reactive, short-lived species. They react with other radicals upon collision to form covalent bonds. They also react with molecules in atom abstraction or addition reactions. These reactions are driven by bond strength energetics.

40 Examples : Cl. + :C 3 methane : : heat or radiation :Cl: +. C 3 methyl radical : : Chlorine radical abstracts a hydrogen atom from methane producing hydrogen chloride and methyl radical. C. +. =... oxygen : : heat or radiation C -. methyl peroxy radical : : : : : thyl radical rapidly reacts with oxygen ( 2 ) to produce the methyl peroxy radical. C. + C 2 =CC 6 5 styrene heat or radiation. C 3 C 2 CC phenyl-1-propyl radical thyl radical adds to styrene

41 chanism of the Free adical Addition eaction The role of the peroxides is to initiate a free radical chain reaction by homolysis of the weak peroxy bond. Initiation : : : : --- heat 2 -. alkoxy radicals : : Δ o ~ 146 kj/mol : : : : : -- : : (Δ o = -366) (Δ o ~ -431) + :. a chain carrier : : Δ o ~ - 65 kj/mol

42 Propagation (1) :. + C 2 =C-C. 3 -C 2 -C-C 3 : : (D o ~ - 251) (D o ~ - 290) Δ o ~ - 39 kj/mol (2). -C 2 -C-C C 2 -C-C 3 + :. : : (D o = - 366) (D o = - 395) Δ o ~ - 29 kj/mol Note: Each propagation step is exothermic so there is no "kinetic bottleneck."

43 egioselectivity of the Addition The regioselectivity in the free radical chain addition of to propene (anti-markovnikov) is determined by the relative stabilities of the alkyl radical intermediates. :. faster. + C 2 =C-C 3 C 2 CC 3 secondary alkyl radical (more stable) : : :. + C 2 =C-C 3 slower. C 2 CC 3 primary alkyl radical (less stable) : : In the absence of peroxide initiators, or with added radical traps, the ionic addition dominates: + - C 3 C=C 2 + C 3 CC 3 C 3 CC 3 more stable carbocation

44 An Evaluation of the Addition of Cl to Propene Why does Cl only react by an ionic mechanism? The energetics of the two propagating steps in the free radical chain mechanism are: (1) : Cl. + C 2 =C-C 3 : :. ClC 2 CC 3 (D o = - 431) (D o = - 395) Δ o ~ + 36 kj/mol (2). Cl-C 2 -C-C 3 + -Cl Cl-C 2 -C-C 3 (D o = - 251) (D o = - 341) + : Cl. : : Δ o ~ - 90 kj/mol

45 Quiz Chapter 10 Section 9 In the presence of peroxides, isobutene reacts with to give 1-bromo-2- methylpropane. Provide the initiation and propagation steps for this reaction. (C 3 ) 2 C=C 2 + (peroxides) (C 3 ) 2 CC 2 Initiation heat or hν Propagation (C 3 ) 2 C = C 2 +. (C 3 ) 2 C-C 2. (C 3 ) 2 C-C 2 + (C 3 ) 2 CC chain carriers

46 Alkene xidations Syn-hydroxylation -, 2 C 2 C 2 + KMn 4 2 C C 2 Two methods: Na 2 S 3 / 2 C 2 C 2 + s 4 2 C C 2 46

47 Alkene oxidations: chanism Mn Mn Mn s s s 47

48 xidative cleavage 3 C 3 C KMn 4, - heat + C 3 C 3 KMn 4, - heat + C 2 ' ' 48

49 Alkyne xidations KMn 4 heat 49

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51 zonolysis Zn ozonide 51

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53 Epoxide formation Let s do the following experiment C + C Epoxide (oxyrane) 53

54 eactions of epoxides ing opening with acid 3 + with base 2 54

55 Stereochemistry of ring opening Acid catalyzed esembles a 3 0 carbocation C 3 (S) 3 + +d C 3 C 3 C 3 () Base catalyzed (S) C 3 C 3 (S) C 3 55

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58 Anti hydroxylation of alkenes C() 3 + trans 58

59 Stereochemistry cis meso 2,3-2,3-butanediol 2S,3S-2,3-butanediol 59

60 racemate trans meso-2,3-butanediol 60

61 (S)-*C() (S) (S) () () ' ' ' () (S) ' () (S) ' 61

62 Addition of halogens Fast, high yield reactions forming vic-dihalides trans 62

63 chanism + anti addition: a stereospecific reaction 63

64 Stereospecificity, S,S trans-1,2-dibromocyclopentane The reaction forms only one steroisomeric form (trans) but is not enantiospecific 64

65 Stereospecificity C 3 3 C 2 2,3S (meso) CCl 4 trans-2-butene C 3 C 3 3 C C 3 C 3 2 CCl C cis-2-butene (MDELS) C 3 2,3 2S,3S C 3 65

66 Stereospecificity Et 2 Et Et 2 Et Et S 66

67 Formation of halohydrins (addition of X 2 and 2 ) minor major Use excess of water to favor halohydrin 67

68 Formation of halohydrins (addition of X 2 and 2 ) mechanism anti attack 68

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72 The acidity of terminal alkynes sp sp 2 sp 3 pk a More s character, more stable anion, weaker conjugated base, better acid Acidity C 3 C 3 < C 2 =C 2 < CC Basicity C 3 C 2 > C 2 =C > CC < < CC < N 2 < C=C 2 < C 2 -C 3 72

73 Alkynes on the same line as alkenes Addition of 2 and Cl Addition of X follows the Markovnikov's rule xidative cleavage ' 3 C 2 + 'C 2 73

74 ydrogenation of alkynes Syn addition for the formation of cis-alkenes 2 Anti addition for the formation of trans-alkenes Li, EtN 2 74

75 Anti addition: the mechanism (reminder) -NEt Li Li -NEt trans 75