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Chapter 14: Q 48P (page 749)

(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

Expert verified

a)

b)

Step by step solution

01

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.

02

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.

03

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

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Most popular questions from this chapter

Question. Grignard reactions are often limited by steric hindrance. While Grignard reagents react in high yield with ethylene oxide and monosubstituted epoxides, yields are often lower with disubstituted epoxides. Tri- and tetrasubstituted epoxides react with difficulty, if at all.

(a) Show how to make these alcohols by a Grignard reacting with an epoxide.

(b) These alcohols cannot be made by a Grignard plus an epoxide. Show the reagents that would be required and why that reaction would be unlikely to succeed.

Propose a fragmentation to account for each numbered peak in the mass spectrum of n-butyl isopropyl ether.

Which of the following ethers can be formed in good yield by condensation of the corresponding alcohols? For those that cannot be formed by condensation, suggest an alternative method that will work.

(a) 诲颈产耻迟测濒鈥夆赌塭迟丑别谤

(b) 别迟丑测濒鈥夆赌塶-辫谤辞辫测濒鈥夆赌塭迟丑别谤

(c) 诲颈-蝉别肠-产耻迟测濒鈥夆赌塭迟丑别谤

An acid-catalyzed reaction was carried out using methyl cellosolve (2-methoxyethanol) as the solvent. When the 2-methoxyethanol was redistilled, a higher-boiling point fraction (bp 162oC) was also recovered. The mass spectrum of this fraction showed the molecular weight to be 134. The IR and NMR spectrum are shown here. Determine the structure for this compound and propose a mechanism for its formation.

Question: (a) Predict the values of m/zand the structures of the most abundant fragments you would observe in the mass spectrum of di-n-propyl-ether.

(b) Give logical fragmentations to account for the following ions observed in the mass spectrum of 2-methoxypentane: 102,87,71,59,31.
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