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Chapter 16: Problem 36

A primary or secondary alcohol can be protected by conversion to its tetrahydropyranyl ether. Why is formation of THP ethers by this reaction limited to primary and secondary alcohols?

Short Answer

Expert verified
Answer: The formation of THP ethers is limited to primary and secondary alcohols due to the less steric hindrance around their hydroxyl groups, which allows for effective nucleophilic attack on the electrophilic center of dihydropyran. In contrast, tertiary alcohols have significant steric hindrance around the hydroxyl group, making it difficult for them to participate in the formation of THP ethers.

Step by step solution

01

Understanding THP ethers and their formation from alcohols

Tetrahydropyranyl (THP) ethers are protecting groups used to prevent the reactivity of alcohols in organic synthesis. Alcohols can be converted to their corresponding THP ethers by reacting them with dihydropyran (DHP) in the presence of an acid catalyst.
02

Identify the characteristics of primary and secondary alcohols

Primary alcohols have the hydroxyl group (-OH) directly bonded to a carbon atom, which is connected to only one other carbon atom. Secondary alcohols have the hydroxyl group (-OH) directly bonded to a carbon atom, which is connected to two other carbon atoms. Tertiary alcohols have the hydroxyl group (-OH) directly bonded to a carbon atom, which is connected to three other carbon atoms.
03

Investigate the reaction mechanism for the formation of THP ethers

The formation of THP ethers involves the following steps: 1. Protonation of the dihydropyran by the acid catalyst, which activates the double bond in dihydropyran and makes it more electrophilic. 2. Nucleophilic attack of the alcohol oxygen atom on the activated double bond of protonated dihydropyran, leading to the formation of an oxonium ion intermediate. 3. Deprotonation of the oxonium ion, resulting in the formation of the THP ether.
04

Understand why THP ether formation is limited to primary and secondary alcohols

The key step in the formation of THP ethers is the nucleophilic attack of the oxygen atom of the alcohol on the activated double bond of dihydropyran. Primary and secondary alcohols, having less steric hindrance around their hydroxyl groups, can easily attack the electrophilic center of dihydropyran. On the other hand, tertiary alcohols have the hydroxyl group connected to a carbon atom that is connected to three other carbon atoms, which creates a significant steric hindrance around the hydroxyl group. This makes it difficult for the oxygen atom of the tertiary alcohol to effectively attack the activated dihydropyran and form the THP ether.
05

Conclusion

Formation of THP ethers by the reaction of dihydropyran with alcohols is limited to primary and secondary alcohols due to the less steric hindrance around their hydroxyl groups, which enables the nucleophilic attack on the electrophilic center of dihydropyran. In contrast, the higher steric hindrance around the hydroxyl group in tertiary alcohols makes it difficult for them to participate in the formation of THP ethers.

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