Chap09 - Elimination Reactions of Alkyl Halides : Chapter 9...

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Chapter 9 1 Elimination Reactions of Alkyl Elimination Reactions of Alkyl Halides : Halides : Chapter 9 Chapter 9 Competition Between Substitution and Competition Between Substitution and Elimination Elimination
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Chapter 9 2 Contents of Chapter 9 The E2 Reaction The E2 Reaction The E1 Reaction The E1 Reaction Competition Between E2 and E1 Reactions Competition Between E2 and E1 Reactions Stereochemistry of Elimination Reactions Stereochemistry of Elimination Reactions Elimination from Cyclic Compounds Elimination from Cyclic Compounds Competition Between Substitution and Competition Between Substitution and Elimination Elimination Substitution and Elimination Reactions in Substitution and Elimination Reactions in Synthesis Synthesis
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Chapter 9 3 Elimination Reactions In this chapter we start with elimination rxns then work subst/elim competition A compound with an electronegative atom bonded to an sp 3 carbon, when approached by a nucleophile/base can undergo either a substitution reaction OR an elimination reaction
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Chapter 9 4 The E2 Reaction
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Chapter 9 5 The E2 Reaction: Regioselectivity 2-bromobutane has two structurally different β -carbons from which to abstract a hydrogen E2 rxns give more stable alkene if possible
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Chapter 9 6 The E2 Reaction: Regioselectivity Zaitsev’s rule: The more substituted alkene will be formed in elimination reactions
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Chapter 9 7 The E2 Reaction: Regioselectivity Zaitsev’s rule does not apply when the base is bulky E2 Rxn is kinetically-controlled
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Chapter 9 8 The E2 Reaction: Regioselectivity Zaitsev’s rule does not apply when the leaving group is poor E2-carbanion mechanism operative
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Chapter 9 9 The E2 Reaction: Regioselectivity Zaitsev’s rule may not apply when conjugated dienes might be formed CH 2 CHCH 2 CHCHCH 3 CH 3 HO Cl CH 2 CH 2 CHCH CHCH 2 CH CHCHCH 3 CH 3 CCH 3 CH 3 + major product
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Chapter 9 10 The E2 Reaction: Regioselectivity The major product of an E2 reaction is the more substituted alkene unless: the base is large (i.e. bulky) the leaving group is poor (i.e. F –) the less substituted β –carbon is allylic or benzylic (ie. more stable)
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Chapter 9 11 The E1 Reaction “E1” stands for “Elimination unimolecular” The E1 reaction is a two-step reaction The first step is rate-determining
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Chapter 9 12 The E1 Reaction Relative reactivities of alkyl halides in an E1 reaction are similar to the relative stabilities of carbocations 3 o benzylic > 3 o allylic > 2 o benzylic > 2 o allylic > 3 o > 1 o benzylic > 1 o allylic 2 o > 1 o > vinyl Increasing reactivity and C+ stability
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Chapter 9 13 The E1 Reaction E1 reaction involves a carbocation Therefore rearrangements must be considered
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Chapter 9 14 Competition Between E2 and
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This note was uploaded on 09/14/2009 for the course CHEM 2201 taught by Professor Davis during the Spring '08 term at Temple.

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Chap09 - Elimination Reactions of Alkyl Halides : Chapter 9...

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