Hemiacetals

Learning Objectives: After this lesson, you should be able to…

• Identify hemiacetals

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As we wrap up Chapter 12, we are going to look at alcohols as nucleophiles in carbonyl addition reactions.

Aldehydes (or ketones) in water will be in equilibrium with their hydrate form. This equilibrium lies to the left (more so for ketones). This should not be surprising since we know that water is a poor nucleophile and that a carbon-oxygen double bond is more stable than two carbon-oxygen single bonds.

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Similarly, when alcohols are the solvent, a hemiacetal and acetal form exist. A hemiacetal is when one group is an OR and the other is an OH. An acetal is when both are OR groups. Like the hydrate, the carbonyl side is typically favored. However, when a cyclic hemiacetal can form, the hemiacetal is favored and more stable. This is the case for sugars, the building blocks for carbohydrates.

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Here is a video overview of how hemiacetals form.

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