© Prentice Hall 2001Chapter 101 On Line Course Evaluation for Chemistry 350/Section We are participating in the online course evaluation Please log on Click on Spring 2003 Evaluation That will take you to Dinput.asp Dinput.asp Your password is your address

© Prentice Hall 2001Chapter 102 Elimination Reactions In this chapter we will focus on elimination reactions 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

© Prentice Hall 2001Chapter 103 The E2 Reaction “E2” stands for “elimination, bimolecular”

© Prentice Hall 2001Chapter 104 The E2 Reaction: Regioselectivity 2-bromobutane has two structurally different  -carbons from which to abstract a hydrogen

© Prentice Hall 2001Chapter 105 The E2 Reaction: Regioselectivity The E2 reaction is kinetically controlled To determine which product will be formed in greater yield, we must know which will be formed faster

© Prentice Hall 2001Chapter 106 The E2 Reaction: Regioselectivity Zaitsev’s rule: The more substituted alkene will be formed in elimination reactions

© Prentice Hall 2001Chapter 107 The E2 Reaction: Regioselectivity Zaitsev’s rule does not apply when the base is bulky

© Prentice Hall 2001Chapter 108 The E2 Reaction: Regioselectivity

© Prentice Hall 2001Chapter 109 The E2 Reaction: Regioselectivity

© Prentice Hall 2001Chapter 1010 The E2 Reaction: Regioselectivity Zaitsev’s rule may not apply when conjugated dienes might be formed major product

© Prentice Hall 2001Chapter 1011 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  –carbon is allylic or benzylic

© Prentice Hall 2001Chapter 1012 The E1 Reaction The reaction of tert-butyl bromide with water is first order

© Prentice Hall 2001Chapter 1013 The E1 Reaction “E1” stands for “Elimination unimolecular” The E1 reaction is a two-step reaction The first step is rate-determining

© Prentice Hall 2001Chapter 1014 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 stability

© Prentice Hall 2001Chapter 1015 The E1 Reaction E1 reaction involves a carbocation Therefore rearrangements must be considered

© Prentice Hall 2001Chapter 1016 The E1 Reaction 1,2-Hydride shifts

© Prentice Hall 2001Chapter 1017 The E1 Reaction: Regioselectivity When two elimination products are possible, the major product generally will be the one which follows Zaitsev’s Rule The transition state has “alkene-like” character The more highly substituted alkene is the more stable The more highly substituted alkene will be formed more quickly

© Prentice Hall 2001Chapter 1018 The E1 Reaction: Regioselectivity

© Prentice Hall 2001Chapter 1019 Competition Between E2 and E1 Reactions Summary of the Reactivity of Alkyl Halides in Elimination Reactions primary alkyl halideE2 only secondary alkyl halideE1 and E2 tertiary alkyl halide E1 and E2

© Prentice Hall 2001Chapter 1020 Competition Between E2 and E1 Reactions For 2 o and 3 o alkyl halides, that undergo both E2 and E1 reactions, the E2 reaction is favored by the same factors that favor S N 2 reactions over S N 1 reactions a high concentration of a strong base (e.g. HO – or – NH 2 ) an aprotic polar solvent (e.g. DMSO, DMF)

© Prentice Hall 2001Chapter 1021 Competition Between E2 and E1 Reactions An E1 reaction is favored by a weak base (e.g. a neutral solvent) a polar protic solvent (e.g. H 2 O or ROH)