In chemistry, an acyl group is a moiety derived by the removal of one or more hydroxyl groups from an oxoacid,[1] including inorganic acids. It contains a double-bonded oxygen atom and an organyl group (R−C=O) or hydrogen in the case of formyl group (H−C=O). In organic chemistry, the acyl group (IUPAC name alkanoyl if the organyl group is alkyl) is usually derived from a carboxylic acid, in which case it has the formula R−C(=O)−, where R represents an organyl group or hydrogen. Although the term is almost always applied to organic compounds, acyl groups can in principle be derived from other types of acids such as sulfonic acids and phosphonic acids. In the most common arrangement, acyl groups are attached to a larger molecular fragment, in which case the carbon and oxygen atoms are linked by a double bond.
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Amides, anhydrides, esters, and acyl chlorides | Organic chemistry | Khan Academy
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Acyl Compounds
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Nomenclature of Acyl Halides
Transcription
What I want to do in this video is talk about a bunch of molecules or classes of molecules that can be derived from carboxylic acid. And just to show a specific example I'll show things that can be derived from acetic acid. And just as a review, acetic acid looks like this. The common name, as I just said, is acetic acid, and if you want to use the systematic name, you look for the longest chain, which is right over there. There's two carbons. So we use the eth- prefix, so it's ethan-, and since this is a carboxylic acid, it is ethanoic acid. Now, the derivatives of acetic acid, and we can later generalize this to all carboxylic acids. We really just have to change what's going on in this carbon chain right here. It won't have to necessarily just be two carbons. It can just keep going. It could have benzene rings, whatever, and that would change the name. But really, I just want to give you the gist and the gist of the naming. So if we were to replace this hydroxyl group with an amine, and in future videos we'll see how that is done, so let me just draw the acyl group. So the acyl group is just that right over there. And we're just going to keep changing what's bonded to the acyl group right over here. So if this is bonded to an amine, so let me draw-- well, this would be the simplest amine right over here, which would be NH2. This thing right here it's called an amide, and if we were to give this its common name, it would be acetamide. This particular example would be acetamide. And if we wanted the systematic name for it, it would be ethanamide. You have two carbons right there so it is ethanamide. Now the natural question is, all amines won't just be primary, you might have other things other than hydrogens attached to it, other radical groups, other carbon chains, so how do you name those? And so if you had a molecule that looked like this, and actually, let me just change things up a little bit so that we diverge a little bit from the ethane route. So let's say you had three carbons bonded or part of the acyl group right there. And then, we are bonded to a nitrogen, which is bonded to a methyl group and then another hydrogen. In this case, you start naming with this methyl group right here and to show that that methyl group is attached to the nitrogen, you call this N-methyl. And then you look at the chain that forms the acyl group, the carbon chain. We have one, two, three carbons so it is propanamide. If you had another methyl here you would say N comma N-dimethyl. If you had a methyl here and a propyl group here, you would've called it N-methyl-N-propyl-propanamide. So hopefully that gives you a sense of amides. Now, and this is something we've seen before so it's a little bit of review, if you have something that looks like this, I'll have it attached to a methyl group right over here. We've seen this before. This is an ester. And if we have an-- let me actually make the part that makes it an ester in blue to diferentiate it. We keep substituting what is attached to the acyl group. Let me label it. This right here is called an acyl group. That right there is an acyl group. So right over here, for the ester, if we were to give it its common name, and we've seen this ester before, it is acetate. And if we wanted to give it its systematic , name you look at the longest chain, one, two carbons so it is ethan-- and you don't call it ethanoic acid anymore. You call it ethanoate, just like that. Now, the next one, and we haven't seen this one before, and it looks complex, but when you really break it down into its constituents, it's not so bad. So let's say we have a molecule that looks like this. So we have one acyl group bonded to an oxygen, which is bonded to another acyl group. So it's almost like you have two carboxylic acids that have been joined together. And you really do have two acyl groups joined by an oxygen here. This is called an anhydride. And they look very complex, but you just have to realize they're two carboxylic acids attached to each other and usually the same one. Most anhydrides