In
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
, enols are a type of
functional group
In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...
or
intermediate in
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
containing a group with the formula (R = many substituents). The term ''enol'' is an abbreviation of ''alkenol'', a
portmanteau
In linguistics, a blend—also known as a blend word, lexical blend, or portmanteau—is a word formed by combining the meanings, and parts of the sounds, of two or more words together. deriving from "-ene"/"alkene" and the "-ol". Many kinds of enols are known.
[
Keto–enol tautomerism refers to a ]chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which both the Reagent, reactants and Product (chemistry), products are present in concentrations which have no further tendency to change with time, so that there is no observable chan ...
between a "keto" form (a carbonyl
In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double bond, double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such a ...
, named for the common ketone
In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...
case) and an enol. The interconversion of the two forms involves the transfer of an alpha hydrogen atom and the reorganisation of bonding electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s. The keto and enol forms are tautomers
In chemistry, tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert.
The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the reloca ...
of each other.
Enolization
Organic
Organic may refer to:
* Organic, of or relating to an organism, a living entity
* Organic, of or relating to an anatomical organ
Chemistry
* Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
ester
In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
s, ketones
In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...
, and aldehydes
In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...
with an α-hydrogen
In the nomenclature of organic chemistry, a locant is a term to indicate the position of a functional group or substituent within a molecule.
Numeric locants
The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of n ...
( bond adjacent to the carbonyl group
In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such as aldehydes ...
) often form enols. The reaction involves migration of a proton () from carbon to oxygen:
:
In the case of ketones, the conversion is called a keto-enol tautomerism, although this name is often more generally applied to all such tautomerizations. Usually the equilibrium constant is so small that the enol is undetectable spectroscopically.
In some compounds with two (or more) carbonyls, the enol form becomes dominant. The behavior of 2,4-pentanedione
Acetylacetone is an organic compound with the chemical formula . It is classified as a 1,3- diketone. It exists in equilibrium with a tautomer . The mixture is a colorless liquid. These tautomers interconvert so rapidly under most conditions that ...
illustrates this effect:
:
Enols are derivatives of vinyl alcohol
Vinyl alcohol, also called ethenol (IUPAC name; not ethanol) or ethylenol, is the simplest enol. With the formula , it is a labile compound that converts to acetaldehyde immediately upon isolation near room temperature. It is not a practical prec ...
, with a connectivity. Deprotonation of organic carbonyls gives the enolate anion
In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl () compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds.
Bonding and structure
Enolate anions are electr ...
, which are a strong nucleophile
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
. A classic example for favoring the keto form can be seen in the equilibrium between vinyl alcohol
Vinyl alcohol, also called ethenol (IUPAC name; not ethanol) or ethylenol, is the simplest enol. With the formula , it is a labile compound that converts to acetaldehyde immediately upon isolation near room temperature. It is not a practical prec ...
and acetaldehyde
Acetaldehyde (IUPAC systematic name ethanal) is an organic compound, organic chemical compound with the chemical formula, formula , sometimes abbreviated as . It is a colorless liquid or gas, boiling near room temperature. It is one of the most ...
(K = nol Nol may refer to:
Surname
* Koen van Nol (born 1974), Dutch judoka
Given name
* Nol Havens (born 1959), lead singer of band The Art Company
* Nol Maassen (1922–2009), Dutch politician
* Nol de Ruiter (born 1940), Dutch football coach and f ...
eto ETO may refer to:
Science and technology
* Emitter turn off thyristor, a semiconductor device
* Ethylene oxide, an organic compound
* RUNX1T1, a gene
* Efforts to Outcomes, software produced by Social Solutions
Sports
* ETO-SZESE Győr FKC, a Hung ...
nbsp;≈ 3). In 1,3-diketones, such as acetylacetone
Acetylacetone is an organic compound with the chemical formula . It is classified as a 1,3-diketone. It exists in equilibrium with a tautomer . The mixture is a colorless liquid. These tautomers interconvert so rapidly under most conditions that ...
(2,4-pentanedione), the enol form is more favored.
