por He acyl group is a molecular fragment that in organic chemistry is usually derived from carboxylic acids. Its formula is RCO, where R is a carbon, alkyl or aryl substituent, covalently bonded 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 that it derives from a carboxylic acid, RCOOH, because it will suffice to remove the hydroxyl group, OH, to obtain the acyl group, RCO. Note that this group comprises a broad family of organic (and inorganic) compounds. Such a family is generally known as acyl (and not asylum) compounds.
In the image above 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. We will see an example of this 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 synthesis.
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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 were not for the electronic characteristics of C=O: the carbon atom has a slight deficit of electrons. This makes it susceptible to attack by nucleophilic agents, rich in electrons. Thus, the acyl group is reactive, being a specific site in which organic synthesis is carried out.
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. We then have some derivatives called acyl chlorides.
Now, changing the identity of R to RCOCl, we get various acyl chlorides:
-HCOCl, R= H, methanoyl chloride, drastically unstable compound
-CH3COCl, R= CH3, acetyl chloride
-CH3CH2COCl, R= CH2CH3, propionyl chloride
-C6H5COCl, R = C6H5 (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
The acyl can momentarily exist as a radical, RCO•, originating from an aldehyde. This species is very unstable, disproportionating immediately into an alkyl radical and carbon monoxide:
RC•=O → R• + C≡O
– Cation
The acyl group can also occur as a cation, RCO+, being an intermediate that reacts to acylate a molecule. This species contains two resonance structures represented in the image below:
Notice how the partial positive charge is distributed between the carbon and oxygen atoms. Of those two structures, [R–C≡O+]with the positive charge on oxygen, is the most predominant.
– Amides
Suppose now that instead of a Cl atom we put an amino group, NH2. We will then have an amide, RCONH2, RC(O)NH2 or RC=ONH2. Thus, finally changing the identity of R, we will obtain a family of amides.
– Aldehydes
If instead of NH2 we place a hydrogen atom, we will obtain an aldehyde, RCOH or RCHO. Note that the acyl group is still present even though it has receded into the background of importance. Both aldehydes and amides are acyl compounds.
– Ketones and esters
Continuing with the same reasoning, we can replace H by 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 extremes 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 easily recognized by the naked eye because it is on the left side of the carbonyl group.
– General comment
The image above 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 their dipole moments. The RCO is of greater interest when it is found as a substituent, or when it is directly linked to a metal (metal acyls).
Depending on what the compound is, RCO can have different names, as we saw in the subsection on acyl chlorides. For example, CH3CO is known as acetyl or ethanoyl, while CH3CH2CO is known as propionyl or propanoyl.
Examples of acyl group
One of the most representative examples of acyl compounds is that of acetyl-CoA (upper image). Note that it is immediately identifiable because it is highlighted in blue. The acyl group of acetyl-CoA, as its name suggests, is acetyl, CH3CO. 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, NH2-CH2-COOH, its acyl group becomes the NH2-CH2-CO segment, and is called glycyl. Meanwhile, for lysine, its acyl group becomes NH2(CH2)4CHNH2CO, which is called lysyl.
Although it is 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 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
Morrison, RT and Boyd, R, N. (1987). organic chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
Carey F. (2008). organic chemistry. (Sixth edition). Mc Graw Hill.
Graham Solomons TW, Craig B. Fryhle. (2011). organic chemistry. (10th ed.). WileyPlus.
Wikipedia. (2020). Acyl group. Retrieved from: en.wikipedia.org
Helmenstine, Anne Marie, Ph.D. (February 11, 2020). Acyl Group Definition and Examples. Retrieved from: thoughtco.com
Laura Foist. (2020). Acyl Group: Definition & Structure. Study. Retrieved from: study.com
Steven A. Hardinger. (2017). Illustrated Glossary of Organic Chemistry: Acyl group. Retrieved from: chem.ucla.edu