you're going to see in organic chemistry are formed from the same carboxylic acid, so how ever many carbons you have on this end, you're normally going to have on this end. So the way the name these is you name it just the same way that you would have named the carboxylic acid, but instead of writing the word acid, you write the word anhydride. So this right here would be acetic anhydride. It's derived from acetic acid. This right here is acetic anhydride. Or the systematic name is, we have one, two carbons so it's ethanoic anhydride. And just to make things clear, if this molecule instead of that, if we had something that looked like this, where the carbons chains on either end had three carbons. One, two, three, one, two, three. We would call this propanoic anhydride. In the unusual circumstance, and it is unusual, where you would see different carbon chains here, you would list each of them. So if this had two here and three here, it would be ethanoic propanoic anhydride. But that is very, very unusual. Normally, these carbon chains on either end of-- or both acyl will groups will contain the same number of carbons. Now, the last carboxylic acid derivative that you should know about, and we've already seen it, are the acyl halides, and, in particular, the acyl chlorides. So let me draw it right over here. So you have your acyl group right there and then it is bonded to a chlorine and this right here is an acyl chloride, maybe the most intuitive name. This right here is an acyl group and then you have a chlorine, so it's an acyl chloride. And we've seen this exact acyl chloride. It's derived from acetic acid, so this is acetyl chloride. But if you wanted to give it its systematic name, and we haven't seen it's systematic name before, we have one, two carbons so it is ethanoyl. This tells us that we are dealing with an acyl group. Ethanoyl chloride is how we would name this. And if this had three carbons, it would be propanoyl chloride. So, hopefully, that gives you at least a good introduction to the differences in structures of all these groups and an introduction to naming them. In the next video, we'll talk a little bit about the relative stabilities and then it'll give you good intuition on which direction a reaction might go. Are you more likely to go from amide to an acyl chloride or from an acyl chloride to an amide or anything in between?
Compounds
Well-known acyl compounds are the acyl chlorides, such as acetyl chloride (CH3COCl) and benzoyl chloride (C6H5COCl). These compounds, which are treated as sources of acylium cations, are good reagents for attaching acyl groups to various substrates. Amides (RC(O)NR′2) and esters (RC(O)OR′) are classes of acyl compounds, as are ketones (RC(O)R′) and aldehydes (RC(O)H), where R and R′ stand for organyl (or hydrogen in the case of formyl).
Acylium cations, radicals, and anions
Acylium ions are cations of the formula RCO+.[2] The carbon–oxygen bond length in these cations is near 1.1 Å (110-112 pm), which is shorter than the 112.8 pm of carbon monoxide and indicates triple-bond character.[3][4][5]
The carbon centres of acylium ions generally have a linear geometry and sp atomic hybridization, and are best represented by a resonance structure bearing a formal positive charge on the oxygen (rather than carbon): [R−C≡O+]. They are characteristic fragments observed in EI-mass spectra of ketones.
Acylium ions are common reactive intermediates, for example in the Friedel–Crafts acylation and many other organic reactions such as the Hayashi rearrangement. Salts containing acylium ions can be generated by removal of the halide from acyl halides:
- RC(O)Cl + SbCl5 → [RCO]+[SbCl6]−
Acyl radicals are readily generated from aldehydes by hydrogen-atom abstraction. However, they undergo rapid decarbonylation to afford the alkyl radical:[6]
- RC(H)=O → RC•=O → R• + C≡O
Acyl anions are almost always unstable—usually too unstable to be exploited synthetically. They readily react with the neutral aldehyde to form an acyloin dimer. Hence, synthetic chemists have developed various acyl anion synthetic equivalents, such as dithianes, as surrogates. However, as a partial exception, hindered dialkylformamides (e.g., diisopropylformamide, HCONiPr2) can undergo deprotonation at low temperature (−78 °C) with lithium diisopropylamide as the base to form a carbamoyl anion stable at these temperatures.[7]
In biochemistry
In biochemistry there are many instances of acyl groups, in all major categories of biochemical molecules.
Acyl-CoAs are acyl derivatives formed via fatty acid metabolism. Acetyl-CoA, the most common derivative, serves as an acyl donor in many biosynthetic transformations. Such acyl compounds are thioesters.
Names of acyl groups of amino acids are formed by replacing the -ine suffix with -yl. For example, the acyl group of glycine is glycyl, and of lysine is lysyl.
Names of acyl groups of ribonucleoside monophosphates such as AMP (5′-adenylic acid), GMP (5′-guanylic acid), CMP (5′-cytidylic acid), and UMP (5′-uridylic acid) are adenylyl, guanylyl, cytidylyl, and uridylyl respectively.
In phospholipids, the acyl group of phosphatidic acid is called phosphatidyl-.
