Acetal vs. Hemiacetal: Synthesis, Examples, Uses

Acetal and Hemiacetal are the main classes of carbohydrates i.e. organic compounds that differ in their functionality based on differences in the number of ether linkages present. Acetals are compounds with two ether (-OR) linkages whereas hemiacetals contain one such linkage. Acetals are the final products of their type and are more stable than hemiacetals. Hemiacetals, on the other hand, are reaction intermediate compounds having relatively less stability.

The acetal and hemiacetal carbon is actually the central carbon derived from the carbonyl group and these forms only exist in cyclic forms i.e. a cyclic form of glucose is a hemiacetal compound. Similarly, fructose is a hemiketal compound.

Difference between Acetal and Hemiacetal

Acetal	Hemiacetal
An acetal contains two ether linkages	A hemiacetal contains one ether linkage
Acetal is the final product	Hemiacetal is an intermediate in the formation of acetals
Acetals are formed by the reaction between aldehydes and alcohols in a process called acetalization	Hemiacetals are generally prepared by the partial hydrolysis of acetals
Acetals are more stable than hemiacetals	Hemiacetals are relatively less stable and they can be separated by the use of stabilizers
The general formula of acetals is as: R'HC-(OR'')₂	The general formula of hemiacetals is as: R'HC-(OR'')(OH)
For example, benzylidene acetal, dioxolane, metaldehyde, paraldehyde, etc	For example, 1-methoxyethan-1-ol, glucose and many other sugars are hemiacetals, etc

Acetal: (Acetals and Ketals)

Acetal functional group refers to the di-ether type linkages present in hydrocarbon chains, provided that the central carbon atom is further bonded to only one carbon atom. The central carbon atom is tetrahedrally (sp³) bonded and is derived from the carbonyl carbon of synthesizing fragments. Acetals are recognized by a general formula RHC-(OR)₂.

Image of acetal functional groups

An acetal may be ‘symmetrical’ if both ether linkages are having the same numbered carbon chains or it may be an ‘asymmetrical’ (mixed) acetal. Generally, acetals are made from fragments like aldehydes and ketones.

Ketals, on the other hand, are a similar functional group with a change in identification of the functional identity of the acetal group. An acetal contains a hydrogen with central carbon atom while a ketal has alkyl groups on both sides of the central carbon atom. They have a general formula of ketals is R₂C-(OR)₂.

Image of Ketal functional groups

Acetals, the final product of acetalization reaction are mostly formed by the reaction between aldehydes and alcohols. If in case, when ketones are used in place of aldehydes, ketals are synthesized as the product.

Formation of Acetals (Acetalization)

Acetalization is a process of acetal synthesis by the nucleophilic addition of an alcoholic functional group onto the electrophilic carbon of the carbonyl groups. This carbonyl shall necessarily be an aldehyde (for acetals) or ketone (for ketals).

The acetalization reaction is the acid-catalyzed reaction consisting of the following mechanism.

The protonation of carbonyl oxygen is done and is followed by an attack of the hydroxyl group from alcohol. This generates a hemiacetal or hemiketal based on the type of carbonyl compound being used. The hemiacetal structure is again protonated leading to an oxonium ion. The reverse reaction takes place at this stage by adding water again which leads to another oxonium ion on the other end of acetal. Finally, deprotonation or regeneration of catalyst (acidic proton) leads to the formation of acetal or a ketal.

The purpose of acetalization is often the creation of protecting groups as acetalization is a reversible reaction.

Related topics

Examples of Acetals

Some examples of acetals are as;

Benzylidene acetal
Metaldehyde
Paraldehyde
Dioxolane
1,3,5-trioxane
1,1-diethoxyethane
Dimethoxymethane
Phenylsulfonyl ethylidene (PSE)
Glycosidic bonds in sugars, etc.

There are some polyacetals like;

Cellulose
Polyoxymethylene, etc.

Hemiacetal: (Hemiacetal and Hemiketal)

Hemiacetals are a class of reaction intermediates synthesized as partial acetalization reactions. Like acetals, hemiacetal compounds are also synthesized by the action of the alcoholic group on electrophilic carbon of aldehydes (for hemiacetals) or ketones (for hemiketals).

Most sugar molecules like glucose are termed hemiacetals. Their general formula is R’HC-(OR”)(OH). The alcoholic (-OH) group attached to the electrophilic carbon is the identification of hemiacetals.

Cyclic hemiacetals and hemiketals exist in 5 or 6 membered rings. Common examples of such cyclic hemiacetals are glucose, etc and that of hemiketals are fructose, etc.

Hemiacetal formation reaction

The most common hemiacetal compound to be found in nature is glucose. The less strained and more favorable structure being the acetal form, the hemiacetal form of compounds converts to acetal form. Usually, this reaction just needs a dehydrating agent.

