Difference Between Acyl and Acetyl

Main Difference – Acyl vs Acetyl

The terms acyl and acetyl refer to two functional groups of organic molecules. Acetyl group is a type of acyl group. These functional groups directly contribute to some reactions used in organic synthesis and identification procedures. The main difference between acyl and acetyl is that acyl may or may not contain a –CH₃ group whereas acetyl group essentially contains a –CH₃ group.

Key Areas Covered

1. What is Acyl
      – Definition, Characteristics, Examples
2. What is Acetyl
      – Definition, Characteristics, Examples
3. Similarities Between Acyl and Acetyl
      – Outline of Common Features
4. Difference Between Acyl and Acetyl
      – Comparison of Key Differences

Key Terms: Acetyl, Acetylation, Acyl, Acylation, Deacetylation, Electrophilic Substitution, Leaving Group, Nucleophilic Substitution, Oxoacid, Substitution

What is Acyl

An acyl group is a functional group having the chemical formula of –C(O)R. The R group can be any alkyl group. The R group is attached to the carbon atom via a single bond and the oxygen atom (O) is bonded to the carbon atom via a double bond. The acyl group is formed from an oxoacid.  

Figure 01: General Structure of Acyl Group

One of the major reactions which molecules containing acyl group undergo is acylation. Acylation is the introduction of an acyl group into a different molecule. The following example shows the acylation of benzene. Here, the “X” group is a leaving group (most of the times a halogen atom). The acyl group is attached to the benzene ring, replacing a hydrogen atom of the benzene ring. This reaction is called electrophilic substitution since “X” group leaves, giving the central carbon atom of acyl group a positive charge. Therefore, the acyl group acts as an electrophile. Then it replaces a hydrogen atom of the benzene ring, which is called a substitution.

Figure 02: Acylation of Benzene

The acyl group can undergo nucleophilic substitution reactions. It is the replacement of the leaving group of the molecule containing acyl group by a nucleophile. For example, a molecule containing an acyl group attached with a halogen can undergo nucleophilic substitution reactions. There, the halogen atom can be replaced with a nucleophile such as amine. Then the end product would be an amide. The byproduct would be hydrogen halide (HX).

Figure 03: Nucleophilic Acyl Substitution

Examples for Molecules Containing Acyl Group

• Acyl chlorides (Ex: Benzoyl chloride)
• Amides (Ex: Benzamide)
• Aldehydes (Ex: Propionaldehyde)

What is Acetyl

Acetyl refers to a functional group with the chemical formula -C(O)CH₃. This indicates that acetyl group is a derivative of the acyl group. The acetyl group essentially has a –CH3 (methyl) group attached to the carbon atom and an oxygen atom (O) attached with a double bond to the carbon atom.

Figure 04: Structure of Acetyl Group

Acetyl groups undergo acetylation. Acetylation is the introduction of an acetyl group into a different molecule. The following example shows the acetylation of 2-bromosorcinol. In this example, the acetyl group has replaced a hydrogen atom of the benzene ring. The leaving group of the molecule containing the acetyl group is –OH group. Therefore, the byproduct would be H₂O molecule. The opposite reaction of acetylation is deacetylation.

Figure 05: Acetylation

Examples for Molecules Containing Acetyl Groups

• Aldehydes (Ex: Acetaldehyde)
• Acyl chlorides (Ex: Acetyl chloride)
• Carboxylic acids (Ex: Acetic acid)
• Esters (Ex: Methyl acetate)
• Amides (Ex: Acetamide)

Similarities Between Acyl and Acetyl

• Both acyl and acetyl has a –C=O double bond.

• Both groups are composed of alkyl groups along with another group.

• The geometry around the carbon atom of the acyl and acetyl groups is the same.

Difference Between Acyl and Acetyl

Definition

Acyl Group: Acyl group is a functional group having the chemical formula of –C(O)R.

Acetyl Group: Acetyl refers to a functional group with the chemical formula -C(O)CH₃.

Presence of Methyl Group

Acyl Group: Acyl group may or may not contain ethyl groups.

Acetyl Group: Acetyl group essentially contains a methyl group.

Chemical Reactions

Acyl Group: Molecules containing acyl group can undergo acylation.

Acetyl Group: Molecules containing acetyl groups can undergo acetylation.

Conclusion

Both acyl and acetyl are derivatives of oxoacids such as carboxylic acid. The main difference between acyl and acetyl is in their composition; acyl may or may not contain a –CH₃ group whereas acetyl group essentially contains a –CH₃ group. Acetyl group is a type of acyl group.

Image Courtesy:

1. “Acyl group” (Public Domain) via Commons Wikimedia
2. “Benzene Friedel-Crafts acylation” By Krishnavedala – Own work (Public Domain) via Commons Wikimedia
3. “Acyl Halide plus Amine” (Public Domain) via Commons Wikimedia
4. “Acetyl group” (Public Domain) via Commons Wikimedia

References:

1. “Introduction to carboxylic acid derivatives and the nucleophilic acyl substitution reaction.” Chemistry LibreTexts. Libretexts, 21 July 2016. Web. Available here. 28 June 2017. 
2. Helmenstine, Anne Marie. “Acyl Group Definition.” ThoughtCo. N.p., n.d. Web. Available here. 28 June 2017.