Question

(a) When ethylene oxide is treated with anhydrous HBr gas, the major product is 1,2-dibromoethane. When ethylene oxide is treated with concentrated aqueous HBr, the major product is ethylene glycol. Use mechanisms to explain these results.

(b) Under base-catalyzed conditions, several molecules of propylene oxide can react to give short polymers. Propose a mechanism for the base-catalyzed formation of the following trimer.

Short answer

a)

b)

Step-by-step solution

Acid-catalyzed ring opening of epoxides

Epoxides react under both acidic and basic conditions. The products of the acid-catalyzed ring opening depend primarily on the solvent used.

Base catalyzed ring opening of epoxides

Epoxides are considered to be more reactive since opening the epoxide ring relieves the strain of the three-membered ring. Strong bases can attack and open epoxides, even though the leaving group is an alkoxide. Amines can also open epoxides.

Synthesis

(a) Ethylene oxide on treatment with anhydrous HBr gas, forms 1,2-dibromoethane as the major product.

ethylene oxide 1,2-dibromoethane

the reaction of ethylene oxide with anhydrous HBr

Ethylene oxide on treatment with concentrated aqueous HBr, forms ethylene glycol as the major product.

ethylene oxide ethylene glycol

the reaction of ethylene oxide with concentrated HBr

(b) Strong bases can attack and open epoxides.

Formation of trimer from propylene oxide molecule