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What is a Hemiacetal? - Formation & Definition

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 will learn about the structure of the hemiacetal and how to identify the hemiacetal. We will also learn how it is formed.

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).

A hemiacetal compound
Hemiacetal

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.

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 is in the cyclical format. When it is in the straight form there is no ether group for it to be a hemiacetal.

Circled in red is the alcohol, R group, and ether. The entire hemiacetal is circled in blue
Cyclic glucose

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.

A hemiacetal formed by a neutral reaction
Neutral reaction

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 is 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 becauseinstead 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.

A hemiacetal formed by an acid catalyzed reaction
Acid catalyzed reaction

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.

Glucose Hemiacetal Formation

In the case of glucose forming the hemiacetal, the alcohol and the aldehyde are on the same molecule. Therefore, it is an intra-molecule reaction, where a molecule reacts with itself. Almost all glucose is in the cyclic form; it is seldom in the straight chain. This shows us how easily this reaction occurs.

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