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Ketone group

In organic chemistry, a ketone (pronounced /ˈkiːtoʊn/ KEE-toan) is a type of compound that features one carbonyl group (C=O) bonded to two other carbon atoms, i.e., R3CCO-CR3 where R can be a variety of atoms and groups of atoms.[1] With carbonyl carbon bonded to two carbon atoms, ketones are distinct from many other functional groups, such as carboxylic acids, aldehydes, esters, amides, and other oxygen-containing compounds. The double-bond of the carbonyl group distinguishes ketones from alcohols and ethers.

A carbon atom adjacent to a carbonyl group is called an α-carbon. Hydrogen atoms attached to these α-carbon centers are called α-hydrogens. Ketones with α-hydrogen centers participate in a so-called keto-enol tautomerism. The reaction with a strong base gives the corresponding enolate, often by deprotonation of the enol.

Contents

Nomenclature

According to the rules of IUPAC nomenclature, ketones are named by changing the suffix -e of the parent alkane to -one. For the most important ketones, however, traditional nonsystematic names are used virtually exclusively, e.g. acetone and benzophenone. These nonsystematic names are considered retained IUPAC names.[2], although some introductory chemistry textbooks use names such as 2-propanone instead of acetone, the simplest ketone (CH3-CO-CH3). The position of the carbonyl group is usually denoted by a number.

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" is the term that describes the ketone functional group. The term "oxo" is used widely through chemistry, for example it also refers to a single oxygen atom coordinated 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.

The ketone carbon is often described as "sp2 hybridized," terminology that describes both their electronic and molecular structure. Ketones are trigonal planar about the ketonic carbon, with C-C-O and C-C-C bond angles of approximately 120°.

The carbonyl group is polar as a consequence of the fact that the electronegativity of the oxygen center is greater than that for carbonyl 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 a hydrogen-bond acceptors. Ketones are not usually hydrogen-bond donors and cannot hydrogen-bond to itself. 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 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 are symmetrical ketones. Acetophenone (C6H5C(O)CH3) is an unsymmetrical ketone. In the area of stereochemistry, unsymmetrical ketones are known for being prochiral.

Diketones

Many kinds of diketones are known, some with unusual properties. The simplest is biacetyl (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 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 3 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, unsymmetrical ketone that is the precursor to other polymers. Muscone, 3-methylpentadecanone, is a animal pheromone.

Keto-enol tautomerization

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

Ketones that have at least one Éø-hydrogen center, undergo keto-enol tautomerization; the tautomer is an enol. Tautomerization may be 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.

Acidity 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 through dissociation. 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 undergo nucleophilic reactions at that position, with either stoichiometric or catalytic base.

Characterization

Spectroscopy

Ketones and aldehydes display an intense absorption in infra-red spectrum near 1700 cm−1. The peak can occur at slightly higher or lower energies, depending 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 NoE 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. Methyl ketones give positive results for the iodoform test.

Synthesis

Many methods exist for the preparation of ketones in industrial scale, biology, and in academic laboratories. In industry, the most important method probably involves oxidation of hydrocarbons. For example, billion kilograms of cyclohexanone are produced annually by aerobic oxidation of cyclohexane. The oxidation of secondary alcohols is also common:

H3C-CH(OH)-CH3 + O → H3C-CO-CH3 + H2O

Typically, such oxidations employ air or oxygen for industrial processes. For specialized applications, such reactions rely on a strong oxidant such as potassium permanganate or a Cr(VI) compound. Milder conditions such as use of the Dess-Martin periodinane or the Moffatt-Swern oxidation are commonly employed in organic synthesis.

Many other methods have been developed including:

Reactions

Ketones engage in many organic reactions but 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:

Biochemistry

Acetone, acetoacetate and beta-hydroxybutyrate are ketones (or ketone bodies) generated from carbohydrates, fatty acids and amino acids in humans and most vertebrates. Ketones are elevated in blood after fasting including a night of sleep, and in both blood and urine in starvation, hypoglycemia due to causes other than hyperinsulinism, various inborn errors of metabolism, and 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. Acetoacetate and beta-hydroxybutyrate are an important fuel for many tissues, especially during fasting and starvation. The brain, in particular, relies heavily on ketone bodies as a substrate for lipid synthesis and for energy during times of reduced food intake. Ketones have been described as "magic" in their ability to increase metabolic efficiency, while decreasing production of free radicals, the damaging byproducts of normal metabolism. Ketone bodies are relevant to neurological diseases such as Alzheimer's and Parkinson's disease,[4] and the heart and brain operate 25% more efficiently using ketones as a source of energy.[5] Research has also shown ketones play a role in reducing epileptic seizures with the so-called high-fat, near-zero carbohydrate Ketogenic Diet. [1]

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. 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 give 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 generally not highly toxic (for instance, the sugar fructose is a ketone). 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).

