PROBLEM 7.80 - Organic Chemistry

Question

Question: As you will learn in Chapter 9, the epoxide is an ether with an oxygen atom in a three-membered ring. Epoxides can be made by intramolecular S_N2  reactions of intermediates that contain a nucleophile and a leaving group on adjacent carbons, as shown.

Assume that each of the following starting materials can be converted to an epoxide by this reaction. Draw the product formed (including stereochemistry) from each starting material. Why might some of these reactions be more difficult than others in yielding nucleophilic substitution products?

a.

b.

c.

d.

Step-by-Step Solution

Answer

a.

b.

c.

d.

Epoxide

An epoxide is represented by the tricyclic ring containing oxygen. The oxide group forms the epoxide by the attack at the electrophilic center via  S_N2 mechanism.

S N 2 mechanism

The S_N2  mechanism is the reaction of nucleophiles directly at the electrophilic center with the opposite stereochemistry if present.

Explanation

a. The given compound has one OH group and one Br group. Therefore, the base NaH abstracts the acidic hydrogen and forms the alkoxide, which attacks the carbon-bearing C-Br bond.  Hence, the final product, that is, epoxide, is formed via S_N2  mechanism.

Formation of epoxide in a 

b.The given compound also forms the epoxide by a similar process as explained in part “a.”

Formation of epoxide in b

c.The given compound also forms the epoxide by a similar process as explained in part “a.”

Formation of epoxide in c

d .The given compound also forms the epoxide by a similar process as explained in part “a.”

Formation of epoxide in d