ACYL GROUP: STRUCTURE, CHARACTERISTICS, COMPOUNDS AND DERIVATIVES - CHEMISTRY - 2025

The acyl group is a molecular fragment that in organic chemistry usually derives from carboxylic acids. Its formula is RCO, where R is a carbon, alkyl or aryl substituent, covalently linked directly to the carbonyl group, C = O. It is usually just a fraction of the structure of an organic compound, such as a biomolecule.

It is said to be derived from a carboxylic acid, RCOOH, because it will suffice to eliminate the hydroxyl group, OH, to obtain the acyl group, RCO. Note that this group comprises a broad family of organic (and inorganic) compounds. This family is generally known as acyl compounds (and not asylum).

Structural formula of the acyl group. Source: Su-no-G via Wikipedia.

In the upper image we have the structural formula of the acyl group. It is easy to recognize it by observing any molecular structure, since it is always located at the ends and is indicated by the carbonyl group. An example of this we will see in the acetyl-CoA molecule, essential for the Krebs cycle.

The incorporation of this group into a molecule is known as an acylation reaction. The acyl group is part of the work routine in organic syntheses.

Structure and characteristics of the acyl group

The structure of the acyl group depends on the identity of R. The carbon atom of said R side chain, as well as the C = O to which it is attached, lie in the same plane. The RCO segment of the first image is therefore flat.

However, this fact might seem insignificant if it weren't for the electronic characteristics of C = O: the carbon atom has a slight electron deficit. This makes it susceptible to attack by nucleophilic agents, rich in electrons. Thus, the acyl group is reactive, being a specific site where organic synthesis takes place.

Compounds and derivatives

Depending on the R chains or the atoms that are placed to the right of RCO, different compounds or derivatives of the acyl group are obtained.

- Chlorides

Suppose for example that a chlorine atom is placed to the right of RCO. This now replaces that sinuosity represented in the first image, remaining as: RCOCl. So we have derivatives called acyl chlorides.

Now, changing the identity of R in RCOCl, we get several acyl chlorides:

-HCOCl, R = H, methanoyl chloride, drastically unstable compound

-CH ₃ COCl, R = CH ₃, acetyl chloride

-CH ₃ CH ₂ COCl, R = CH ₂ CH ₃, propionyl chloride

-C ₆ H ₅ COCl, R = C ₆ H ₅ (benzene ring), benzoyl chloride

The same reasoning applies to acyl fluorides, bromides, and iodides. These compounds are used in acylation reactions, with the purpose of incorporating RCO as a substituent to a larger molecule; for example, to a benzene ring.

- Radical

Acyl can exist momentarily as a radical, RCO •, originating from an aldehyde. This species is very unstable, and is immediately disproportionate to an alkyl radical and carbon monoxide:

RC • = O → R • + C≡O

- Cation

The acyl group can also appear as a cation, RCO ⁺, being an intermediate that reacts to acylate a molecule. This species contains two resonance structures represented in the image below:

Acyl cation resonance structures. Source: Jü

Notice how the positive partial charge is distributed between the carbon and oxygen atoms. Of these two structures, with the positive charge on oxygen, it is the most predominant.

- Amides

Now suppose that instead of a Cl atom we place an amino group, NH ₂. We will then have an amide, RCONH ₂, RC (O) NH ₂ or RC = ONH ₂. Thus, finally changing the identity of R, we will obtain a family of amides.

- Aldehydes

If instead of NH ₂ we place a hydrogen atom, we will obtain an aldehyde, RCOH or RCHO. Note that the acyl group is still present even when it has passed into the background of importance. Both aldehydes and amides are acyl compounds.

- Ketones and esters

Continuing with the same reasoning, we can substitute H for another side chain R, which will give rise to a ketone, RCOR 'or RC (O) R'. This time the acyl group is more "hidden", since either of the two ends could be considered as RCO or R'CO.

On the other hand, R 'can also be substituted for OR', giving rise to an ester, RCOOR '. In esters, the acyl group is visible to the naked eye because it is on the left side of the carbonyl group.

- General comment

The acyl group present in various compounds. Source: Jü.

The top image globally represents everything discussed in this section. The acyl group is highlighted in blue, and starting from the top corner, from left to right, we have: ketones, acyl cation, acyl radical, aldehyde, esters, and amides.

Although the acyl group is present in these compounds, as well as in carboxylic acids and thioesters (RCO-SR '), the carbonyl group is usually given more importance when defining its dipole moments. RCO is of greater interest when it is found as a substituent, or when it is directly attached to a metal (metal acyls).

Depending on the compound, RCO can have different names, as seen in the subsection on acyl chlorides. For example, CH ₃ CO is known as acetyl or ethanoyl, while CH ₃ CH ₂ CO, propionyl or propanoyl.

Examples of acyl group

Acetyl-CoA structural formula. Source: User: Bryan Derksen (original) and DMacks (talk) (color-change).

One of the most representative examples of acyl compounds is acetyl-CoA (top image). Note that it is immediately identified because it is highlighted in blue. The acyl group of acetyl-CoA, as its name implies, is acetyl, CH ₃ CO. Although it may not seem like it, this group is essential in the Krebs cycle of our body.

Amino acids also contain the acyl group, only, again, it tends to go unnoticed. For example, for glycine, NH ₂ -CH ₂ -COOH, its acyl group becomes the NH ₂ -CH ₂ -CO segment, and is called glycyl. Meanwhile, for lysine, its acyl group becomes NH ₂ (CH ₂) ₄ CHNH ₂ CO, which is called lysyl.

Although not usually discussed very regularly, acyl groups can also come from inorganic acids; that is, the central atom does not have to be carbon, but can also be made of other elements. For example, an acyl group could also be RSO (RS = O), derived from sulfonic acid, or RPO (RP = O), derived from phosphonic acid.

References

1. Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. (10th edition.). Wiley Plus.
4. Wikipedia. (2020). Acyl group. Recovered from: en.wikipedia.org
5. Helmenstine, Anne Marie, Ph.D. (February 11, 2020). Acyl Group Definition and Examples. Recovered from: thoughtco.com
6. Laura Foist. (2020). Acyl Group: Definition & Structure. Study. Recovered from: study.com
7. Steven A. Hardinger. (2017). Illustrated Glossary of Organic Chemistry: Acyl group. Recovered from: chem.ucla.edu