References

  1. ^ International Union of Pure and Applied Chemistry. "ketones". Compendium of Chemical Terminology Internet edition.
  2. ^ List of retained IUPAC names retained IUPAC names Link
  3. ^ Haller-Bauer Reaction
  4. ^ Y. Kashiwaya, T. Takeshima, N. Mori, K. Nakashima, K. Clarke and R. L. Veech (2000). "D-beta -Hydroxybutyrate protects neurons in models of Alzheimer's and Parkinson's disease". PNAS 97 (10): 5440–5444. doi:10.1073/pnas.97.10.5440. PMID 10805800. 
  5. ^ Y. Kashiwaya, K. Sato, N. Tsuchiya, S. Thomas, D. A. Fell, R. L. Veech and J. V. Passonneau (1994). "Control of glucose utilization in working perfused rat heart". J. Biol. Chem. 269 (41): 25502–25514. PMID 7929251. http://www.jbc.org/cgi/content/abstract/269/41/25502. 

Wikibooks

Up to date as of January 23, 2010

From Wikibooks, the open-content textbooks collection

A ketone. Carbonyl is in between two hydrocarbons.

A ketone is another functional group under the general groups of carbonyls. In a ketone, the carbonyl carbon is doubly-bonded to an oxygen, and single bonded to two alkyl groups, which can be either identical or different. It is important for both “R” groups to be alkyl groups – if the “R” group is an “H”, it becomes an aldehyde, a carboxylic acid if the “R” group is an “OH”, and an ester if the “R” group is “OR.”

The carbon atom next to the carbonyl carbon is the alpha-carbon. By analogy, the hydrogen’s attached to the alpha-carbon are the alpha-hydrogens.

Nomenclature

Examples of IUPAC nomenclature for ketones.
File:O P281777.jpg
Acetone.

IUPAC Nomenclature
1. In terms of functional group priority, ketones take priority over everything but carboxylic groups and aldehydes.If an aldehyde or carboxylic acid group is there, the ketone is no longer the highest priority and thus, becomes an oxo- prefix.
2. First count the longest carbon chain which contains the carbonyl compound. Name this chain according to normal IUPAC rules.
3. Number the chain such that the carbonyl carbon has the lowest number.
4. The oxygen is designated in the name with the number of the carbon it is on.
5. The suffix of ketones is "-one" so the "e" in the alkyl chain is replaced with "-one."

Another method of naming ketones exists though this method is not used as much. In this method of nomenclature, the two alkyl groups on either side of the carbonyl group are mentioned separately. For example, look at the example of propan-2-one or 2-propanone. Another way of naming this, would be to take the carbons #2 and #3 as one group and #1 as another. The carbonyl carbon becomes a part of the longer alkyl chain - in this example, both are equivalent so it does not matter. Then each side is named and the word "ketone" put at the end of it. Thus, 2-propanone could also be referred to as ethyl methyl ketone.

Common Names
Like other carbonyl compounds, ketones have common names that are used more often in everyday jargon. An example is acetone. The chemical formular of acetone is C3H6O. The IUPAC name is propanone but is often referred to as acetone.

Reactivity

Like the other carbonyl groups, the carbonyl carbon has a large positive charge due to the very electronegative oxygen atom. This characteristic of the ketones allows it to undergo numerous reactions as detailed below:

1. Nucleophilic addition – nucleophilic addition is very common for ketones. Because of the polarity of the carbonyl carbon, nucleophilic attack is a typical reaction ketones undergo. Thus, the carbonylcarbon is subject to nucleophilic attack from other excellent nucleophiles such as cyanide and hydroxide ions. The lone pair of electrons on the nucleophile attacks the positively charged carbon, which moves one of the two bonds shared between the carbon and oxygen onto the oxygen, in the process giving oxygen a full negative charge. For example, a reaction of a ketone and water results in geminal diols.

2. Gnignard – Ketones can be completely transformed, resulting in another functional group. For example, gnignard reagents or organometallic reagants will result in a tertiary alcohol. Of course, this is after an aqueous workup is used – aqueous work up in this case is a solution of acid and water. When using gnignard reagants it is important to remember that a carbon-carbon bond is formed.

An example of a Grignard reaction







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