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Chapter 6 1
Elimination Reactions
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In addition to substitution, an alkyl halide can undergo an elimination reaction
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Chapter 6 3
Elimination Reactions
• Elimination reactions produce double bonds. • The alkyl halides loses a hydrogen and the
halide. • Also called dehydrohalogenation (-HX).
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The E2 Reaction
Second-order reaction
E2: elimination, bimolecular.
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The weaker the base, the better it is as a leaving group
The role of the leaving group
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Any good leaving group will lead to a fast elimination.
The role of the leaving group
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The Regioselectivity of the E2 Reaction
The major product of an E2 reaction is the most stable alkene
The greater the number of substituents, the more stable is the alkene
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Reaction coordinate diagram for the E2 reaction of 2-bromobutane and methoxide ion
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The Zaitsev Rule
The more substituted alkene product is obtained when a proton is removed from the β-carbon that is bonded to the fewest hydrogens
The most stable alkene is generally (but not always)a the most substituted alkene
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Hofmann and Saytsev Traditionally, these two opposite preferences—for the more or the less substituted alkenes—have been called ‘Saytsev’s rule’ and ‘Hofmann’s rule’, respectively.
Zaitsev and Saytsev have different spelling but they are the same
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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
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Steric hindrance also affects the product distribution
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If the alkyl halide is not sterically hindered and the base is only moderately hindered, the major product will still be the more stable product. For example, the major product obtained from the reaction of 2-iodobutane and tert-butoxide ion is 2-butene. In other words, it takes a lot of hindrance for the less stable product to be the major product.
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Chapter 7 18
Hofmann Products Bulky bases, such as potassium tert-butoxide,
abstract the least hindered H+ giving the less substituted alkene as the major product (Hofmann product).