Finally, many saccharides are acylated.
In organometallic chemistry and catalysis
Acyl ligands are intermediates in many carbonylation reactions, which are important in some catalytic reactions. Metal acyls arise usually via insertion of carbon monoxide into metal–alkyl bonds. Metal acyls also arise from reactions involving acyl chlorides with low-valence metal complexes or by the reaction of organolithium compounds with metal carbonyls. Metal acyls are often described by two resonance structures, one of which emphasizes the basicity of the oxygen center. O-alkylation of metal acyls gives Fischer carbene complexes.[8]
Nomenclature
The common names of acyl groups are derived typically by replacing the -ic acid suffix of the corresponding carboxylic acid's common name with -yl (or -oyl), as shown in the table below.
In the IUPAC nomenclature of organic chemistry, the systematic names of acyl groups are derived exactly by replacing the -yl suffix of the corresponding hydrocarbyl group's systemic name (or the -oic acid suffix of the corresponding carboxylic acid's systemic name) with -oyl, as shown in the table below.
The acyls are between the hydrocarbyls and the carboxylic acids.
The hydrocarbyl group names that end in -yl are not acyl groups, but alkyl groups derived from alkanes (methyl, ethyl, propyl, butyl), alkenyl groups derived from alkenes (propenyl, butenyl), or aryl groups (benzyl).
| Corresponding hydrocarbyl group name RC– |
Acyl group name RC(O)– |
Corresponding carboxylic acid name RC(O)O-H | |||
|---|---|---|---|---|---|
| common | systematic | common | systematic | common | systematic |
| methyl | formyl | methanoyl | formic acid | methanoic acid | |
| ethyl | acetyl | ethanoyl | acetic acid | ethanoic acid | |
| propyl | propionyl | propanoyl | propionic acid | propanoic acid | |
| butyl | butyryl | butanoyl | butyric acid | butanoic acid | |
| propenyl | acrylyl or acryloyl | propenoyl | acrylic acid | propenoic acid | |
| crotyl | butenyl | crotonyl | butenoyl | crotonic acid | butenoic acid |
| benzyl | benzoyl | benzoic acid | |||
Acyl species
In acyloxy groups the acyl group is bonded to oxygen: R−C(=O)−O−R′ where R−C(=O) is the acyl group.
Acylium ions are cations of the type R−C+=O ↔ R−C≡O+ and play an important role as intermediates in organic reactions[1] for example the Hayashi rearrangement.
See also
References
- ^ a b IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Acyl groups". doi:10.1351/goldbook.A00123
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Acyl species". doi:10.1351/goldbook.A00129
- ^ Chevrier, B.; Carpentier, J. M. Le; Weiss, R. (1972). "Synthesis of two crystalline species of the Friedel–Crafts intermediate antimony pentachloride-p-toluoyl chloride. Crystal structures of the donor–acceptor complex and of the ionic salt". J. Am. Chem. Soc. 94 (16): 5718–5723. doi:10.1021/ja00771a031.
- ^ Davlieva, Milya G.; Lindeman, Sergey V.; Neretin, Ivan S.; Kochi, Jay K. (2004). "Structural effects of carbon monoxide coordination to carbon centers. π and σ bindings in aliphatic acyl versus aromatic aroylcations". New J. Chem. 28: 1568–1574. doi:10.1039/B407654K.
- ^ Hermannsdorfer, André; Driess, Matthias (2021). "Silicon Tetrakis(trifluoromethanesulfonate): A Simple Neutral Silane Acting as a Soft and Hard Lewis Superacid". Angew. Chem. Int. Ed. 60 (24): 13656–13660. doi:10.1002/anie.202103414. PMC 8252640. PMID 33826216.
- ^ Smith, Michael B. (2013). March's Advanced Organic Chemistry. Hoboken, NJ: Wiley. p. 857. ISBN 978-0-470-46259-1.
- ^ Fraser, Robert R.; Hubert, Patrick R. (1974-01-01). "Direct Formation of the Carbonyl Anion of Diisopropyl Formamide". Canadian Journal of Chemistry. 52 (1): 185–187. doi:10.1139/v74-029. ISSN 0008-4042.
- ^ Elschenbroich, C. (2006). Organometallics. Weinheim: Wiley-VCH. ISBN 3-527-29390-6.
External links
Media related to Acyl groups at Wikimedia Commons
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This page was last edited on 12 February 2024, at 18:41