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Chapter 14: Q 39 P (page 747)

Question. The following reaction resembles the acid-catalyzed cyclization of squalene oxide. Propose a mechanism for this reaction.

Short Answer

Expert verified

Epoxide ring is highly strained due to which it readily opens up so as to release strain, thus oxygen in epoxide ring abstracts acidic proton from acid, and oxygen acquires positive charge due to which strain on ring further increases, hence it opens up and alkene carbon which is electron rich acts as a nucleophile and is present in same system thus intramolecularly attacks at carbocation generated.

Abstraction of proton and attack of nucleophile at carbocation

Step by step solution

01

Step-1. Abstraction of acidic hydrogen and ring opening of squalene oxide:

Epoxide ring is highly strained due to which it readily opens up so as to release strain, thus oxygen in epoxide ring abstracts acidic proton from acid, and oxygen acquires positive charge due to which strain on ring further increases, hence it opens up and alkene carbon which is electron rich acts as a nucleophile and is present in same system thus intramolecularly attacks at carbocation generated.

Abstraction of proton and attack of nucleophile at carbocation

02

Step-2. Attack of water as a nucleophile at carbocation and formation of product:

Water acts as a nucleophile and attacks at carbocation center, then after neutralization of positive charge on oxygen, we get our required product, that is, 3-hydroxymethyl-1-methyl-cyclopentan-1-ol.

Attack of water as a nucleophile and formation of final product

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Most popular questions from this chapter

Question. Predict the major products of the following reactions, including stereochemistry where appropriate.

  1. 2,2-dimethyloxirane +(oxygen-labelled water)
  2. 2,2-dimethyloxirane +
  3. (2S,3R)-2-ethyl-2,3-dimethyloxirane +

(2S,3R)-2-ethyl-2,3-dimethyloxirane +

Question. Under the right conditions, the following acid-catalyzed double cyclization proceeds in remarkably good yields. Propose a mechanism. Does this reaction resemble a biological process you have seen?

(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.

Show how you would synthesize the following ethers in good yields from the indicated starting materials and any additional reagents needed.

(a) Cyclopentyl n propyl etherfromcyclopentanolandpropan-1-ol.

(b) n-butyl phenyl etherfromphenolandbutan-1-ol.

(c) 2-ethoxyoctanefrom anoctene

(d) 1-methoxydecanefrom adecene

(e) 1-ethoxy-1-methylcyclohexanefrom 2-methylcyclohexanol.

(f) trans-2,3-epoxyoctane from octane-2-ol.

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.
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