The acid-catalyzed conversion of an enol to the keto form proceeds by proton transfer from O to carbon. The process does not occur intramolecularly, but requires participation of solvent or other mediators.
Stereochemistry of ketonization
If R1 and R2 (note equation at top of page) are different substituents, there is a new stereocenter formed at the alpha position when an enol converts to its keto form. Depending on the nature of the three R groups, the resulting products in this situation would be diastereomer
In stereochemistry, diastereomers (sometimes called diastereoisomers) are a type of stereoisomer. Diastereomers are defined as non-mirror image, non-identical stereoisomers. Hence, they occur when two or more stereoisomers of a compound have di ...
s or enantiomer
In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...
s.
Enediols
Enediols are alkenes with a hydroxyl group on each carbon of the C=C double bond. Normally such compounds are disfavored components in equilibria with acyloin
In organic chemistry, acyloins or α-hydroxy ketones are a class of organic compounds of the general form , composed of a hydroxy group () adjacent to a ketone group (). The name ''acyloin'' is derived from the fact that they are formally deri ...
s. One special case is catechol
Catechol ( or ), also known as pyrocatechol or 1,2-dihydroxybenzene, is an organic compound with the molecular formula . It is the ''ortho'' isomer of the three isomeric benzenediols. This colorless compound occurs naturally in trace amounts. It ...
, where the C=C subunit is part of an aromatic ring. In some other cases however, enediols are stabilized by flanking carbonyl groups. These stabilized enediols are called reductone A reductone is a special class of organic compounds. They are enediols with a carbonyl group adjacent to the enediol group, i.e. RC(OH)=C(OH)-C(O)R. The enediol structure is stabilized by the resonance resulting from the tautomerism with the adjace ...
s. Such species are important in glycochemistry, e.g., the Lobry de Bruyn–Van Ekenstein transformation.
:
Ribulose-1,5-bisphosphate
Ribulose 1,5-bisphosphate (RuBP) is an organic substance that is involved in photosynthesis, notably as the principal acceptor in plants. It is a colourless anion, a double phosphate ester of the ketopentose (ketone-containing sugar with five c ...
is a key substrate in the Calvin cycle
The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into ...
of photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
. In the Calvin cycle, the ribulose equilibrates with the enediol, which then binds carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
. The same enediol is also susceptible to attack by oxygen (O2) in the (undesirable) process called photorespiration
Photorespiration (also known as the oxidative photosynthetic carbon cycle or C2 cycle) refers to a process in plant physiology, plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. Th ...
.
Phenols
Phenol
Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire.
The molecule consists of a phenyl group () ...
s represent a kind of enol. For some phenols and related compounds, the keto tautomer plays an important role. Many of the reactions of resorcinol
Resorcinol (or resorcin) is a phenolic compound. It is an organic compound with the formula C6H4(OH)2. It is one of three isomeric benzenediols, the 1,3-isomer (or ''meta- (chemistry), meta''-isomer). Resorcinol crystallizes from benzene as co ...
involve the keto tautomer, for example. Naphthalene-1,4-diol exists in observable equilibrium with the diketone tetrahydronaphthalene-1,4-dione.
:
Biochemistry
Keto–enol tautomerism is important in several areas of biochemistry
Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, a ...
.
The high phosphate-transfer potential of phosphoenolpyruvate
Phosphoenolpyruvate (2-phosphoenolpyruvate, PEP) is the carboxylic acid derived from the enol of pyruvate and a phosphate anion. It exists as an anion. PEP is an important intermediate in biochemistry. It has the high-energy phosphate, highest-e ...
results from the fact that the phosphorylated compound is "trapped" in the less thermodynamically favorable enol form, whereas after dephosphorylation it can assume the keto form.
The enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
enolase
Phosphopyruvate hydratase, usually known as enolase, is a metalloenzyme () that catalyses the conversion of 2-phosphoglycerate (2-PG) to phosphoenolpyruvate (PEP), the ninth and penultimate step of glycolysis. The chemical reaction is:
:2-ph ...
catalyzes the dehydration of 2-phosphoglyceric acid
2-Phosphoglyceric acid (2PG), or 2-phosphoglycerate, is a glyceric acid which serves as the substrate in the ninth step of glycolysis. It is catalyzed by enolase into phosphoenolpyruvate (PEP), the penultimate step in the conversion of glucose to ...
to the enol phosphate ester. Metabolism of PEP to pyruvic acid
Pyruvic acid (CH3COCOOH) is the simplest of the keto acids, alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate acid, conjugate base, CH3COCOO−, is an metabolic intermediate, intermediate in several m ...
by pyruvate kinase
Pyruvate kinase is the enzyme involved in the last step of glycolysis. It catalyzes the transfer of a phosphate group from phosphoenolpyruvate (PEP) to adenosine diphosphate (ADP), yielding one molecule of pyruvate and one molecule of ATP. Pyruv ...
(PK) generates adenosine triphosphate
Adenosine triphosphate (ATP) is a nucleoside triphosphate that provides energy to drive and support many processes in living cell (biology), cells, such as muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all known ...
(ATP) via substrate-level phosphorylation
Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP supported by the energy released from another high-energy bond that leads to phosphorylation of ADP or GDP to ATP or GTP (note that the rea ...
.
Reactivity
Addition of electrophiles
The terminus of the double bond in enols is nucleophilic
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they a ...
. Its reactions with electrophilic
In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carr ...
organic compounds is important in biochemistry
Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, a ...
as well as synthetic organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...
. In the former area, the fixation of carbon dioxide involves addition of CO2 to an enol.
Deprotonation: enolates
Deprotonation of enolizable ketones, aldehydes, and esters gives enolate
In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl () compounds. Rarely isolated, they are widely used as reagents in the Organic synthesis, synthesis of organic compounds.
Bonding and structure
Enolate ...
s. Enolates can be trapped by the addition of electrophiles at oxygen. Silylation gives silyl enol ether
In organosilicon chemistry, silyl enol ethers are a class of organic compounds that share the common functional group , composed of an enolate () bonded to a silane () through its oxygen end and an ethene group () as its carbon end. They are i ...
. Acylation gives
esters such as vinyl acetate
Vinyl acetate is an organic compound with the Chemical formula, formula CH3CO2CH=CH2. This colorless liquid is the precursor to polyvinyl acetate, ethylene-vinyl acetate copolymers, polyvinyl alcohol, and other important industrial polymers.
Prod ...
.
Stable enols
In general, enols are less stable than their keto equivalents because of the favorability of the C=O double bond over C=C double bond. However, enols can be stabilized kinetically or thermodynamically.
Some enols are sufficiently stabilized kinetically so that they can be characterized.
Delocalization can stabilize the enol tautomer. Thus, very stable enols are phenol
Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire.
The molecule consists of a phenyl group () ...
s. Another stabilizing factor in 1,3-dicarbonyls is intramolecular hydrogen bonding. Both of these factors influence the enol-dione equilibrium in acetylacetone.
See also
* Alkenal
* Enolase
Phosphopyruvate hydratase, usually known as enolase, is a metalloenzyme () that catalyses the conversion of 2-phosphoglycerate (2-PG) to phosphoenolpyruvate (PEP), the ninth and penultimate step of glycolysis. The chemical reaction is:
:2-ph ...
* Ketone
In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...
* Ynol
In organic chemistry, alkynols (''hydroxyalkynes'') are organic compounds that contain both alkyne and alcohol functional groups. Thus, as structural features, they have a C≡C triple bond and a hydroxyl group. Some alkynols play a role as inte ...
* Geminal diol
A geminal diol (or gem-diol for short) is any organic compound having two hydroxyl functional groups () bound to the same carbon atom. Geminal diols are a subclass of the diols, which in turn are a special class of alcohols. Most of the geminal ...
, another form of ketones and aldehydes in water solutions
* Regioselectivity
In organic chemistry, regioselectivity is the preference of chemical bonding or breaking in one direction over all other possible directions. It can often apply to which of many possible positions a reagent will affect, such as which proton a str ...
References
External links
Enols and enolates in biological reactions
{{DEFAULTSORT:Keto-Enol Tautomerism
Functional groups
Metabolism
Reactive intermediates
Alcohols
Alkene derivatives
Organic reactions