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Chapter 8 RX and Elimination Rxns. The E2 Mechanism.

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Presentation on theme: "Chapter 8 RX and Elimination Rxns. The E2 Mechanism."— Presentation transcript:

1 Chapter 8 RX and Elimination Rxns

2 The E2 Mechanism

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4 Energy Diagram for the E2 Mechanism

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6 Effect of the Substrate on E2 Reactivity

7 Effect of the LG on E2 Reactivity

8 The E2 and S N 2 mechanisms share many of the same characteristics

9 Here are four characteristics that the E2 / S N 2 mechanisms have in common.

10 The E1Mechanism

11 Let ‘s look at an E1 reaction mechanism using the rxn shown below.

12 The E1 Mechanism

13 The first step of the E1 and S N 1 mechanisms are identical.

14 The E1 Mechanism

15 Energy Diagram for E1

16

17 The E1 and S N 1 mechanisms share many of the same characteristics

18 Here are four characteristics that the E1 / S N 1 mechanisms have in common.

19 What about dehydrohalogenations involving RX with hydrogen atoms on different  carbon atoms The Zaitsev Rule

20 The Zaitsev Rule- A Regioselective Rxn

21 The Zaitsev Rule (Z-rule) According to the Z-rule, the major product in a dehydrohalgenation is the ….. most stable product.

22 Let’s ask a different question about the products.

23

24 More R groups increase the stability of the T.S.

25 Let look at another example from the Smith text.

26 The Zaitsev (Saytzeff) Rule – A Stereoselective RXN

27 E2 Stereochemistry

28 The LG and the H atom can be syn or anti to each other

29 Bases Used in Elimination Rxns

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31 Sterically Hindered Strong Bases KOC(CH 3 ) 3, LiN(CH(CH 3 ) 2 ) 2 or LDA (lithium diisopropyl amide)

32 Exam II Review Problems

33 Predict the organic product(s) and name the mechanism

34 Methylenecyclohexane is not the major product in this rxn. Give the chemical structure of the major product and explain why it would be the major product.

35 Alkyl Halides and Elimination Reactions Elimination reactions involve the loss of elements from the starting material to form a new  bond in the product. General Features of Elimination

36 Alkyl Halides and Elimination Reactions Equations [1] and [2] illustrate examples of elimination reactions. In both reactions a base removes the elements of an acid, HX, from the organic starting material. General Features of Elimination

37 Alkyl Halides and Elimination Reactions Removal of the elements HX is called dehydrohalogenation. Dehydrohalogenation is an example of  elimination. The curved arrow formalism shown below illustrates how four bonds are broken or formed in the process. General Features of Elimination

38 Alkyl Halides and Elimination Reactions The most common bases used in elimination reactions are negatively charged oxygen compounds, such as HO ¯ and its alkyl derivatives, RO ¯, called alkoxides. General Features of Elimination

39 Alkyl Halides and Elimination Reactions To draw any product of dehydrohalogenation—Find the  carbon. Identify all  carbons with H atoms. Remove the elements of H and X form the  and  carbons and form a  bond. General Features of Elimination

40 Alkyl Halides and Elimination Reactions Recall that the double bond of an alkene consists of a  bond and a  bond. Alkenes—The Products of Elimination

41 Alkyl Halides and Elimination Reactions Alkenes are classified according to the number of carbon atoms bonded to the carbons of the double bond. Alkenes—The Products of Elimination

42 Alkyl Halides and Elimination Reactions Recall that rotation about double bonds is restricted. Alkenes—The Products of Elimination

43 Alkyl Halides and Elimination Reactions Because of restricted rotation, two stereoisomers of 2- butene are possible. cis-2-Butene and trans-2-butene are diastereomers, because they are stereoisomers that are not mirror images of each other. Alkenes—The Products of Elimination

44 Alkyl Halides and Elimination Reactions Whenever the two groups on each end of a carbon- carbon double bond are different from each other, two diastereomers are possible. Alkenes—The Products of Elimination

45 Alkyl Halides and Elimination Reactions In general, trans alkenes are more stable than cis alkenes because the groups bonded to the double bond carbons are further apart, reducing steric interactions. Alkenes—The Products of Elimination

46 Alkyl Halides and Elimination Reactions The stability of an alkene increases as the number of R groups bonded to the double bond carbons increases. Alkenes—The Products of Elimination The higher the percent s-character, the more readily an atom accepts electron density. Thus, sp 2 carbons are more able to accept electron density and sp 3 carbons are more able to donate electron density. Consequently, increasing the number of electron donating groups on a carbon atom able to accept electron density makes the alkene more stable.

