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From Wikipedia, the free encyclopedia

Ketone group
Ketone group

In chemistry, a ketone (alkanone) /ˈktn/ is an organic compound with the structure RC(=O)R', where R and R' can be a variety of carbon-containing substituents. Ketones and aldehydes are simple compounds that contain a carbonyl group (a carbon-oxygen double bond). They are considered "simple" because they do not have reactive groups like −OH or −Cl attached directly to the carbon atom in the carbonyl group, as in carboxylic acids containing −COOH.[1] Many ketones are known and many are of great importance in industry and in biology. Examples include many sugars (ketoses) and the industrial solvent acetone, which is the smallest ketone.

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Transcription

In the last video, we learned a little bit about aldehydes. And we saw that they have the general structure where you have a carbonyl group bonded to some type of carbon chain, or I guess in the simplest form, this could just be a hydrogen over here, and then they definitely will have at least one hydrogen bonded to the carbon in the carbonyl group, so this was an aldehyde. Now, very closely related to an aldehyde is a type of molecule called a ketone. And let's draw a couple of ketones just to make things clear, and then we'll think about what the difference is between a ketone and an aldehyde. So this right here, CH3, CH3, carbon right over here. This right here is a ketone. And you say, hey, Sal, that looks very similar to an aldehyde. I have a carbonyl group in both. So this right here is a carbonyl group. This right here is a carbonyl group. Let me write this: carbonyl. We have a carbonyl group in both. We have a carbon chain over here. This could be a general carbon chain here. We have a methyl group. So how is this different? And I think you'll see. It's this part that I haven't highlighted yet. In an aldehyde, there's two ways to think about an aldehyde. You could either say that, look, the carbonyl group is at the end of a carbon chain. So the next thing over is going to be a hydrogen. Or you could say that in an aldehyde, you have at least one hydrogen bonded to the carbonyl carbon. And the way I remember that, so you have a hydrogen there, and in the ketone you don't. The carbonyl group is embedded in a carbon chain. It is bonded to-- at least it is bonded to a carbon on either side, so over here you have a carbon. And the way that I remember the difference, and this is really just a little bit of a mnemonic just to memorize it, is aldehyde has an "h" in it. There's an "h" right over there, and an aldehyde has a hydrogen bonded to this carbon. So now that we at least have a reasonable understanding of what a ketone is, let's name a few, just to familiarize ourselves. So this right here is the simplest possible ketone. And it's called acetone, or that's its common name. And actually, the word ketone comes from the German word for acetone, which was I think instead of a "c" there, they had a "k," so it's like aketone. And they said, oh, this is a ketone. So this is the common name. Another kind of traditional or common way of naming a ketone is to name each of these groups, and it's kind of similar to the way that we named ethers. But instead of writing the word ether at the end, we write the word ketone. So here we have a methyl group, and we have another methyl group, so we have two methyl groups right over here. So this would be dimethyl ketone. This right here is dimethyl ketone. And then if you wanted the systematic way of naming it, you just look at the longest carbon chain, which is one, two, three carbons, so it's prop. Propa-, and instead of calling it propane, we get rid of that "e" over there and we would call it propanone. That tells us that this right here is a ketone. And you have to know where this double bond is. And actually, for propanone, you don't have to specify it, because if you know it's a ketone, you know that it has to have a carbon on either side of the carbonyl group, so you actually don't even have to specify where the carbonyl group is. But if you wanted to, you could say, OK, that's going to be on the two carbon. No matter what direction you start counting from, it's going to be on the two carbon. But the two is kind of optional for propanone. Let's do a couple of other ones. So let's say we had a molecule that looks like this. So the traditional way of naming it, you'd say, OK, on this end of the ketone, I have one, two, three carbons. So on that end, I have three carbons. That is a propyl group. And on this other side of the ketone right over here, I have only one carbon. That is a methyl group. So then you would just name them. And you name then in order of increasing chain size, molecule size, or group size. So this one you'd write methyl first. Methyl, because it's only one carbon. So this is methyl propyl ketone. This is kind of the traditional or the common way, often kind of the most used way, of naming this molecule. But the systematic way of naming it, you look at the longest carbon chain and you say, OK, I have one, two, three, four, five carbons. So it's going to be pent-. And then you want to start numbering it so that the carbonyl carbon has the lowest possible number. So you want to start numbering on the right side: one, two, three, four and five. So this right here, so we said the prefix would be pent-. So it's penton, and instead of saying it's pentane, you say it's pentanone. And to specify where the carbonyl group is, you say it's 2. This is 2-pentanone. And you might also see it written like this: pentan-2-one. Either one of these right here would be acceptable. Let's do a slightly more complicated example. Let's say we had something that looked like this. So we have something, a molecule, that looks like this. And let me stick some chlorines over here. So what would this be? Well, our longest chain, once again, is this cyclohexane: one, two, three, four, five, six carbons. And I'll just name this systematically right here. And the more complicated things get, the more systematic people will want to name it. So if we have six carbons right here, and they're in a chain, so this is cyclohexane. You'd put the "e" there if this carbonyl group wasn't there. But since it is, we would call this cyclohexanone, So this right here tells us to name it cyclohexanone. And then in a ring like this, this would implicitly be the number one carbon. So if this is the number one carbon and we want to number in the direction so that the next groups have the lowest possible number, so we want to make this the two carbon. So this is 2,2-dichlorocyclohexanone. Now, there's two more. And I'll just show these to you because these tend to be referred to by their common names. So I just want to show them to you real fast. One is this molecule right here, where we have a methyl group on this side: CH3. And over here, we have a benzene ring. Now, that first super simple ketone that we saw, we called this acetone. And so the common name here is actually derived from acetone. Instead of calling it acetone, because it doesn't have just a methyl group here, this is called aceto-, and instead of acetone, it's acetophenone, because we have this phenyl group, that benzene ring right there. Acetophenone, which is a pretty common molecule, and you'll see it referred to this way. Now, the other one that you might see every now and then, and I just want to expose it to you, is a molecule that looks like this, that has two benzene rings on it. It looks like that. And this is benzophenone. These last two I just really wanted to expose you to their common names. But, in general, I think you have a decent idea at this point of how to name at least the simpler chains, either with the common names, for example, propyl, or methyl propyl ketone, or 2-pentanone. And these are the more typical or maybe the easier naming examples.

