Chapter 6 1

Elimination Reactions

In addition to substitution, an alkyl halide can undergo an elimination reaction

Chapter 6 3

Elimination Reactions

•  Elimination reactions produce double bonds. •  The alkyl halides loses a hydrogen and the

halide. •  Also called dehydrohalogenation (-HX).

The E2 Reaction

Second-order reaction

E2: elimination, bimolecular.

The weaker the base, the better it is as a leaving group

The role of the leaving group

Any good leaving group will lead to a fast elimination.

The role of the leaving group

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

Reaction coordinate diagram for the E2 reaction of 2-bromobutane and methoxide ion

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

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

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

Steric hindrance also affects the product distribution

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.

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).