Organic Chemistry

PROBLEM 10

What alcohol would you treat with phosphorus oxychloride and pyridine to form each of

the following alkenes?

PROBLEM-SOLVING STRATEGY

Propose a mechanism for the following reaction:

Even the most complicated-looking mechanism can be worked out if you proceed one step

at a time, keeping in mind the structure of the final product. The oxygen is the only basic

atom in the starting material, so that is where protonation occurs. Loss of water forms a

tertiary carbocation.

Because the starting material contains a seven-membered ring and the final product has a

six-membered ring, a ring-contraction rearrangement must occur. When doing a ring-

contraction(or a ring-expansion) rearrangement, it can be helpful to label the equivalent

carbons in the reactant and product. Of the two possible pathways for ring contraction, one

leads to a tertiary carbocation and the other leads to a primary carbocation. The correct path-

way must be the one that leads to the tertiary carbocation, since it has the same arrangement

of atoms as the product, and the primary carbocation would be too unstable to form.

The final product can now be obtained by removing a proton from the rearranged carbocation.

Now continue on to Problem 11.

H 2 O

2

1

CH 3

CH 2

1

CH 3

H 3 C H 3 C

CH 2

+

2

H

+ H 3 O+

+

2

1

CH 3

H 3 C

CH 3

2

+

1

+

+

2

1 2

1

CH 2

tertiary carbocation

primary carbocation

CH 3

CH 3

H 3 C

CH 3

CH 3

H 3 C CH 3

CH 3

H 3 C

H 3 C CH 3

CH 3

OH

+

H

H 3 C CH 3

CH 3

+

+ H 2 O

OH

∆

H+

CH 3

CH 3 CH^3

CH 2

H 3 C

H 3 C

CH 3 CH CHCH 2 CH 3

CH 3 CH 2

CH 3

a.CH 3 CH 2 C CH 2

b. d.

c.

450 CHAPTER 12 Reactions of Alcohols, Ethers, Epoxides, and Sulfur-Containing Compounds

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