Contents

Nomenclature and etymology

The word ketone is derived from Aketon, an old German word for acetone.[2][3]

According to the rules of IUPAC nomenclature, ketones are named by changing the suffix -ane of the parent alkane to -anone. The position of the carbonyl group is usually denoted by a number. For the most important ketones, however, traditional nonsystematic names are still generally used, for example acetone and benzophenone. These nonsystematic names are considered retained IUPAC names,[4] although some introductory chemistry textbooks use systematic names such as "2-propanone" or "propan-2-one" for the simplest ketone (CH3−CO−CH3) instead of "acetone".

The common names of ketones are obtained by writing separately the names of the two alkyl groups attached to the carbonyl group, followed by "ketone" as a separate word. The names of the alkyl groups are written alphabetically. When the two alkyl groups are the same, the prefix is added before the name of alkyl group. The positions of other groups are indicated by Greek letters, the α-carbon being the atom adjacent to carbonyl group. If both alkyl groups in a ketone are the same then the ketone is said to be symmetrical, otherwise unsymmetrical.

Although used infrequently, oxo is the IUPAC nomenclature for a ketone functional group. Other prefixes, however, are also used. For some common chemicals (mainly in biochemistry), keto or oxo refer to the ketone functional group. The term oxo is used widely through chemistry. For example, it also refers to an oxygen atom bonded to a transition metal (a metal oxo).

Structure and properties

Representative ketones, from the left: acetone, a common solvent; oxaloacetate, an intermediate in the metabolism of sugars; acetylacetone in its (mono) enol form (the enol highlighted in blue); cyclohexanone, precursor to nylon; muscone, an animal scent; and tetracycline, an antibiotic.
Representative ketones, from the left: acetone, a common solvent; oxaloacetate, an intermediate in the metabolism of sugars; acetylacetone in its (mono) enol form (the enol highlighted in blue); cyclohexanone, precursor to nylon; muscone, an animal scent; and tetracycline, an antibiotic.

The ketone carbon is often described as "sp2 hybridized", a description that includes both their electronic and molecular structure. Ketones are trigonal planar around the ketonic carbon, with C−C−O and C−C−C bond angles of approximately 120°. Ketones differ from aldehydes in that the carbonyl group (CO) is bonded to two carbons within a carbon skeleton. In aldehydes, the carbonyl is bonded to one carbon and one hydrogen and are located at the ends of carbon chains. Ketones are also distinct from other carbonyl-containing functional groups, such as carboxylic acids, esters and amides.[5]

The carbonyl group is polar because the electronegativity of the oxygen is greater than that for carbon. Thus, ketones are nucleophilic at oxygen and electrophilic at carbon. Because the carbonyl group interacts with water by hydrogen bonding, ketones are typically more soluble in water than the related methylene compounds. Ketones are hydrogen-bond acceptors. Ketones are not usually hydrogen-bond donors and cannot hydrogen-bond to themselves. Because of their inability to serve both as hydrogen-bond donors and acceptors, ketones tend not to "self-associate" and are more volatile than alcohols and carboxylic acids of comparable molecular weights. These factors relate to the pervasiveness of ketones in perfumery and as solvents.