47 Alkyl Halides and Elimination Reactions trans-2-Butene (a disubstituted alkene) is more stable than cis-2-butene (another disubstituted alkene), but both are more stable than 1-butene (a monosubstituted alkene). Alkenes—The Products of Elimination

48 Alkyl Halides and Elimination Reactions There are two mechanisms of elimination—E2 and E1, just as there are two mechanisms of substitution, S N 2 and S N 1. E2 mechanism—bimolecular elimination E1 mechanism—unimolecular elimination The E2 and E1 mechanisms differ in the timing of bond cleavage and bond formation, analogous to the S N 2 and S N 1 mechanisms. E2 and S N 2 reactions have some features in common, as do E1 and S N 1 reactions. Mechanisms of Elimination

49 Alkyl Halides and Elimination Reactions The most common mechanism for dehydrohalogenation is the E2 mechanism. It exhibits second-order kinetics, and both the alkyl halide and the base appear in the rate equation i.e. Mechanisms of Elimination—E2 rate = k[(CH 3 ) 3 CBr][ ¯ OH] The reaction is concerted—all bonds are broken and formed in a single step.

50 The Zaitsev (Saytzeff) Rule

51 Zaitsev Rule in an E1 Type Reaction

52 Alkyl Halides and Elimination Reactions There are close parallels between E2 and S N 2 mechanisms in how the identity of the base, the leaving group and the solvent affect the rate. The base appears in the rate equation, so the rate of the E2 reaction increases as the strength of the base increases. E2 reactions are generally run with strong, negatively charged bases like ¯ OH and ¯ OR. Two strong sterically hindered nitrogen bases called DBN and DBU are also sometimes used. Mechanisms of Elimination—E2

53 Alkyl Halides and Elimination Reactions Mechanisms of Elimination—E2

54 Alkyl Halides and Elimination Reactions The increase in E2 reaction rate with increasing alkyl substitution can be rationalized in terms of transition state stability. In the transition state, the double bond is partially formed. Thus, increasing the stability of the double bond with alkyl substituents stabilizes the transition state (i.e. lowers E a, which increases the rate of the reaction according to the Hammond postulate). Mechanisms of Elimination—E2

55 Alkyl Halides and Elimination Reactions Increasing the number of R groups on the carbon with the leaving group forms more highly substituted, more stable alkenes in E2 reactions. In the reactions below, since the disubstituted alkene is more stable, the 3 0 alkyl halide reacts faster than the 1 0 alkyl halide. Mechanisms of Elimination—E2

56 Alkyl Halides and Elimination Reactions Mechanisms of Elimination—E2

57 Alkyl Halides and Elimination Reactions Recall that when alkyl halides have two or more different  carbons, more than one alkene product is formed. When this happens, one of the products usually predominates. The major product is the more stable product—the one with the more substituted double bond. This phenomenon is called the Zaitsev rule. The Zaitsev (Saytzeff) Rule

58 Alkyl Halides and Elimination Reactions When a mixture of stereoisomers is possible from a dehydrohalogenation, the major product is the more stable stereoisomer. A reaction is stereoselective when it forms predominantly or exclusively one stereoisomer when two or more are possible. The E2 reaction is stereoselective because one stereoisomer is formed preferentially. The Zaitsev (Saytzeff) Rule

59 Alkyl Halides and Elimination Reactions The dehydrohalogenation of (CH 3 ) 3 CI with H 2 O to form (CH 3 )C=CH 2 can be used to illustrate the second general mechanism of elimination, the E1 mechanism. An E1 reaction exhibits first-order kinetics: Mechanisms of Elimination—E1 rate = k[(CH 3 ) 3 CI] The E1 reaction proceed via a two-step mechanism: the bond to the leaving group breaks first before the  bond is formed. The slow step is unimolecular, involving only the alkyl halide. The E1 and E2 mechanisms both involve the same number of bonds broken and formed. The only difference is timing. In an E1, the leaving group comes off before the  proton is removed, and the reaction occurs in two steps. In an E2 reaction, the leaving group comes off as the  proton is removed, and the reaction occurs in one step.

60 Alkyl Halides and Elimination Reactions The rate of an E1 reaction increases as the number of R groups on the carbon with the leaving group increases. Mechanisms of Elimination—E1 The strength of the base usually determines whether a reaction follows the E1 or E2 mechanism. Strong bases like ¯ OH and ¯ OR favor E2 reactions, whereas weaker bases like H 2 O and ROH favor E1 reactions.

61 Alkyl Halides and Elimination Reactions Table 8.3 summarizes the characteristics of the E1 mechanism. Mechanisms of Elimination—E1

62 Alkyl Halides and Elimination Reactions S N 1 and E1 reactions have exactly the same first step—formation of a carbocation. They differ in what happens to the carbocation. S N 1 and E1 Reactions Because E1 reactions often occur with a competing S N 1 reaction, E1 reactions of alkyl halides are much less useful than E2 reactions.

