Chapter 6 1

Elimination Reactions

Chapter 6 2

E1 Mechanism

•  Step 1: halide ion leaves, forming a carbocation. •  Step 2: Base abstracts H+ from adjacent carbon

forming the double bond.

Chapter 6 3

E1 Energy Diagram

The E1 and the SN1 reactions have the same first step: carbocation formation is the rate determining step for both mechanisms.

E1: elimination, unimolecular.

Chapter 6 4

A Closer Look

Reaction coordinate diagram for the E1 reaction of 2-chloro-2-methylbutane

E1 reactions can be regioselective

The role of the leaving group Since the leaving group is involved in the rate-determining step of both E1 and E2, in general, any good leaving group will lead to a fast elimination.

Because the E1 reaction forms a carbocation intermediate, we need to consider carbocation rearrangement

• The rate of an E1 reaction increases as the number of R groups on the carbon with the leaving group increases.

Characteristics of E1 Reactions

• Because the base does not appear in the rate equation, weak bases favor E1 reactions. • E1 reactions are regioselective, favoring formation of the more substituted, more stable alkene.

Competition Between E2 and E1 Reactions

p481a

• Only the E2 is affected by the concentration of base, so at high base concentration E2 is favoured.

• Strong bases at whatever concentration will favour E2 over E1. If you see a strong base being used for an elimination, it is certainly an E2 reaction.

• A tertiary alkyl halide can eliminate by either E2 (with strong bases) or E1 (with weak bases).

• Primary alkyl halides only ever eliminate by E2 because the primary carbocation required for E1 would be too unstable.

Substrate structure

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Polar solvents also favour E1 reactions because they stabilize the intermediate carbocation. E1 eliminations from alcohols in aqueous or alcohol solution are particularly common, and very useful. An acid catalyst is used to promote loss of water, and in the absence of good nucleophiles ensures that substitution does not compete.

Solvent

Mechanism Comment E2 mechanism

Table 8.4 A Comparison of the E1 and E2 Mechanisms

• Much more common and useful • Favored by strong, negatively charged bases, especially –OH and –OR • The reaction occurs with 1°, 2°, and 3° alkyl halides. Order of reactivity:R

3CX > R

2CHX > RCH

2X.

E1 mechanism • Much less useful because a mixture of SN

1 and E1 products usually results • Favored by weaker, neutral bases, such as H

2O and

ROH • This mechanism does not occur with 1° RX because they form highly unstable 1° carbocations.

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Regioselectivity

  Regioselective: Term describing a reaction that can produce two (or more) constitutional isomers but gives one of them in greater amounts than the other.

  In other words, a regioselective reaction selects for a particular constitutional isomer.

a regioselective reaction

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  A stereoselective reaction: is one in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amounts than any other i.e. a stereoselective reaction selects for a particular stereoisomer.

Stereoselectivity

a stereoselective reaction

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 the more stable conformation

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

•  Trans alkenes are more stable than cis alkenes because they have fewer steric interactions.

In summary:

Reaction coordinate diagram for the E2 reaction of 2-bromopentane and ethoxide ion

Stereochemistry of the E1 Reaction

The new π bond can only form if the vacant p orbital of the carbocation and the breaking C–H bond are aligned parallel.

E1 reactions can be stereoselective

p488b

E1 reactions can be stereoselective

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 isotope is 7.1, indicating that C–H (or C–D) bond must be broken in the rate-determining step

kH

kD

Kinetic isotope effects.

In the example in the previous slide, the kinetic isotope effect tells us that the C–H (or C–D) bond is being broken during the rate-determining step, and so the reaction must be an E2 elimination.

Most often, however, we will have to rely on other criteria to predict the outcome of these reactions. To determine the product of a reaction with an alkyl halide:

[1] Classify the alkyl halide as 1°, 2°, or 3°. [2] Classify the base or nucleophile as strong, weak, or bulky.

When Is the Reaction SN1, SN2, E1, or E2?

Competition Between Substitution and Elimination

Alkyl halides can undergo SN2, SN1, E2 and E1

1) decide whether the reaction conditions favor SN2/E2 or SN1/E1

• SN2/E2 reactions are favored by a high concentration of strong nucleophile/ base

• SN1/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

A bulky base encourages elimination over substitution

Consider the SN2/E2 conditions

DBU and DBN are two amidine bases that are particularly basic, with pKaHs of about 12.5. There is not much chance of getting those voluminous fused rings into tight corners—so they pick off the easy-to-reach protons rather than attacking carbon atoms in substitution reactions.

Consider SN1/E1 conditions Now let’s look at what happens when conditions favor SN1/E1 reactions (a poor nucleophile/weak base). In SN1/E1 reactions, the alkyl halide dissociates to form a carbocation, which can then either combine with the nucleophile to form the substitution product or lose a proton to form the elimination product.

Remember that primary alkyl halides do not undergo SN1/E1 reactions because primary carbocations are too unstable to be formed.

E1 elimination mechanism usually has substitution products too.

Table 11.6 summarizes the products obtained when alkyl halides react with nucleophiles/bases under SN2/E2 and SN1/E1 conditions.