Classes of ketones

Ketones are classified on the basis of their substituents. One broad classification subdivides ketones into symmetrical and asymmetrical derivatives, depending on the equivalency of the two organic substituents attached to the carbonyl center. Acetone and benzophenone (C6H5C(O)C6H5) are symmetrical ketones. Acetophenone (C6H5C(O)CH3) is an asymmetrical ketone. In the area of stereochemistry, asymmetrical ketones are known for being prochiral.

Diketones

Many kinds of diketones are known, some with unusual properties. The simplest is diacetyl (CH3C(O)C(O)CH3), once used as butter-flavoring in popcorn. Acetylacetone (pentane-2,4-dione) is virtually a misnomer (inappropriate name) because this species exists mainly as the monoenol CH3C(O)CH=C(OH)CH3. Its enolate is a common ligand in coordination chemistry.

Unsaturated ketones

Ketones containing alkene and alkyne units are often called unsaturated ketones. The most widely used member of this class of compounds is methyl vinyl ketone, CH3C(O)CH=CH2, which is useful in the Robinson annulation reaction. Lest there be confusion, a ketone itself is a site of unsaturation; that is, it can be hydrogenated.

Cyclic ketones

Many ketones are cyclic. The simplest class have the formula (CH2)nCO, where n varies from 2 for cyclopropanone to the teens. Larger derivatives exist. Cyclohexanone, a symmetrical cyclic ketone, is an important intermediate in the production of nylon. Isophorone, derived from acetone, is an unsaturated, asymmetrical ketone that is the precursor to other polymers. Muscone, 3-methylpentadecanone, is an animal pheromone. Another cyclic ketone is cyclobutanone, having the formula C4H6O.

Keto-enol tautomerization

Keto-enol tautomerism. 1 is the keto form; 2 is the enol.
Keto-enol tautomerism. 1 is the keto form; 2 is the enol.

Ketones that have at least one alpha-hydrogen, undergo keto-enol tautomerization; the tautomer is an enol. Tautomerization is catalyzed by both acids and bases. Usually, the keto form is more stable than the enol. This equilibrium allows ketones to be prepared via the hydration of alkynes.

Acid/base properties of ketones

Ketones are far more acidic (pKa ≈ 20) than a regular alkane (pKa ≈ 50). This difference reflects resonance stabilization of the enolate ion that is formed upon deprotonation. The relative acidity of the α-hydrogen is important in the enolization reactions of ketones and other carbonyl compounds. The acidity of the α-hydrogen also allows ketones and other carbonyl compounds to react as nucleophiles at that position, with either stoichiometric and catalytic base. Using very strong bases like lithium diisopropylamide (LDA, pKa of conjugate acid ~36) under non-equilibrating conditions (–78 °C, 1.1 equiv LDA in THF, ketone added to base), the less-substituted kinetic enolate is generated selectively, while conditions that allow for equilibration (higher temperature, base added to ketone, using weak or insoluble bases, e.g., NaOEt in EtOH, or NaH) provides the more-substituted thermodynamic enolate.

Ketones are also weak bases, undergoing protonation on the carbonyl oxygen in the presence of Brønsted acids. Ketonium ions (i.e., protonated ketones) are strong acids, with pKa values estimated to be somewhere between –5 and –7.[6][7] Although acids encountered in organic chemistry are seldom strong enough to fully protonate ketones, the formation of equilibrium concentrations of protonated ketones is nevertheless an important step in the mechanisms of many common organic reactions, like the formation of an acetal, for example. Acids as weak as pyridinium cation (as found in pyridinium tosylate) with a pKa of 5.2 are able to serve as catalysts in this context, despite the highly unfavorable equilibrium constant for protonation (Keq < 10–10).

Characterization

An aldehyde differs from a ketone because of its hydrogen atom attached to its carbonyl group, making aldehydes easier to oxidize. Ketones don't have a hydrogen atom bonded to the carbonyl group, and are more resistant to oxidation. They are only oxidized by powerful oxidizing agents which have the ability to cleave carbon-carbon bonds.

Spectroscopy

Ketones and aldehydes absorb strongly in the infra-red spectrum near 1700 cm−1. The exact position of the peak depends on the substituents.

