Ketone

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Ketone

In

steroids (e.g., testosterone), and the solvent acetone.[1]

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, ketone names are derived by changing the suffix -ane of the parent alkane to -anone. Typically, the position of the carbonyl group is denoted by a number, but traditional nonsystematic names are still generally used for the most important ketones, 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−C(=O
)−CH3) instead of "acetone".

The derived names of ketones are obtained by writing separately the names of the two

Greek letters
, the α-carbon being the atom adjacent to carbonyl group.

Although used infrequently, oxo is the

IUPAC nomenclature for the oxo group (=O) and used as prefix when the ketone does not have the highest priority. Other prefixes, however, are also used. For some common chemicals (mainly in biochemistry), keto refer to the ketone functional group
.

Structure and properties

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

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

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 unsymmetrical derivatives, depending on the equivalency of the two organic substituents attached to the carbonyl center. Acetone and benzophenone ((C6H5)2CO) are symmetrical ketones. Acetophenone (C6H5C(O)CH3) is an unsymmetrical ketone.

Diketones

Many kinds of diketones are known, some with unusual properties. The simplest is

coordination chemistry
.

Unsaturated ketones

Ketones containing

hydrogenated
.

Cyclic ketones

Many ketones are cyclic. The simplest class have the formula (CH2)nCO, where n varies from 2 for cyclopropanone ((CH2)2CO) to the tens. Larger derivatives exist. Cyclohexanone ((CH2)5CO), 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 (CH2)3CO.

Keto-enol tautomerization

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

Ketones that have at least one

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

C−H bonds adjacent to the carbonyl in ketones are more acidic

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., CH3CH2ONa in CH3CH2OH, or NaH
) provides the more-substituted thermodynamic enolate.

Ketones are also weak bases, undergoing

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 in that it has a hydrogen atom attached to its carbonyl group, making aldehydes easier to oxidize. Ketones do not have a hydrogen atom bonded to the carbonyl group, and are therefore more resistant to oxidation. They are

oxidizing agents which have the ability to cleave
carbon–carbon bonds.

Spectroscopy

Ketones (and aldehydes) absorb strongly in the infra-red spectrum near 1750 cm−1, which is assigned to νC=O ("carbonyl stretching frequency"). The energy of the peak is lower for aryl and unsaturated ketones.[8]

Whereas

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

Ketones also give positive results when treated with 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

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:[10]

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

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:[10]

  • With strong oxidizing agents to give carboxylic acids. Ketones are generally oxidized under vigorous conditions, i.e., strong oxidizing agents and at elevated temperatures. Their oxidation involves carbon-carbon bond cleavage to afford a mixture of carboxylic acids having lesser number of carbon atoms than the parent ketone.

Biochemistry

Ketones are pervasive in nature. The formation of organic compounds in

Krebs cycle which releases energy from sugars and carbohydrates.[22]

In medicine,

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

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. ^ Raymond, Kenneth W. (2010). General Organic and Biological Chemistry (3rd ed.). Wiley. p. 297.
  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 Archived 9 July 2023 at the Wayback Machine
  5. ^ Evans, David A. (4 November 2005). "Evans pKa table" (PDF). Evans group website. Archived from the original (PDF) on 19 June 2018. Retrieved 14 June 2018.
  6. .
  7. ^ Smith, Brian. "The C=O Bond, Part VIII: Review". Spectroscopy. Archived from the original on 13 February 2024. Retrieved 12 February 2024.
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  19. ^ Haller–Bauer Reaction. homeip.net
  20. .
  21. ^
    ISBN 9783527306732.{{cite book}}: CS1 maint: DOI inactive as of January 2024 (link
    )

External links

  • Media related to Ketones at Wikimedia Commons