The acyl group It 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 and directly linked 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 be enough 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).
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.
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.
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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, are located 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 in which organic syntheses are carried out.
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.
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
-CH3COCl, R = CH3, acetyl chloride
-CH3CHtwoCOCl, R = CHtwoCH3, 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.
Acyl can exist momentarily as a radical, RCO •, originating from an aldehyde. This species is very unstable, and it is immediately disproportionate to an alkyl radical and carbon monoxide:
RC • = O → R • + C≡O
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 positive partial charge is distributed between the carbon and oxygen atoms. Of these two structures, [R-C≡O+], with the positive charge on oxygen, is the most predominant.
Now suppose that instead of a Cl atom we place an amino group, NHtwo. We will then have an amide, RCONHtwo, RC (O) NHtwo or RC = ONHtwo. Thus, finally changing the identity of R, we will obtain a family of amides.
If instead of NHtwo 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.
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 recognized by the naked eye because it is on the left side of the carbonyl group.
The top image globally represents everything discussed in this section. The acyl group is highlighted in blue, and starting from the upper 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 linked to a metal (metal acyls).
Depending on the compound, RCO can have different names, as seen in the subsection on acyl chlorides. For example, CH3CO is known as acetyl or ethanoyl, while CH3CHtwoCO, propionyl or propanoyl.
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 indicates, 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, NHtwo‐CHtwo‐COOH, its acyl group becomes the NH segmenttwo‐CHtwo‐CO, and is called glycyl. Meanwhile, for lysine, its acyl group becomes NHtwo(CHtwo)4CHNHtwoCO, 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 made of 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..
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