Difference Between Acetal and Hemiacetal

Main Difference – Acetal vs Hemiacetal

Acetal and hemiacetal are recognized as functional groups. Hemiacetal is an intermediate formed during the formation of acetal. The formation of acetal is known as acetalisation. Here, the reaction between an aldehyde and an alcohol is used for the synthesis of acetal. Partial hydrolysis of acetal can also be used to form a hemiacetal. Acetal is more stable than hemiacetal. Both groups are composed of sp³ hybridized carbon atoms at the center of the group. The main difference between acetal and hemiacetal is that acetals contain two –OR groups whereas hemiacetals contain one –OR and one –OH group.

Key Areas Covered

1. What is Acetal
      – Definition, Formation
2. What is Hemiacetal
      – Definition, Formation
3. What is the Difference Between Acetal and Hemiacetal
      – Comparison of Key Differences

Key Terms: Acetal, Acetalisation, Alcohol, Aldehyde, Hemiacetal, Hemiketal, Ketal, Ketone

What is Acetal

Acetal is a group of atoms that is represented by a central carbon atom bonded to two –OR groups, -R group and a –H group. The central carbon atom is sp³ hybridized. The R groups are alkyl groups. All R groups can be equal or different from each other. The general formula of an acetal can be given as RHC(OR’)₂. An acetal is a geminal diether derivative of an aldehyde or a ketone.

Figure 1: General Structure of Acetal

Alkyl groups bonded to oxygen atoms are often similar. The carbon atom bearing the two oxygen atoms are known the acetal carbon. The acetal group is considered as a functional group. Ketal is a type of acetal group. Here, the central carbon atom is bonded to two –OR groups and two –R groups. No hydrogen atoms are bonded to the central carbon there.

The geometry around the central carbon atom is tetrahedral. If the two alkyl groups bonded to oxygen atoms are equivalent, then it is called a symmetric acetal. If they are different from each other, then it is called a mixed acetal. But sometimes, one of these fragments can be a hydrogen atom rather than an alkyl group. This is called a hemiacetal.

Acetals are stable when compared to hemiacetals. The formation of acetals is known as acetalisation. This process can be used to form both acetals and ketals. The most common method for acetal formation is the nucleophilic addition of an alcohol to an aldehyde or a ketone. The reaction is catalyzed by acids and gives water as a byproduct. Water should be removed in order to obtain the acetal. Otherwise, water can hydrolyze the acetal easily.

What is Hemiacetal

Hemiacetal is a group of atoms composed of a central carbon atom bonded to four groups: an –OR group, -OH group, -R group and a –H group. Hemiacetal is formed from aldehydes. If it is formed from a ketone, then it is called a hemiketal. The general formula of a hemiacetal is given as RHC(OH)OR’.         

Figure 2: General Structure of a Hemiacetal

Here, the –C-OR group represents an ether group whereas the –C-OH group represents an alcohol group. Therefore, the hemiacetal is formed by the addition of alcohol to a carbonyl group. Here, the carbonyl group belongs to an aldehyde. The R group is an alkyl group. The –OH group of alcohol can attack the carbonyl carbon. It is a nucleophilic addition reaction.

Hemiacetals are formed as an intermediate in the formation of acetals from aldehydes. Therefore hemiacetals can also be formed by partial hydrolysis of an acetal. The reaction between a hemiacetal and an alcohol can form an acetal.

Difference Between Acetal and Hemiacetal

Definition

Acetal: Acetal is a group of atoms that is represented by a central carbon atom bonded to two –OR groups, -R group and a –H group.

Hemiacetal: Hemiacetal is a group of atoms composed of a central carbon atom bonded to four groups; an –OR group, -OH group, -R group and a –H group.

General Formula

Acetal: The general formula of an acetal can be given as RHC(OR’)₂.

Hemiacetal: The general formula of a hemiacetal is given as RHC(OH)OR’.

Formation

Acetal: Acetal can be formed from the reaction between an aldehyde and an alcohol.

Hemiacetal: Hemiacetal is formed as an intermediate of the reaction between an aldehyde and an alcohol.

Stability

Acetal: Acetal is more stable when compared to a hemiacetal.

Hemiacetal: Hemiacetal is less stable when compared to an acetal.

Conclusion

Acetal and hemiacetal are groups of atoms considered as functional groups. There is a slight difference between their chemical structures. The main difference between acetal and hemiacetal is that acetals contain two –OR groups whereas hemiacetals contain one –OR and one –OH group.

Reference:

1. Helmenstine, Anne Marie. “Acetal Definition.” ThoughtCo, Available here.
2. “Acetal.” OChemPal, Available here.
3. “On Acetals and Hemiacetals.” Master Organic Chemistry RSS, Available here.

Image Courtesy:

1. “Generic Acetal” By Jeff Dahl – Own work (Public Domain) via Commons Wikimedia
2. “Hemiacetal-2D-skeletal” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia