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1 Chapter 8 Alkyl Halides and Elimination reactions.

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1 1 Chapter 8 Alkyl Halides and Elimination reactions

2 2 Alkyl Halides and Elimination Reactions General Features of Elimination It is well known that elimination reactions (E) often compete successfully with S N reactions, because nucleophiles are also Brønsted-Lowry bases (S N 2 versus E2) and carbocations are prone to the elimination of a proton (again involving a Brønsted-Lowry base: S N 1 versus E1).

3 3 Alkyl Halides and Elimination Reactions General Features of Elimination

4 4 Alkyl Halides and Elimination Reactions Removal of the elements HX is called dehydrohalogenation. Dehydrohalogenation is an example of  elimination (1,2-elimination). General Features of Elimination

5 5 Alkyl Halides and Elimination Reactions bases used in elimination reactions : RO ¯ ( alkoxides). General Features of Elimination

6 6 Alkyl Halides and Elimination Reactions How to draw any product of dehydrohalogenation 1.Find the  carbon. 2.Identify all  carbons with H atoms. 3.Remove the elements of H and X from the  and  carbons and form a  bond. General Features of Elimination

7 7 Alkyl Halides and Elimination Reactions Alkenes—The Products of Elimination

8 8 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

9 9 Alkyl Halides and Elimination Reactions rotation about double bonds is restricted. Alkenes—The Products of Elimination diastereomers

10 10 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. Stability of Alkenes

11 11 Alkyl Halides and Elimination Reactions The stability of an alkene increases as the number of R groups bonded to the double bond carbons increases. 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. Stability of Alkenes

12 12 Alkyl Halides and Elimination Reactions Stability of Alkenes

13 13 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

14 14 Alkyl Halides and Elimination Reactions The most common mechanism for dehydrohalogenation second-order kinetics : 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] One step mechanism : all bonds are broken and formed in a single step. -- The reaction is concerted

15 15 Alkyl Halides and Elimination Reactions Mechanisms of Elimination—E2

16 16 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. Strong sterically hindered (non- nucleophilic) nitrogen bases (DBN and DBU) are also sometimes used. Mechanisms of Elimination—E2

17 17 Alkyl Halides and Elimination Reactions 1,5-diazabicyclo[4.3.0]non-5-ene and 1.8-diazobicylco[5.4.0]undec-7-ene

18 18 Alkyl Halides and Elimination Reactions Mechanisms of Elimination—E2 : the leaving group the solvent Acetone, DMF, DMSO

19 19 Alkyl Halides and Elimination Reactions Substitution v.s. elimination Mechanisms of Elimination—E2 : nature of R group i.e. more substitution lowers Ea

20 20 Alkyl Halides and Elimination Reactions Mechanisms of Elimination—E2 slower faster

21 21 Alkyl Halides and Elimination Reactions Table 8.2 summarizes the characteristics of the E2 mechanism. Mechanisms of Elimination—E2

22 22 Alkyl Halides and Elimination Reactions Mechanisms of Elimination—E2

23 23 Alkyl Halides and Elimination Reactions The major product is the more stable product—the one with the more substituted double bond. This phenomenon is called the Zaitsev rule. (1875) The Zaitsev (Saytzeff) Rule

24 24 Alkyl Halides and Elimination Reactions The Zaitsev rule: the major product in  elimination has the more substituted double bond. A reaction is regioselective when it yields predominantly or exclusively one constitutional isomer when more than one is possible. Thus, the E2 reaction is regioselective. The Zaitsev (Saytzeff) Rule

25 25 If the stereochemistry is known, the outcome is not always the same.

26 26 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. cf. stereoselective Why?

27 27 Alkyl Halides and Elimination Reactions In the transition state of an E2 reaction all involved atoms should be aligned in the same plane. one hydrogen atom, two carbon atoms, the leaving group (X) 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 is all about orbital alignment. 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.

28 28 Stereochemistry of the E2 Reaction Alkyl Halides and Elimination Reactions

29 29 Alkyl Halides and Elimination Reactions In cyclic compounds, the stereochemical requirement of an anti periplanar geometry in an E2 reaction has important consequences. Stereochemistry of the E2 Reaction

30 30 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.

31 31 Alkyl Halides and Elimination Reactions Stereochemistry of the E2 Reaction The cis isomer

32 32 Alkyl Halides and Elimination Reactions Stereochemistry of the E2 Reaction

33 33 Alkyl Halides and Elimination Reactions The trans isomer. Stereochemistry of the E2 Reaction This is not predicted by the Zaitsev rule.

