Acetals contain two –OR groups, one –R group and a –H atom. In hemiacetals, one of the –OR groups in acetals is replaced by a –OH group. This is the key difference between acetal and hemiacetal.
Acetals and hemiacetals are two functional groups which are most commonly found in natural products. Hemiacetal is an intermediate chemical compound formed during the chemical process of acetal formation. Therefore, these two groups have a slight difference in their chemical structure. In detail, the central carbon atom in both of these compounds is a sp3-C atom bonded to four bonds, and out of these four bonds, only one bonding type is different.
CONTENTS
1. Overview and Key Difference
2. What is Acetal
3. What is Hemiacetal
4. Side by Side Comparison – Acetal vs Hemiacetal in Tabular Form
5. Summary
What is Acetal?
Acetal is a functional group where the central carbon atom has four bonds; -OR1, -OR2, -R3 and H (where R1, R2 and R3groups are organic fragments). The two –OR groups may be equivalent to each other (symmetric acetals) or different (mixed acetal).
Figure 1: Acetal
The central carbon atom is said to be saturated since it has four bonds and this provides the central carbon atom a tetrahedral geometry. Acetals can be formed from aldehydes. The formation of an acetal can take place when the hydroxyl group of a hemiacetal becomes protonated to lose a water molecule. The resulting carbocation is then rapidly attacked by an alcohol molecule. During the final step, acetal formation completes after receiving a proton from the alcohol. The mechanism for the formation of acetals can be explained as follows.
Figure 2: Formation of Acetals
In addition, acetals are used to protect carbonyl groups in organic synthesis because they are stable with many oxidizing and reducing agents and in hydrolysis in the basic medium.
Examples
Some examples of chemical compounds containing acetal functional groups are as below.
- Dimethoxymethane: a solvent
- Dioxolane
- Metaldehyde
- Paraldehyde
- Most glycosidic bonds in carbohydrates and other polysaccharides are acetal linkages.
- Cellulose is a ubiquitous example of a polyacetal.
- Polyoxymethylene (POM): a formaldehyde polymer which is used as plastic.
- 1,1-Diethoxyethane (acetaldehyde diethyl acetal), is an important flavoring agent in distilled beverages.
What is Hemiacetal?
Hemiacetals are derived from aldehydes and the term Hemiacetal comes from Greek word “hemi” means “half”.
Figure 3: Hemiacetal
Hemiacetals can be synthesized using several methods; by the nucleophilic addition of alcohol to an aldehyde, by the nucleophilic addition of alcohol to a resonance stabilized hemiacetal cation and by partial hydrolysis of an acetal.
Figure 4: Formation of Hemiacetal
The main structural feature of a hemiacetal molecule is having a central carbon atom with four different bonds; -OR1 group, -R2 group, -H group and a –OH group.
Examples
Most of the hemiacetals are found as common functional groups in natural products. Some examples are;
- Glucose
- Mycorrhizin A
- Thromboxane B2
What is the Difference Between Acetal and Hemiacetal?
Acetal functional group has a sp3 hybridized carbon atom that is bonded to two –OR groups, a hydrogen atom, and a –R group. In contrast, the central atom of hemiacetals contain a sp3-C atom bonded to four different chemical groups; they are –OR, -R, -OH and –H.
Acetals are chemically stable compared to hemiacetals. However, acetals readily hydrolyze back to their parent alcohol and the carbonyl compound in the presence of aqueous acids. In general, we usually consider hemiacetals as unstable chemical compounds, therefore, they tend to form ring structures to raise the stability. In this case, the formation of 5 or 6- membered rings are possible, and this happens by the reaction between the –OH group with a carbonyl group. Two examples of cyclic hemiacetals are glucose and aldose.
Summary – Acetal vs Hemiacetal
Acetals contain two –OR groups, one –R group and a –H atom. In hemiacetals, one of the –OR groups in acetals is replaced by a –OH group. This is the basic difference between acetal and hemiacetal.
References:
1. Acetal. (n.d.). Retrieved October 11, 2016, from here.
2. Hemiacetal. (n.d.). Retrieved October 11, 2016, from here