Organic Chemistry

PROBLEM-SOLVING STRATEGY

Propose a mechanism for the following reaction:

Because the given reagent is an acid, start by protonating the molecule at the position that

allows the most stable carbocation to be formed. By protonating the group, a tertiary

allylic carbocation is formed in which the positive charge is delocalized over two other car-

bons. Then move the electrons so that the 1,2-methyl shift required to obtain the product

can take place. Loss of a proton gives the final product.

Now continue on to Problem 7.

PROBLEM 7

Propose a mechanism for the following reaction:

11.4 Competition Between E2 and E1 Reactions

Primaryalkyl halides undergo only E2 elimination reactions. They cannot undergo E1

reactions because of the difficulty encountered in forming primary carbocations.

Secondaryand tertiaryalkyl halides undergo both E2 and E1 reactions (Table 11.3).

For those alkyl halides that can undergo both E2 and E1 reactions, the E2 reaction

is favored by the same factors that favor an reaction and the E1 reaction is favored

by the same factors that favor an reaction. Thus,an E2 reaction is favored by a

high concentration of a strong base and an aprotic polar solvent (e.g., DMSO or

DMF), whereas an E1 reaction is favored by a weak base and a protic polar solvent

(e.g., or ROH).How the solvent affects the mechanism of the reaction was dis-

cussed in Section 10.10.

H 2 O

SN 1

SN 2

CH 3

CH 3 CH 3

CH 3

C

Cl

CH 3 OH

CH 3

CH 3

H 3 C

HO

H

H

H OH 2

CH 2

H 3 C CH 3

CH 3

H 3 C CH 3

CH 3

CH 3

H 3 C

+ H 3 O+

+

+ +

p

CH 2

CH 3

CH 3

H 3 C

H 3 O+

CH 2

H 3 C CH 3

412 CHAPTER 11 Elimination Reactions of Alkyl Halides • Competition Between Substitution and Elimination

Tutorial:

Common terms for

E1 and E2 reactions

Table 11.3 Summary of the Reactivity of Alkyl Halides in Elimination Reactions

Primary alkyl halide E2only

Secondary alkyl halide E1and E2

Tertiary alkyl halide E1and E2

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