34 34 If the stereochemistry is known, the outcome is not always the same.

35 35 Alkyl Halides and Elimination Reactions An E1 reaction exhibits first-order kinetics: Mechanisms of Elimination— E1 mechanism rate = k[(CH 3 ) 3 CCI] The E1 reaction proceeds 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.

36 36 Alkyl Halides and Elimination Reactions

37 37 Alkyl Halides and Elimination Reactions E1 Mechanisms

38 38 The rate of an E1 reaction increases as the number of R groups on the carbon with the leaving group increases. Other characteristics of E1 reactions 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. Alkyl Halides and Elimination Reactions

39 39 Alkyl Halides and Elimination Reactions E1 reactions are regioselective, favoring formation of the more substituted, more stable alkene. Zaitsev’s rule applies to E1 reactions also. E1 Mechanisms

40 40 Alkyl Halides and Elimination Reactions Table 8.3 summarizes the characteristics of the E1 mechanism. Mechanisms of Elimination—E1 No need of antiperiplanar arrangement

41 41 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.

42 42 Alkyl Halides and Elimination Reactions S N 1 and E1 Reactions

43 43 Alkyl Halides and Elimination Reactions Strong bases favor the E2 mechanism. Weak bases favor the E1 mechanism. When is the Mechanism E1 or E2?

44 44 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

45 45 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

46 46 Alkyl Halides and Elimination Reactions Stronger bases are needed to synthesize alkynes by dehydrohalogenation than are needed to synthesize alkenes. Because sp2 hybridized C—H bonds are stronger than sp3 hybridized C—H bonds. As a result, a stronger base is needed to cleave this bond. 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

47 47 Alkyl Halides and Elimination Reactions E2 Reactions and Alkyne Synthesis

48 48 Alkyl Halides and Elimination Reactions Predicting the Mechanism from the Reactants—S N 1, S N 2, E1 or E2.

49 49 Alkyl Halides and Elimination Reactions Good nucleophiles that are weak bases favor substitution over elimination —Certain anions generally give products of substitution because they are good nucleophiles but weak bases. These include I ¯, Br ¯, HS ¯, ¯ CN, and CH 3 COO ¯. Predicting the Mechanism from the Reactants—S N 1, S N 2, E1 or E2.

50 50 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.

51 51 Alkyl Halides and Elimination Reactions Predicting the Mechanism from the Reactants—S N 1, S N 2, E1 or E2. Tertiary Alkyl Halides

52 52 Predicting the Mechanism from the Reactants—S N 1, S N 2, E1 or E2. Alkyl Halides and Elimination Reactions Tertiary Alkyl Halides E2 will occur preferentially if a strong base is used (OH-, OR-). Reaction in neutral (weakly basic) conditions leads to a mixture of SN1 and E1 products, with usually the SN1 product favored.

53 53 Predicting the Mechanism from the Reactants—S N 1, S N 2, E1 or E2. Alkyl Halides and Elimination Reactions Primary Alkyl Halides SN2 is favored by use of a good nucleophile (RS-, I-, CN-, Br-) and polar aprotic solvent. E2 is favored by use of a strong, hindered base (tert-butoxide).

54 54 Predicting the Mechanism from the Reactants—S N 1, S N 2, E1 or E2. Alkyl Halides and Elimination Reactions Secondary Alkyl Halides SN2 and E2 compete. If a weakly basic strong nucleophile (CH3COO-, Br-, I-) and polar aprotic solvent is used, SN2 dominates. If a strong base (OR-) in a protic solvent is used, E2 is dominant.

55 55 Predicting the Mechanism from the Reactants—S N 1, S N 2, E1 or E2. Alkyl Halides and Elimination Reactions Secondary Alkyl Halides If only a weak nucleophile or base is present then SN1 competes with E1 and it is difficult to predict which will be favored. This is a situation that is best avoided.

56 56 8.18, 8.22, 8.24, 8.31, 8.33, 8.40, 8.41, 8.45, 8.49, 8.50, 8.53, 8.54, 8.57 Homework

57 57 Preview of Chapter 9 Alcohols, Ethers and Epoxides Preparation of alcohols, ethers, and epoxides Synthesize an alcohol and an ether from alkylhalide. Reactions of alcohols, ethers and epoxides What is dehydration reaction? What is 1,2-hydride shift? What reagents are used to convert alcohols into halides?


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