Estabilidade Relativa dos Dienos

Apresentação em tema: "Estabilidade Relativa dos Dienos"— Transcrição da apresentação:

1 Estabilidade Relativa dos Dienos
Dr. Wolf's CHM 201 & 202 10-44 3

2 Calor de Hidrogenação 1,3-pentadieno é 26 kJ/mol mais estável do que o 1,4-pentadieno, mas parte desta estabilização é devida à presença de uma dupla ligação mais substituída. 252 kJ/mol 226 kJ/mol Dr. Wolf's CHM 201 & 202 10-45 4

3 Calor de Hidrogenação 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol
Dr. Wolf's CHM 201 & 202 10-46 5

4 Calor de Hidrogenação 126 kJ/mol 111 kJ/mol 126 kJ/mol 115 kJ/mol
Dr. Wolf's CHM 201 & 202 10-47 5

5 este adicional de 15 kJ/mol é conhecido por nomes diferentes:
Calor de Hidrogenação 126 kJ/mol 111 kJ/mol este adicional de 15 kJ/mol é conhecido por nomes diferentes: Energia de estabilização Energia de deslocalização Energia de Ressoância Dr. Wolf's CHM 201 & 202 10-49 5

6 Heats of Hydrogenation
Cumulated double bonds have relatively high heats of hydrogenation H2C C + CH2 2H2 CH3CH2CH3 DH° = -295 kJ + H2C CH2CH3 H2 CH3CH2CH3 DH° = -125 kJ Dr. Wolf's CHM 201 & 202 10-50 7

7 10.7 Bonding in Conjugated Dienes
Dr. Wolf's CHM 201 & 202 10-51 8

8 Dieno isolado 1,4-pentadieno 1,3-pentadieno Dieno conjugado 10-52 10
Dr. Wolf's CHM 201 & 202 10-52 10

9 Ligações p são independentes uma da outra
Dieno isolado Ligações p são independentes uma da outra 1,3-pentadiene Dieno conjugado Dr. Wolf's CHM 201 & 202 10-53 10

10 Ligações p são independentes uma da outra
Dieno isolado Ligações p são independentes uma da outra Orbitais p se sobrepõe dando uma extensão da Ligãção p envolvendo quatro carbonos Dieno conjugado Dr. Wolf's CHM 201 & 202 10-54 10

11 Menor deslocaização dos elétrons; Menos estável
Dieno isolado Menor deslocaização dos elétrons; Menos estável Maior deslocalização dos elétrons; mais estável Dieno conjugado Dr. Wolf's CHM 201 & 202 10-55 10

12 Conformations of Dienes
H H s-trans s-cis s prefix designates conformation around single bond s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case) Dr. Wolf's CHM 201 & 202 10-56 11

13 Conformations of Dienes
H H s-trans s-cis s prefix designates conformation around single bond s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case) Dr. Wolf's CHM 201 & 202 10-57 11

14 Conformations of Dienes
s-trans s-cis Both conformations allow electron delocalization via overlap of p orbitals to give extended p system Dr. Wolf's CHM 201 & 202 10-58 11

15 s-trans is more stable than s-cis
Interconversion of conformations requires two p bonds to be at right angles to each other and prevents conjugation 12 kJ/mol Dr. Wolf's CHM 201 & 202 10-59 12

16 Dr. Wolf's CHM 201 & 202 10-60 12

17 16 kJ/mol 12 kJ/mol Dr. Wolf's CHM 201 & 202 10-61 15

18 10.8 Bonding in Allenes Dr. Wolf's CHM 201 & 202 10-62 16

19 cumulated dienes are less stable than isolated and conjugated dienes
(see Problem 10.7 on p 375) Dr. Wolf's CHM 201 & 202 10-63 17

20 linear arrangement of carbons
Structure of Allene 118.4° 131 pm linear arrangement of carbons nonplanar geometry Dr. Wolf's CHM 201 & 202 10-64 19

21 linear arrangement of carbons
Structure of Allene 118.4° 131 pm linear arrangement of carbons nonplanar geometry Dr. Wolf's CHM 201 & 202 10-65 19