Whereas 1H NMR spectroscopy is generally not useful for establishing the presence of a ketone, 13C NMR spectra exhibit signals somewhat downfield of 200 ppm depending on structure. Such signals are typically weak due to the absence of nuclear Overhauser effects. Since aldehydes resonate at similar chemical shifts, multiple resonance experiments are employed to definitively distinguish aldehydes and ketones.

Qualitative organic tests

Ketones give positive results in Brady's test, the reaction with 2,4-dinitrophenylhydrazine to give the corresponding hydrazone. Ketones may be distinguished from aldehydes by giving a negative result with Tollens' reagent or with Fehling's solution. Methyl ketones give positive results for the iodoform test.[8] Ketones also give positive results when treated with Metadinitrobenzene(m-dinitrobenzene) in presence of dilute Sodium Hydroxide to give violet coloration.

Synthesis

Many methods exist for the preparation of ketones in industrial scale and academic laboratories. Ketones are also produced in various ways by organisms, see the section on biochemistry below.

In industry, the most important method probably involves oxidation of hydrocarbons, often with air. For example, a billion kilograms of cyclohexanone are produced annually by aerobic oxidation of cyclohexane. Acetone is prepared by air-oxidation of cumene.

For specialized or small scale organic synthetic applications, ketones are often prepared by oxidation of secondary alcohols:

R2CH(OH) + O → R2C=O + H2O

Typical strong oxidants (source of "O" in the above reaction) include potassium permanganate or a Cr(VI) compound. Milder conditions make use of the Dess–Martin periodinane or the Moffatt–Swern methods.

Many other methods have been developed, examples include:[9]

Reactions

The Haller-Bauer reaction occurs between a non-enolizable ketone and a strong amide base.  In this prototypical example involving benzophenone, the tetrahedral intermediate expels phenyl anion to give benzamide and benzene as the organic products
The Haller-Bauer reaction occurs between a non-enolizable ketone and a strong amide base. In this prototypical example involving benzophenone, the tetrahedral intermediate expels phenyl anion to give benzamide and benzene as the organic products

Ketones engage in many organic reactions. The most important reactions follow from the susceptibility of the carbonyl carbon toward nucleophilic addition and the tendency for the enolates to add to electrophiles. Nucleophilic additions include in approximate order of their generality:[9]

Biochemistry

Ketones are pervasive in nature. The formation of organic compounds in photosynthesis occurs via the ketone ribulose-1,5-bisphosphate. Many sugars are ketones, known collectively as ketoses. The best known ketose is fructose, which exists as a cyclic hemiketal, which masks the ketone functional group. Fatty acid synthesis proceeds via ketones. Acetoacetate is an intermediate in the Krebs cycle which releases energy from sugars and carbohydrates.[22]

In medicine, acetone, acetoacetate, and beta-hydroxybutyrate are collectively called ketone bodies, generated from carbohydrates, fatty acids, and amino acids in most vertebrates, including humans. Ketone bodies are elevated in the blood (ketosis) after fasting, including a night of sleep; in both blood and urine in starvation; in hypoglycemia, due to causes other than hyperinsulinism; in various inborn errors of metabolism, and intentionally induced via a ketogenic diet, and in ketoacidosis (usually due to diabetes mellitus). Although ketoacidosis is characteristic of decompensated or untreated type 1 diabetes, ketosis or even ketoacidosis can occur in type 2 diabetes in some circumstances as well.

Applications

Ketones are produced on massive scales in industry as solvents, polymer precursors, and pharmaceuticals. In terms of scale, the most important ketones are acetone, methylethyl ketone, and cyclohexanone.[23] They are also common in biochemistry, but less so than in organic chemistry in general. The combustion of hydrocarbons is an uncontrolled oxidation process that gives ketones as well as many other types of compounds.

Toxicity

Although it is difficult to generalize on the toxicity of such a broad class of compounds, simple ketones are, in general, not highly toxic. This characteristic is one reason for their popularity as solvents. Exceptions to this rule are the unsaturated ketones such as methyl vinyl ketone with LD50 of 7 mg/kg (oral).[23]

