10 Lecture

Post on 11-Dec-2015

223 views 1 download

description

pwrpoint

Transcript of 10 Lecture

© 2014 Pearson Education, Inc.

Elimination Reactions of Alkyl Halides

Competition Between Substitution and

Elimination

Paula Yurkanis BruiceUniversity of California,

Santa Barbara

Chapter 10

© 2014 Pearson Education, Inc.

Alkyl Halides UndergoSubstitution and Elimination Reactions

In an elimination reaction, a halogen is removed from one carbonand a hydrogen is removed from an adjacent carbon.

A double bond is formed between the two carbonsfrom which the atoms were removed.

© 2014 Pearson Education, Inc.

An E2 Reaction

© 2014 Pearson Education, Inc.

Mechanism for an E2 Reaction

© 2014 Pearson Education, Inc.

The Halogen Comes off the Alpha Carbon;the Hydrogen Comes off the Beta Carbon

dehydrohalogenation

© 2014 Pearson Education, Inc.

The Weakest Base Is the Best Leaving Group

© 2014 Pearson Education, Inc.

An E2 Reaction is Regioselective

The most stable alkene is (generally) obtained by removing a hydrogen from the beta carbon that is bonded to the fewest hydrogens.

The major product is the most stable alkene.

© 2014 Pearson Education, Inc.

The More Stable AlkeneHas the More Stable Transition State

© 2014 Pearson Education, Inc.

Alkene-Like Transition State

© 2014 Pearson Education, Inc.

More E2 Reactions

© 2014 Pearson Education, Inc.

Relative Reactivities of Alkyl Halides in an E2 Reaction

© 2014 Pearson Education, Inc.

The More Stable Alkene is the Major Product

The conjugated alkene is the more stable alkene (an exception to Zaitsev’s rule).

© 2014 Pearson Education, Inc.

The More Stable Alkene is Not the Major Product

A sterically hindered alkyl halide and a sterically hindered base forms the less stable alkene (another exception to Zaitsev’s rule).

© 2014 Pearson Education, Inc.

Need a Lot of Steric Hindrance

forms the more stable alkene

© 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

Fluoride Ion is a Poor Leaving Group

This is another exception to Zaitsev’s rule.

© 2014 Pearson Education, Inc.

The Transition State is Carbanion-Like

© 2014 Pearson Education, Inc.

Relative Stabilities of Carbocations

© 2014 Pearson Education, Inc.

Relative Stabilities of Carbanions

© 2014 Pearson Education, Inc.

How the Leaving Group Affects the Product Distribution

© 2014 Pearson Education, Inc.

An E1 Reaction

© 2014 Pearson Education, Inc.

The Mechanism for an E1 Reaction

© 2014 Pearson Education, Inc.

How Does a Weak Base (Like Water) Remove a Proton From an sp3 Carbon?

1. The presence of the positive charge greatly reduces the pKa.2. Hyperconjugation weakens the C—H bond by draining electron density.

© 2014 Pearson Education, Inc.

The most stable alkene is obtained by removing a hydrogen from the beta carbon that is bonded to the fewest hydrogens.

An E1 Reaction is Regioselective

The major product is the more stable alkene.

© 2014 Pearson Education, Inc.

The More Stable Alkene is the Major Product

© 2014 Pearson Education, Inc.

The Weakest Base is the Best Leaving Group

© 2014 Pearson Education, Inc.

Benzylic and Allylic Halides Undergo E2 Reactions

© 2014 Pearson Education, Inc.

Benzylic and Allylic Halides Undergo E1 Reactions

© 2014 Pearson Education, Inc.

The E1 Reaction of Allylic Halides Can Form Two Products

© 2014 Pearson Education, Inc.

The Reactivity of Alkyl Halides in Elimination Reactions

© 2014 Pearson Education, Inc.

The Bonds to Be Broken Must Be in the Same Plane

© 2014 Pearson Education, Inc.

Anti Elimination is Preferred

1. Anti requires the molecule to be in a staggered conformation.2. Back-side attack achieves the best overlap of interacting orbitals (see Figure 9.1).3. It avoids repulsion of the electron-rich base with the electron-rich leaving group.

© 2014 Pearson Education, Inc.

Anti Elimination

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.

© 2014 Pearson Education, Inc.