22 Bonding in Allene sp 2 sp sp 2 Dr. Wolf's CHM 201 & 202 10-66 19

23 Bonding in Allene Dr. Wolf's CHM 201 & 202 10-67 19

24 Bonding in Allene Dr. Wolf's CHM 201 & 202 10-68 19

25 Bonding in Allene Dr. Wolf's CHM 201 & 202 10-69 19

26 Allenes of the type shown are chiral
Chiral Allenes Allenes of the type shown are chiral A X C B Y A ¹ B; X ¹ Y Have a stereogenic axis Dr. Wolf's CHM 201 & 202 10-70 25

27 analogous to difference between:
Stereogenic Axis analogous to difference between: a screw with a right-hand thread and one with a left-hand thread a right-handed helix and a left-handed helix Dr. Wolf's CHM 201 & 202 10-71 26

28 10.9 Preparation of Dienes Dr. Wolf's CHM 201 & 202 10-72 1

29 used to prepare synthetic rubber (See "Diene Polymers" box)
1,3-Butadiene °C CH3CH2CH2CH3 H2C CHCH CH2 chromia- alumina + 2H2 More than 4 billion pounds of 1,3-butadiene prepared by this method in U.S. each year used to prepare synthetic rubber (See "Diene Polymers" box) Dr. Wolf's CHM 201 & 202 10-73 2

30 Dehydration of Alcohols
KHSO4 OH heat Dr. Wolf's CHM 201 & 202 10-74 3

31 Dehydration of Alcohols
KHSO4 OH heat major product; 88% yield Dr. Wolf's CHM 201 & 202 10-75 3

32 Dehydrohalogenation of Alkyl Halides
KOH Br heat Dr. Wolf's CHM 201 & 202 10-76 3

33 Dehydrohalogenation of Alkyl Halides
KOH Br heat major product; 78% yield Dr. Wolf's CHM 201 & 202 10-77 3

34 isolated dienes: double bonds react independently of one another
Reactions of Dienes isolated dienes: double bonds react independently of one another cumulated dienes: specialized topic conjugated dienes: reactivity pattern requires us to think of conjugated diene system as a functional group of its own Dr. Wolf's CHM 201 & 202 10-78 6

35 Tipos de Reações Orgânicas
Reações de : Adição – duas moléculas se combinam Eliminação – uma molécula quebra em duas Substituição – partes de duas moléculas trocam Rearranjo – a molécula sofre mudanças no modo como seus átomos são conectados.

36 10.10 Addition of Hydrogen Halides to Conjugated Dienes
Dr. Wolf's CHM 201 & 202 10-79 7

37 Electrophilic Addition to Conjugated Dienes
+ H X H Proton adds to end of diene system Carbocation formed is allylic Dr. Wolf's CHM 201 & 202 10-80 8

38 Example: H HCl Cl H H Cl ? ? Dr. Wolf's CHM 201 & 202 10-81 9

39 Example: H HCl Cl H Dr. Wolf's CHM 201 & 202 10-82 9

40 via: H + H H X H + Dr. Wolf's CHM 201 & 202 10-83 10

41 and: H + Cl H Cl– 3-Chlorocyclopentene H + H H Cl H H H H H 10-84 10
Dr. Wolf's CHM 201 & 202 10-84 10

42 1,2-Addition versus 1,4-Addition
1,2-addition of XY X Y Dr. Wolf's CHM 201 & 202 10-85 12

43 1,2-Addition versus 1,4-Addition
1,2-addition of XY 1,4-addition of XY X Y X Y Dr. Wolf's CHM 201 & 202 10-86 12

44 1,2-Addition versus 1,4-Addition
1,2-addition of XY 1,4-addition of XY X Y X Y via X + Dr. Wolf's CHM 201 & 202 10-87 12

45 HBr Addition to 1,3-Butadiene
H2C CHCH CH2 HBr Br CH2 CH3CHCH + CHCH2Br CH3CH electrophilic addition 1,2 and 1,4-addition both observed product ratio depends on temperature Dr. Wolf's CHM 201 & 202 10-88 13

46 Rationale 3-Bromo-1-butene is formed faster than 1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. Br CH2 CH3CHCH + CHCH2Br CH3CH via: + + CH2 CH3CHCH CHCH2 CH3CH Dr. Wolf's CHM 201 & 202 10-89 13

47 Rationale 3-Bromo-1-butene is formed faster than 1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. Br CH2 CH3CHCH + CHCH2Br CH3CH formed faster Dr. Wolf's CHM 201 & 202 10-90 13