See also

References

  1. ^ an introduction to aldehydes and ketones. Chemguide.co.uk. Retrieved on 2016-10-28.
  2. ^ Harper, Douglas. "ketone". Online Etymology Dictionary.
  3. ^ The word "ketone" was coined in 1848 by the German chemist Leopold Gmelin. See: Leopold Gmelin, ed., Handbuch der organischen Chemie: Organische Chemie im Allgemeinen … (Handbook of organic chemistry: Organic chemistry in general … ), 4th ed., (Heidelberg, (Germany): Karl Winter, 1848), volume 1, p. 40. From page 40: "Zu diesen Syndesmiden scheinen auch diejenigen Verbindungen zu gehören, die als Acetone im Allegemeinen (Ketone?) bezeichnet werden." (To these syndesmides*, those compounds also seem to belong, which are designated as acetones in general (ketones?).") [*Note: In 1844, the French chemist Auguste Laurent suggested a new nomenclature for organic compounds. One of his new classes of compounds was "syndesmides", which were compounds formed by the combination of two or more simpler organic molecules (from the Greek σύνδεσμος (syndesmos, union) + -ide (indicating a group of related compounds)). For example, acetone could be formed by the dry distillation of metal acetates, so acetone was the syndesmide of two acetate ions. See: Laurent, Auguste (1844) "Classification chimique," Comptes rendus, 19 : 1089–1100 ; see especially p. 1097.
  4. ^ List of retained IUPAC names retained IUPAC names  Link
  5. ^ McMurry, John E. (1992), Organic Chemistry (3rd ed.), Belmont: Wadsworth, ISBN 0-534-16218-5
  6. ^ Evans, David A. (November 4, 2005). "Evans pKa table" (PDF). Evans group website. Retrieved June 14, 2018.
  7. ^ Smith, Michael B. (2013). March's Advanced Organic Chemistry (7th ed.). Hoboken, N.J.: Wiley. pp. 314–315. ISBN 978-0-470-46259-1.
  8. ^ Mendham, J.; Denney, R. C.; Barnes, J. D.; Thomas, M. J. K. (2000), Vogel's Quantitative Chemical Analysis (6th ed.), New York: Prentice Hall, ISBN 0-582-22628-7
  9. ^ a b Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7
  10. ^ Marvel, C. S.; Sperry, W. M. (1928). "Benzophenone". Organic Syntheses. 8: 26. doi:10.15227/orgsyn.008.0026.
  11. ^ a b c d e Furniss, Brian; Hannaford, Antony; Smith, Peter; Tatchell, Austin (1996). Vogel's Textbook of Practical Organic Chemistry (5th ed.). London: Longman Science & Technical. pp. 612–623, 976–977, 982–983. ISBN 9780582462366.
  12. ^ Allen, C. F. H.; Barker, W. E. (1932). "Desoxybenzoin". Organic Syntheses. 12: 16. doi:10.15227/orgsyn.012.0016.
  13. ^ Gulati, K. C.; Seth, S.R.; Venkataraman, K. (1935). "Phloroacetophenone". Organic Syntheses. 15: 70. doi:10.15227/orgsyn.015.0070.
  14. ^ Tietze, Lutz F.; Bratz, Matthias (1993). "Dialkyl Mesoxalates by Ozonolysis of Dialkyl Benzalmalonates: Dimethyl Mesoxalate". Organic Syntheses. 71: 214. doi:10.15227/orgsyn.071.0214.
  15. ^ Heinzelman, R. V. (1955). "o-Methoxyphenylacetone". Organic Syntheses. 35: 74. doi:10.15227/orgsyn.035.0074.
  16. ^ Wiley, Richard H.; Borum, O. H. (1953). "3-Acetamido-2-butanone". Organic Syntheses. 33: 1. doi:10.15227/orgsyn.033.0001.
  17. ^ Moffett, R. B.; Shriner, R. L. (1941). "ω-Methoxyacetophenone". Organic Syntheses. 21: 79. doi:10.15227/orgsyn.021.0079.
  18. ^ Thorpe, J. F.; Kon, G. A. R. (1925). "Cyclopentanone". Organic Syntheses. 5: 37. doi:10.15227/orgsyn.005.0037.
  19. ^ Fieser, Louis F. (1937). "1,2-Naphthoquinone". Organic Syntheses. 17: 68. doi:10.15227/orgsyn.017.0068.
  20. ^ Herbst, R. M.; Shemin, D. (1939). "Phenylpyruvic acid". Organic Syntheses. 19: 77. doi:10.15227/orgsyn.019.0077.
  21. ^ Haller–Bauer Reaction. homeip.net
  22. ^ Nelson, D. L.; Cox, M. M. (2000) Lehninger, Principles of Biochemistry. 3rd Ed. Worth Publishing: New York. ISBN 1-57259-153-6.
  23. ^ a b Siegel, Hardo; Eggersdorfer, Manfred (2000). "Ketones". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a15_077. ISBN 9783527306732.
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