The conversion of hemiacetal to acetal form is accomplished in the same environment as during the conversion of aldehyde to hemiacetal, or ketone to hemiketal.

Protonation leading to the nucleophilic attack of hydroxyl (-OH) group on electrophilic carbon leads to deprotonation and the hemiacetal form gets converted to acetal form.

Examples of Hemiacetals

Hemiacetals exist only in cyclic forms of compounds like dextrose (glucose). Some examples are given as;

1-methoxyethan-1-ol
Common sugars like glucose, etc.

Alpha(α) and Beta(β) Glucose are hemiacetals, and in acidic environments, these hemiacetals change to acetals.

Industrial applications of Acetals and Hemiacetals

There are a large number of large and small products made from acetals. Mechanical products like handles, hinges, glides, gears, wheel fans, valve bodies, nuts, screws, and bolts are made up of materials containing polymeric acetal compounds. Medicinal delivery devices like insulin delivery drugs, and some electrical insulators, connectors, and devices are produced from acetal compounds. Several toy components, jewelry items like watch bands, and food delivery convoy systems also contain acetal compounds as a major component.

The useful properties of acetals are further enhanced by PTFE (Polytetrafluoroethylene) to improve lubricity and fiberglass is used to enhance its stiffness.

Synthetic polymer acetals are homopolymers and copolymers. Industrially, acetals are sometimes referred to as Delrin®, Celcon®, Ultraform®, and Duracon®, etc.

Delrin® acetal homopolymer plastic

It is an ideal material to replace metals. It gives low friction, high wear resistance with high strength and stiffness. Delrin plastic is commonly available in sheets, tubes, and rods. It is also sometimes known as Black Delrin® plastic DuPont acetal.

Acetal Acetron® GP Barra

It is an industrial-grade material with a good coefficient of friction. Its applications include exposure to moisture conditions.

Delrin® Acetal Sheet Homopolymer Colored (Red Acetal)

It is a strong, stiff, easy-to-machine type of plastic. It is usually available in red, orange, yellow, and blue colors.

Polyoxymethylene homopolymer

(POM) Polyoxymethylene homopolymer is also termed as acetal, polyacetal, and polyformaldehyde. It is an engineering thermoplastic used for precision instruments. It is industrially called with names Delrin, Kocetal, Ultrafoprm, Celcon, Ramtal, Duracon, Kepital, Polypenco, Tenac, and Hostaform.

Black acetal

Black acetal is a thermoplastic based acetal used in engineering. It is based on polymerization based technology. Black acetal is also named Celcon®, and Ultraform® buy several companies.

Acetal Natural (White)

It is an engineering thermoplastic based on formaldehyde polymerization technology. This acetal copolymer has an exceptional balance of tensile properties, shear strength, stiffness, and toughness. White acetal is sometimes called natural acetal.

Polyethylene-Vinyl Acetate (PEVA)

It is a copolymer of ethylene and vinyl acetate. Ethylene-Vinyl Acetate (EVA) is commonly referred to as thermoplastic ethylene-vinyl acetate copolymer and it is a thermoplastic type elastomer material.

Key Takeaway(s)

Concepts Berg

Why are acetals more stable than hemiacetals?

Acetals are complete products whereas hemiacetals are intermediates in the production of acetals. Furthermore, the tendency of hemiacetals to get protonated makes them less stable.

What’s the difference between hemiacetal and acetal?

Acetal is a diether linked complete product whereas hemiacetal is a reaction intermediate with one ether linkage.

How are acetals formed?

Acetals are formed by the acidic hydroxylation of aldehydes.

How do you identify acetals?

Acetals are necessarily having a diether linkage. They are complete reaction products.

How do you go from hemiacetal to acetal?

Protonation, then hydroxylation, and then deprotonation leads a hemiacetal to acetal.

Is glucose an acetal or hemiacetal?

Glucose is a common hemiacetal but due to more stability, it usually converts to acetal form.

Hemiacetal vs Acetal Sugars

Hemiacetal sugars are reducing while acetals consist of non-reducing sugars. Acetal forms being stable do not convert back to hemiacetal or aldehydic forms. Alpha and beta sugars are mostly hemiacetals that convert to acetals in acidic environments.

Ketal vs Acetal

A ketal is a ketone derivative whereas an acetal belongs to the aldehydic family.

What is anomeric carbon?

An anomeric carbon is a stereogenic center of carbonyl compounds which then transform into any carbon of sugar molecules. Whatever the geometry, this carbon can still make two optical isomers based on the position of hydroxyl group. This gives rise to alpha and sugars.

Why are 5 or 6-membered cyclic hemiacetals more stable than acyclic hemiacetals?

Five or six membered cyclic hemiacetals are relatively more stable than acyclic hemiacetals because of less entropy and no change in intermolecular bond enthalpy during cyclization.

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