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Alkyl Halides and Elimination Reactions Dehydrohalogenation is an example of  elimination.

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Presentation on theme: "Alkyl Halides and Elimination Reactions Dehydrohalogenation is an example of  elimination."— Presentation transcript:

1 Alkyl Halides and Elimination Reactions Dehydrohalogenation is an example of  elimination.

2 Negatively charged oxygen compounds like HO ¯ and its alkyl derivatives, RO ¯ (called alkoxides) are common bases used in elimination reactions.

3 Example products of dehydrohalogenation. Notice the same starting material can produce different products

4 A double bond consists of a  bond and a  bond. Alkenes—Orbital view

5 Alkenes are classified by the number of carbon atoms bonded to the carbons of the double bond.

6 Free Rotation around sigma (single) bonds Pi bonds are not free to rotate

7 Because of this lack of free-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 & Stereoisomers

8 When two groups on each end of a carbon-carbon double bond are different from each other, two diastereomers are possible.

9 Cis and trans--stability Trans alkenes are more stable than cis alkenes because the groups bonded to the double bond carbons are further apart, reducing steric interactions. * Go back to the chain orientation during elimination To see how these two are formed.

10 The greater 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. Number of substituents and stability

11 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, (similar to the S N 2 and S N 1 mechanisms). E2 and S N 2 reactions have some features in common, and so do E1 and S N 1 reactions. Mechanisms of Elimination

12 E2 is the most common and it follows second-order kinetics with both the alkyl halide and the base in the rate equation Mechanism of E2 Elimination rate = k[(CH 3 ) 3 CBr][ ¯ OH] The reaction is concerted—all bonds are broken and formed in a single step.

13 Mechanism of E2 Elimination Example energy diagram for an E2 reaction:

14 The type of the base, the leaving group and the solvent affect the rate. Since the base appears in the rate equation, the rate of the E2 reaction increases as the strength of the base increases. Strong, negatively charged bases like ¯ OH and ¯ OR favor E2 mechanisms. Mechanisms of E2 Elimination

15 Mechanism of E2 Elimination

16 The S N 2 and E2 mechanisms differ in how the R group impacts reaction rate. E2 vs SN2 reactions (elimination vs substitution) Why ?...

17 The increase in E2 reaction rate with increasing alkyl substitution can be seen when looking at stability of the transition state. Since the double bond is partially formed in the TS, increasing the stability of the double bond with alkyl substituents stabilizes the transition state (lowers E a, and increasing the rate of the reaction). E2 transition state stability vs rxn rate

18 Examples:

19 Overview of E2 mechanism

20 When more than one product can form, the major product is the more stable product—the one with the more substituted double bond. Zaitsev’s (Saytzeff) Rule

21 A reaction is regioselective when it yields mostly one constitutional isomer when more than one product is possible. Zaitsev’s (Saytzeff) Rule: “E2 Rxns are Regioselective”

22 A reaction is stereoselective when it forms mostly or exclusively one stereoisomer when two or more are possible. The E2 reaction is stereoselective because one stereoisomer is formed preferentially. Zaitsev’s (Saytzeff) Rule

23 The E1 reaction

24 The dehydrohalogenation of (CH 3 ) 3 CCI with H 2 O to form (CH 3 ) 2 C=CH 2 follows an E1 mechanism with first-order kinetics: The E1 reaction,…remember your lab experiment? rate = k[(CH 3 ) 3 CCI] As with SN1, the E1 reaction is 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 difference is timing.

25 E1 mechanism (similar to SN1)

26 Mechanism of E1 elimination An example energy diagram for an E1 reaction:

27 The rate of an E1 reaction increases as the number of R groups on the carbon with the leaving group increases (same with E2). The E1 reaction  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.

28 E1 reactions are also regioselective, favoring formation of the more substituted, more stable alkene. E1 reactions: Zaitsev’s rule also applies for E1

29 Summary of the E1 mechanism.

30 NOTE: 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 vs E1 Reactions Because E1 reactions occur with a competing S N 1 reaction.

31 The transition state of an E2 reaction consists of four atoms from an alkyl halide 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 which allows the molecule to react in the lower energy staggered conformation, letting the incoming base and leaving group to be further away from each other.

32 Stereochemistry of the E2 Reaction Two possible geometries for the E2 reaction

33 The stereochemical requirement of an anti periplanar geometry in an E2 reaction has big consequences for six-membered rings. Stereochemistry of the E2 Reaction For E2 elimination, the C-Cl bond must be anti periplanar to the C—H bond on a  carbon, which 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.

34 Stereochemistry of the E2 Reaction The trans diaxial geometry for the E2 elimination in chlorocyclohexane

35 What about the E2 dehydrohalogenation of cis- and trans-1- chloro-2-methylcyclohexane ? Stereochemistry of the E2 Reaction This cis isomer exists as two conformations, A and B, each of which as one group axial and one group equatorial. E2 reaction must result from conformation B, which contains an axial Cl atom.

36 Because conformation “B” has two different axial  hydrogens, labeled H a and H b, so the E2 reaction occurs in two different directions giving two different alkenes. * Zaitsev says the major product contains the more stable trisubstituted double bond. Stereochemistry of the E2 Reaction

37 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.

38 Because conformer D has only one axial  H, the E2 reaction occurs only in one direction to give only a single product. Not predicted by the Zaitsev rule. Stereochemistry of the E2 Reaction

39 The strength of the base is the most important factor. Strong bases favor the E2 mechanism. Weak bases favor the E1 mechanism. When is the Mechanism E1 or E2?

40 Two consecutive elimination reactions produce two  bonds of an alkyne. E2 Reactions & the synthesis of Alkynes

41 NOTE: 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 & the synthesis of Alkynes

42 Very strong bases (such as ¯NH2 or KOC(CH3)3 in DMSO) are needed to synthesize alkynes by dehydrohalogenation. E2 Reactions & the synthesis of Alkynes

43 Very strong bases are needed because the transition state for the second elimination reaction includes partial cleavage of the C—H bond which is sp 2 hybridized (and sp 2 hybridized C—H bonds are stronger than sp 3 hybridized C—H bonds). E2 Reactions & the synthesis of Alkynes

44 Examples of dehydrohalogenation of dihalides to produce alkynes E2 Reactions & the synthesis of Alkynes

45 Good nucleophiles that are weak bases favor substitution over elimination—(examples: I ¯, Br ¯, HS ¯, ¯ CN, and CH 3 COO ¯ ) Predicting the Mechanism— S N 1, S N 2, E1 or E2

46 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— S N 1, S N 2, E1 or E2

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50 Elimination Rxns


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