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What Is a Hemiacetal? - Formation & Definition

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Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

The hemiacetal is part of what makes sugar sweet. In this lesson, we'll learn about the structure of the hemiacetal and how to identify the hemiacetal. We'll also learn how it's formed. Updated: 10/29/2020

Hemiacetal: Definition

We all love eating a piece of candy or a cake, but what makes it taste so sweet? The sugar in these products have OH groups oriented in the perfect way to interact with receptors on our tongue, which creates the sweet taste that we love so much!

Part of the orientation of these OH groups is a hemiacetal. In fact, the sugar glucose may be the most commonly known hemiacetal. A hemiacetal is a carbon connected to two oxygen atoms, where one oxygen is an alcohol (OH) and the other is an ether (OR).

Remember that ''R'' is short hand to denote any carbon chain. The carbon chain can be hundreds of carbon atoms long or as short as one carbon atom. This carbon chain could also only have hydrogen atoms attached, or it could have other functional groups such as alcohols.

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  • 0:04 Hemiacetal Definition
  • 0:55 Hemiacetal Examples
  • 1:21 Hemiacetal Formation
  • 2:48 Glucose Hemiacetal Formation
  • 3:56 Lesson Summary
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Hemiacetal: Examples

The simplest hemiacetal is 1-methoxyethan-1-ol. In this case each ''R'' group is a methane, a CH3.

1-methoxyethan-1-ol is a simple hemiacetal
1-methoxyethan-1-ol

We can see that glucose is a hemiacetal when it's in the cyclical format. When it's in the straight form there is no ether group for it to be a hemiacetal.

Cyclic glucose

In this image the alcohol, R group, and ether are circled in red. The entire hemiacetal is circled in blue.

Hemiacetal: Formation

A hemiacetal forms when an aldehyde reacts with an alcohol. There are two different ways this can occur, as a neutral reaction or catalyzed with an acid.

The neutral reaction only involves the alcohol and the aldehyde. The alcohol attacks the carbon. Those extra electrons are pushed onto the oxygen. So, there's a negative charge on the oxygen and a positive charge on the ether.

The extra proton (hydrogen) on the ether can then transfer over to the oxygen forming the hemiacetal. The reaction occurs slowly because instead of transferring the hydrogen from the ether to the oxygen, the extra electrons on the oxygen could simply go back onto the carbon, kicking off the ether, and we end up with our original reactants.

This reaction can and does go backwards frequently, making it happen very slowly.

Since the acid catalyzes the reaction, it occurs faster. The acid catalyzed reaction puts a hydrogen on the aldehyde oxygen to start out with. This way it starts with a positive charge, so the reaction is much more likely to occur because oxygen does not like to have a positive charge.

The alcohol can quickly react with the carbon. Once those electrons are pushed onto the oxygen it no longer has the positive charge. But the ether still has a positive charge. So now, the conjugate base from the previous acid can remove that hydrogen and the hemiacetal is formed.

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