48 Rationale 1-Bromo-2-butene is more stable than 3-bromo-1-butene because it has a more highly substituted double bond. Br CH2 CH3CHCH + CHCH2Br CH3CH more stable Dr. Wolf's CHM 201 & 202 10-91 13

49 Rationale The two products equilibrate at 25°C. Once equilibrium is established, the more stable isomer predominates. Br CH2 CH3CHCH CHCH2Br CH3CH major product at -80°C major product at 25°C (formed faster) (more stable) Dr. Wolf's CHM 201 & 202 10-92 13

50

51 Alkene Stabilities from DH’s:

52 Hiperconjugação Elétrons no orbital vizinho  (lig. C-H) estabiliza o orbital antiligante  vazio – Interação com efeito líquido positivo interaction Grupos Alquila interagem melhor que H

53 Kinetic Control versus Thermodynamic Control
Kinetic control: major product is the one formed at the fastest rate Thermodynamic control: major product is the one that is the most stable Dr. Wolf's CHM 201 & 202 10-93 16

54 CH2 CH3CHCH CHCH2 CH3CH + HBr H2C CHCH CH2 10-94 17
Dr. Wolf's CHM 201 & 202 10-94 17

55 higher activation energy CH3CHCH CH2
+ higher activation energy CH3CHCH CH2 + CH3CH CHCH2 formed more slowly Br CH2 CH3CHCH CHCH2Br CH3CH Dr. Wolf's CHM 201 & 202 10-95 17

56 Addition of hydrogen chloride to 2-methyl-1,3-butadiene is a kinetically controlled reaction and gives one product in much greater amounts than any isomers. What is this product? + HCl ? Dr. Wolf's CHM 201 & 202 10-96 19

57 + HCl Think mechanistically.
Protonation occurs: at end of diene system in direction that gives most stable carbocation Kinetically controlled product corresponds to attack by chloride ion at carbon that has the greatest share of positive charge in the carbocation + HCl Dr. Wolf's CHM 201 & 202 10-97 19

58 Think mechanistically
Cl + + one resonance form is tertiary carbocation; other is primary Dr. Wolf's CHM 201 & 202 10-98 20

59 Think mechanistically
Cl Cl H + + + + one resonance form is tertiary carbocation; other is primary one resonance form is secondary carbocation; other is primary Dr. Wolf's CHM 201 & 202 10-99 20

60 Think mechanistically
Cl More stable carbocation Is attacked by chloride ion at carbon that bears greater share of positive charge + + one resonance form is tertiary carbocation; other is primary Dr. Wolf's CHM 201 & 202 10-100 20

61 Think mechanistically
Cl Cl– Cl + + one resonance form is tertiary carbocation; other is primary major product Dr. Wolf's CHM 201 & 202 10-101 20

62 Terpenos e Terpenóides
Regra do isopreno - Wallach (1887) O nome terpeno deriva da terebentina (turpentine) de onde foram isolados a cânfora e o a-pineno. As estruturas foram elucidadas em torno de 1894.

63 Borracha Natural

64 Borracha Natural Gutta percha History Properties Applications
Palaquium gutta trees from the Malay peninsula (1843) Gutta percha was used to make knife handles Properties trans-1,4-polyisoprene Softened in hot water and then pressed into desired shape Solid at room temperature and is ductile and strong At higher temperature it can be drawn out into strips with no recoil like rubber Highly inert and resists vulcanization Applications Excellent insulator for Transatlantic Cables (Used until the 1930s)

65 Natural rubber Natural rubber consists of 1,4-cis-polyisoprene, extracted from the rubber tree (Hevea brasiliensis). It is produced in the tree by the biocatalyst hydroxynitrilelyase (2-hydroxyisobutyronitrile acetone-lyase). Synthetic rubber accounts for 75 % of rubber usage. However, natural rubber has advantages of elasticity, resilience and thermal properties. Natural rubber is easily broken down in the environment, however vulcanisation (treatment with sulfur) renders it resistant to biodegradation.

66 Tomates Licopeno Pigmento vermelho dos tomates Polieno Conjugado
Antioxidante, neutraliza radicais Livres

67 Sunscreen Octyl methoxycinnamate Conjugated cinnamate ester
Found in Coppertone Sport, Bullfrog Sunblock, Hawaiian Tropic Water Sport Absorbs radiation, preventing skin damage