The More Stable Product is Easier to Form

© 2014 Pearson Education, Inc.

E2 and E1 Reactions are Regioselective

© 2014 Pearson Education, Inc.

E2 and E1 Reactions are Regioselective

© 2014 Pearson Education, Inc.

The Major Product of an E2 Reaction (Largest Groups Are on Opposite Sides)

© 2014 Pearson Education, Inc.

When Only One Hydrogen is Bonded to the β-Carbon, the Major Product of an E2 Reaction Depends on the

Structure of the Alkene

© 2014 Pearson Education, Inc.

When Only One Hydrogen is Bonded to theβ-Carbon, the Major Product is

Still the More Stable Alkene

© 2014 Pearson Education, Inc.

Summary of Stereochemistry

© 2014 Pearson Education, Inc.

E2 Elimination from Six-Membered Rings

Both groups being eliminated must be in axial positions.

© 2014 Pearson Education, Inc.

H and Cl Must Both Be Axial

© 2014 Pearson Education, Inc.

The Value of Keq Depends on Whether the ReactionTakes Place Through the More Stable

Conformer or Through the Less Stable Conformer

Keq is large Keq is small

© 2014 Pearson Education, Inc.

Neomenthyl Chloride is Faster

Elimination occurs through the more stable conformer.

© 2014 Pearson Education, Inc.

Menthyl Chloride is Slower

Elimination occurs through the less stable conformer.

© 2014 Pearson Education, Inc.

E1 Elimination from Six-Membered Rings

The H and Cl do not have to be in axial positions because the reaction is not concerted.

© 2014 Pearson Education, Inc.

Proof That the E2 Reaction is Concerted

The deuterium kinetic isotope effect = 7.1. Therefore, the C—D bond is broken in the rate-limiting step.

A carbon deuterium bond (C—D) is stronger than a carbon hydrogen bond (C—H).

© 2014 Pearson Education, Inc.

Alkyl Halides in SN2 and E2 Reactions

© 2014 Pearson Education, Inc.

Under SN2/E2 Conditions Primary Alkyl Halide = Primarily Substitution

© 2014 Pearson Education, Inc.

Steric Hindrance Favors Elimination

© 2014 Pearson Education, Inc.

Under SN2/E2 Conditions Secondary Alkyl Halide = Substitution and Elimination

Substitution is favored by a weak base.Elimination is favored by a strong base.

© 2014 Pearson Education, Inc.

Bulky Bases are Used to Encourage Elimination

© 2014 Pearson Education, Inc.

Although They are Neutral Bases, They are Strong Bases

© 2014 Pearson Education, Inc.

A High Temperature Favors Elimination

Why? Because elimination has a greater ∆S‡.

© 2014 Pearson Education, Inc.

Under SN2/E2 ConditionsTertiary Alkyl Halide = Only Elimination

© 2014 Pearson Education, Inc.

Under SN1/E1 Conditions Tertiary Alkyl Halides Undergo Substitution and Elimination

© 2014 Pearson Education, Inc.

Tertiary (SN1/E1): Substitution is FavoredTertiary (SN2/E2): Only Elimination

© 2014 Pearson Education, Inc.

Summary of the Products Obtained From Substitution and Elimination Reactions

© 2014 Pearson Education, Inc.

William Ether Synthesis:an SN2 Reaction

© 2014 Pearson Education, Inc.

Forming an Alkoxide Ion

© 2014 Pearson Education, Inc.

Synthesizing Butyl Propyl Ether

© 2014 Pearson Education, Inc.

The less hindered group should be provided by the alkyl halide.

Synthesizing tert-Butyl Ethyl Ether

© 2014 Pearson Education, Inc.

Synthesizing an Alkene

The more hindered group should be provided by the alkyl halide.

© 2014 Pearson Education, Inc.

If the reactant is a tertiary alkyl halide, use SN2/E2 conditions because it gives only elimination.

Synthesizing an Alkene

© 2014 Pearson Education, Inc.

Synthesizing an Alkyne

© 2014 Pearson Education, Inc.

Converting an Alkene to an Alkyne

© 2014 Pearson Education, Inc.

Designing a Synthesis

© 2014 Pearson Education, Inc.

Designing a Synthesis

© 2014 Pearson Education, Inc.

Designing a Synthesis

© 2014 Pearson Education, Inc.

Designing a Synthesis