63 Alkyl Halides and Elimination Reactions The transition state of an E2 reaction consists of four atoms from an alkyl halide—one hydrogen atom, two carbon atoms, and the leaving group (X)—all aligned in a plane. There are two ways for the C—H and C—X bonds to be coplanar. Stereochemistry of the E2 Reaction E2 elimination occurs most often in the anti periplanar geometry. This arrangement allows the molecule to react in the lower energy staggered conformation, and allows the incoming base and leaving group to be further away from each other.

64 Stereochemistry of the E2 Reaction

65 Alkyl Halides and Elimination Reactions The stereochemical requirement of an anti periplanar geometry in an E2 reaction has important consequences for compounds containing six-membered rings. Consider chlorocyclohexane which exists as two chair conformers. Conformer A is preferred since the bulkier Cl group is in the equatorial position. Stereochemistry of the E2 Reaction For E2 elimination, the C-Cl bond must be anti periplanar to the C—H bond on a  carbon, and this occurs only when the H and Cl atoms are both in the axial position. The requirement for trans diaxial geometry means that elimination must occur from the less stable conformer, B.

66 Alkyl Halides and Elimination Reactions Stereochemistry of the E2 Reaction

67 Alkyl Halides and Elimination Reactions Now consider the E2 dehydrohalogenation of cis- and trans-1- chloro-2-methylcyclohexane. Stereochemistry of the E2 Reaction This cis isomer exists as two conformers, A and B, each of which as one group axial and one group equatorial. E2 reaction must occur from conformer B, which contains an axial Cl atom.

68 Alkyl Halides and Elimination Reactions Because conformer B has two different axial  hydrogens, labeled H a and H b, E2 reaction occurs in two different directions to afford two alkenes. The major product contains the more stable trisubstituted double bond, as predicted by the Zaitsev rule. Stereochemistry of the E2 Reaction

69 Alkyl Halides and Elimination Reactions The trans isomer of 1-chloro-2-methylcyclohexane exists as two conformers: C, having two equatorial substituents, and D, having two axial substituents. Stereochemistry of the E2 Reaction E2 reaction must occur from D, since it contains an axial Cl atom.

70 Alkyl Halides and Elimination Reactions Because conformer D has only one axial  H, E2 reaction occurs only in one direction to afford a single product. This is not predicted by the Zaitzev rule. Stereochemistry of the E2 Reaction

71 Alkyl Halides and Elimination Reactions The strength of the base is the most important factor in determining the mechanism for elimination. Strong bases favor the E2 mechanism. Weak bases favor the E1 mechanism. When is the Mechanism E1 or E2

72 Alkyl Halides and Elimination Reactions A single elimination reaction produces a  bond of an alkene. Two consecutive elimination reactions produce two  bonds of an alkyne. E2 Reactions and Alkyne Synthesis

73 Alkyl Halides and Elimination Reactions Two elimination reactions are needed to remove two moles of HX from a dihalide substrate. Two different starting materials can be used—a vicinal dihalide or a geminal dihalide. E2 Reactions and Alkyne Synthesis

74 Alkyl Halides and Elimination Reactions Stronger bases are needed to synthesize alkynes by dehydrohalogenation than are needed to synthesize alkenes. The typical base used is ¯ NH 2 (amide), used as the sodium salt of NaNH 2. KOC(CH 3 ) 3 can also be used with DMSO as solvent. E2 Reactions and Alkyne Synthesis

75 Alkyl Halides and Elimination Reactions The reason that stronger bases are needed for this dehydrohalogenation is that the transition state for the second elimination reaction includes partial cleavage of the C—H bond. In this case however, the carbon atom is sp 2 hybridized and sp 2 hybridized C—H bonds are stronger than sp 3 hybridized C—H bonds. As a result, a stronger base is needed to cleave this bond. E2 Reactions and Alkyne Synthesis

76 Alkyl Halides and Elimination Reactions E2 Reactions and Alkyne Synthesis

77 Alkyl Halides and Elimination Reactions Good nucleophiles that are weak bases favor substitution over elimination—Certain anions always give products of substitution because they are good nucleophiles but weak bases. These include I ¯, Br ¯, HS ¯, and CH 3 COO ¯. Predicting the Mechanism from the Reactants—S N 1, S N 2, E1 or E2.

78 Alkyl Halides and Elimination Reactions Bulky nonnucleophilic bases favor elimination over substitution—KOC(CH 3 ) 3, DBU, and DBN are too sterically hindered to attack tetravalent carbon, but are able to remove a small proton, favoring elimination over substitution. Predicting the Mechanism from the Reactants—S N 1, S N 2, E1 or E2.

79 Alkyl Halides and Elimination Reactions

80 Predicting the Mechanism from the Reactants—S N 1, S N 2, E1 or E2. Alkyl Halides and Elimination Reactions

81 Stereochemistry of the E2 Reaction

82 Is it S N 1, S N 2, E1 or E2?


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