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.

What type of molecules can form a stable hemiacetal?

When a five- or six-membered ring can be formed. Think glucose, fructose, etc.

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.

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.

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.

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.

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.

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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