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© 2011 Pearson Education, Inc. Chapter 9 Elimination Reactions of Alkyl Halides Competition Between Substitution and Elimination Organic Chemistry 6 th Edition Paula Yurkanis Bruice 1
© 2011 Pearson Education, Inc. In addition to substitution, an alkyl halide can undergo an elimination reaction 2
© 2011 Pearson Education, Inc. The E2 Reaction Hydroxide cannot act as a nucleophile in this reaction because of the bulky tertiary halide. Rather, hydroxide acts as a base and abstracts a proton. 3
© 2011 Pearson Education, Inc. The removal of a proton and a halide ion is called dehydrohalogenation: 4
© 2011 Pearson Education, Inc. An E2 reaction is also called a -elimination or a 1,2-elimination reaction: The weaker the base, the better it is as a leaving group 5
© 2011 Pearson Education, Inc. The Regioselectivity of the E2 Reaction The major product of an E2 reaction is the most stable alkene: 6
© 2011 Pearson Education, Inc. Reaction coordinate diagram for the E2 reaction of 2-bromobutane and methoxide ion 7
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The Zaitsev Rule The more substituted alkene product is obtained when a proton is removed from the -carbon that is bonded to the fewest hydrogens The most stable alkene is generally (but not always) the most substituted alkene 9
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© 2011 Pearson Education, Inc. Conjugated alkene products are preferred over the more substituted alkene product: Do not use Zaitsev’s rule to predict the major product in these cases 11
© 2011 Pearson Education, Inc. Bulky bases affect the product distribution resulting in the Hofmann product, the least substituted alkene: However, it takes a lot of steric hindrance for the less stable product to be the major product: Hofmann product Zaitsev product 12
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© 2011 Pearson Education, Inc. Another exception to Zaitsev’s rule 14
© 2011 Pearson Education, Inc. Consider the elimination of 2-fluoropentane… A carbanion-like transition state 15
© 2011 Pearson Education, Inc. Therefore, the major product of an E2 elimination reaction is the more stable alkene except if the reactants are sterically hindered or the leaving group is poor 16
© 2011 Pearson Education, Inc. The E1 Reaction A unimolecular reaction: 17
© 2011 Pearson Education, Inc. The E1 reaction, like the S N 1 reaction, involves a carbocation intermediate: 18
© 2011 Pearson Education, Inc. How does a weak base like water remove a proton from an sp 3 carbon? 1) The presence of a positive charge greatly reduces the pK a 2) Hyperconjugation weakens the C-H bond by electron density 19
© 2011 Pearson Education, Inc. The major product in an E1 reaction is generally the more substituted alkene 20
© 2011 Pearson Education, Inc. Reaction coordinate diagram for the E1 reaction of 2-chloro-2-methylbutane 21
© 2011 Pearson Education, Inc. Because the first step is the rate-determining step, the rate of an E1 reaction depends both on the ease with which the carbocation is formed and how readily the leaving group leaves 22
© 2011 Pearson Education, Inc. Because the E1 reaction forms a carbocation intermediate, we need to consider carbocation rearrangement 23
© 2011 Pearson Education, Inc. Competition Between E2 and E1 Reactions An E2 is favored by a high concentration of strong base and an aprotic polar solvent An E1 is favored by a weak base and a protic polar solvent Review Section
© 2011 Pearson Education, Inc. Stereochemistry of the E2 Reaction The bonds to the eliminated groups (H and X) must be in the same plane The anti elimination is favored over the syn elimination 25
© 2011 Pearson Education, Inc. Another view of the E2 reaction mechanism The best overlap of the interacting orbitals is achieved through back-side attack Anti elimination avoids repulsion of the electron-rich base 26
© 2011 Pearson Education, Inc. Consider the stereoselectivity of the E2 reaction The alkene with the bulkiest groups on opposite sides of the double bond will be formed in greater yield, because it is the more stable alkene 27
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© 2011 Pearson Education, Inc. Reaction coordinate diagram for the E2 reaction of 2-bromopentane and ethoxide ion 29
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© 2011 Pearson Education, Inc. When only one hydrogen is bonded to the -carbon, the major product of an E2 reaction depends on the structure of the alkene 31
© 2011 Pearson Education, Inc. Stereochemistry of the E1 Reaction The major stereoisomer obtained from an E1 reaction is the alkene in which the bulkiest substituents are on opposite sides of the double bond Both syn and anti elimination can occur in an E1 reaction 32
© 2011 Pearson Education, Inc. Elimination from Cyclic Compounds In an E2 reaction, groups to be eliminated must be in axial positions 33
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© 2011 Pearson Education, Inc. The hydrogen that is removed and the leaving group both have to be in the axial position, therefore… 38
© 2011 Pearson Education, Inc. E1 Elimination from Cyclic Compounds No axial orientation required for the E1 reaction. 39
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© 2011 Pearson Education, Inc. Utilization of kinetic isotope effect to study a reaction mechanism A carbon deuterium bond (C–D) is stronger than a carbon hydrogen bond (C–H) The deuterium kinetic isotope effect is 7.1, indicating that a C–H (or C–D) bond must be broken in the rate-determining step 42
© 2011 Pearson Education, Inc. Competition Between Substitution and Elimination Alkyl halides can undergo S N 2, S N 1, E2, and E1 1) decide whether the reaction conditions favor S N 2/E2 or S N 1/E1 S N 2/E2 reactions are favored by a high concentration of a good nucleophile/strong base S N 1/E1 reactions are favored by a poor nucleophile/weak base 2) decide how much of the product will be the substitution product and how much of the product will be the elimination product 43
© 2011 Pearson Education, Inc. Consider the S N 2/E2 conditions 44
© 2011 Pearson Education, Inc. A bulky alkyl halide or a sterically hindered nucleophile encourages elimination over substitution 45
© 2011 Pearson Education, Inc. A strong or a bulky base encourages elimination over substitution 46
© 2011 Pearson Education, Inc. High temperature favors elimination over substitution: Why? Because elimination is entropically favorable. 47
© 2011 Pearson Education, Inc. Tertiary alkyl halides undergo only elimination under S N 2/E2 conditions: 48
© 2011 Pearson Education, Inc. Consider S N 1/E1 conditions: Primary alkyl halides do not form carbocations; therefore they cannot undergo S N 1 and E1 reactions. The elimination reaction favored at higher temperatures. 49
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© 2011 Pearson Education, Inc. Substitution and Elimination Reactions in Synthesis 51
© 2011 Pearson Education, Inc. In synthesizing an ether, the less hindered group should be provided by the alkyl halide 52
© 2011 Pearson Education, Inc. However, the most hindered alkyl halide should be used if you want to synthesize an alkene 53
© 2011 Pearson Education, Inc. Hydroxide Water The E2 reaction should be used if you want to synthesize an alkene: 54
© 2011 Pearson Education, Inc. Consecutive E2 Elimination Reactions Requires a strong base NaNH 2. Reaction carried out in liquid ammonia. 55
© 2011 Pearson Education, Inc. Synthesis Example 56
© 2011 Pearson Education, Inc. Synthesis Example Racemic 57
© 2011 Pearson Education, Inc. Synthesis Example 58
© 2011 Pearson Education, Inc. Synthesis Example 59
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