Aldehydes and Ketones - What Are Hemiacetals and Acetals?

8 important questions on Aldehydes and Ketones - What Are Hemiacetals and Acetals?

What is a hemiacetal?

A hemiacetal has an - OH and an -OR or -OAr group bonded to the same carbon.
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What type of molecules can form a stable hemiacetal?

When a five- or six-membered ring can be formed. Think glucose, fructose, etc.
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Under what conditions are acetals usually synthesized?

The formation of acetals is usually carried out with alcohol as solvent, and as acid-catalyst either dry HCl or arenesulfonic acid, ArSO3H is used.
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How do you determine the reactants used to synthesize a hemiacetal or an acetal?

1. Look for the carbon bonded to two oxygen atoms. This carbon belongs to het carbonyl group.
2. Remove both C-O bonds. Add a proton to both oxygen atoms. Convert the previous identified carbonyl-carbon to a carbonyl group.
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Why are acetals used to protect the carbonyl groups of aldehydes and ketones?

Because the acetals are, like ethers, unreactive to bases, reducing agents such as H2/M, to Grignard reagents and to oxidizing agents.
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Why can a Grignard reagent not be used in the reaction between benzaldehyde and 4-Bromobutanal to form 5-hydroxy-5phenylpentanal?

When using a Grignard reagent for this reaction the Grignard reagent prepaired from the 4-Bromobutanal will react with the carbonyl group of other 4-Bromobutanal Grignard reagents. Thereby self-destructing.
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How can the carbonyl group of 4-Bromobutanal be protected against reacting with the Grignard reagent?

The carbonyl group of the 4-Bromobutanal can be converted to an acetal. Usually a cyclic acetal.
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How will the cyclic acetal then react with the benzaldehyde?

The cyclic acetal will be converted to a Grignard reagent which than reacts with the benzaldehyde.
The formed magnesium alkoxide will be treated with aqueous acid resulting in a hydrolysis and recovery of the carbonyl group.
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