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Chapter 11: Problem 32

Starting with cis-3-hexene, show how to prepare the following diols. (a) Meso 3,4-hexanediol (b) Racemic 3,4-hexanediol

Short Answer

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Question: Provide the step-by-step procedures for synthesizing (a) meso 3,4-hexanediol and (b) racemic 3,4-hexanediol from cis-3-hexene. Answer: (a) Meso 3,4-hexanediol: 1. Epoxidation of cis-3-hexene using a peracid such as MCPBA 2. Acid-catalyzed ring-opening with an aqueous acidic solution (e.g., H3O+ in water) (b) Racemic 3,4-hexanediol: 1. Epoxidation of cis-3-hexene using a peracid such as MCPBA 2. Nucleophilic ring-opening using a hydroxide ion (OH-) generated by a base like NaOH in water.

Step by step solution

01

Epoxidation of cis-3-hexene

Treat cis-3-hexene with a peracid, such as MCPBA (meta-chloroperoxybenzoic acid), to create the epoxide. This will form a three-membered ring containing an oxygen atom.
02

Acid-catalyzed ring-opening

To open the epoxide ring, use an aqueous acidic solution (such as H3O+ in water). This will attack the epoxide's less hindered carbon due to steric reasons. The acid-catalyzed reaction will proceed with syn stereochemistry, which means that both hydroxyl groups will attach to the same face of the starting material. This will produce meso 3,4-hexanediol. (b) Racemic 3,4-hexanediol
03

Epoxidation of cis-3-hexene

As in the previous synthesis, begin by treating cis-3-hexene with a peracid like MCPBA to generate the epoxide.
04

Nucleophilic ring-opening

To create a racemic mixture of 3,4-hexanediol, we must open the epoxide ring using separate nucleophiles for each enantiomer. A suitable nucleophile for this reaction is hydroxide ion (OH-), which can be generated by using a base, such as NaOH, in water. Hydroxide will attack the epoxide's less hindered carbon, but due to the absence of the acid catalyst, the reaction will proceed with anti stereochemistry. This means that one hydroxyl group will attach from the opposite face of the cis-3-hexene's alkene. The reaction will generate a racemic mixture, as both enantiomers will be formed in equal amounts.

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Using your roadmap as a guide, show how to convert 1-butanol and ethanol into racemic 2-ethoxy-1-butanol. You must use 1-butanol and ethanol as the source of all carbon atoms in the ether product. Show all required reagents and all molecules synthesized along the way.

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