Organic Chemistry

Section 11.6 Elimination from Cyclic Compounds 417

In contrast, we just saw that an E2 reaction forms both the Eand Zproducts only if the

-carbon from which the proton is removed is bonded to two hydrogens. If it is bond-

ed to only one hydrogen, only one product is obtained because anti elimination is

favored.

PROBLEM 11 SOLVED

For each of the following alkyl halides, determine the major product that is formed when

that alkyl halide undergoes an E1 reaction:

a. c.

b. d.

SOLUTION TO 11a First, we must consider the regiochemistry of the reaction: More

2-hexene will be formed than 1-hexene because 2-hexene is more stable. Next, we must

consider the stereochemistry of the reaction: Of the 2-hexene that is formed, more

(E)-2-hexene will be formed than (Z)-2-hexene because (E)-2-hexene is more stable. Thus,

(E)-2-hexene is the major product of the reaction.

11.6 Elimination from Cyclic Compounds

E2 Elimination from Cyclic Compounds

Elimination from cyclic compounds follows the same stereochemical rules as elimina-

tion from open-chain compounds. To achieve the anti-periplanar geometry that is pre-

ferred for an E2 reaction, the two groups that are being eliminated from a cyclic

compound must be trans to one another. In the case of six-membered rings, the groups

being eliminated will be anti-periplanar only if both are in axial positions.

The more stable conformer of chlorocyclohexane does not undergo an E2 reac-

tion, because the chloro substituent is in an equatorial position. (Recall from

Section 2.13 that the more stable conformer of a monosubstituted cyclohexane is the

one in which the substituent is in an equatorial position because there is more room

groups to be eliminated must

both be in axial positions

groups to be

eliminated

must be trans

H H

Br

X

Cl CH 3

CH 3 CH 2 CH 2 CCH 3

Cl

CH 3

CH 3 CH 2 CH 2 CHCHCH 2 CH 3

I

CH 3

CH 3 CH 2 CH 2 CH 2 CHCH 3

Br

b

CC CC

CH 3 CH 2 CH 2 CH 2 CHCH 3

Br

2-hexene 1-hexene

E1 CH

3 CH 2 CH 2 CH CHCH 3 + CH 3 CH 2 CH 2 CH 2 CH CH 2

(E)-2-hexene

CH 3 CH 2 CH 2 H

H CH 3

(Z)-2-hexene

CH 3 CH 2 CH 2

H H

CH 3

In an E2 reaction of a substituted

cyclohexane, the groups being

eliminated must both be in axial

positions.

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