Tautomers are
isomerIn chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...
s of
organic compoundAn organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s that readily interconvert by a
chemical reactionA chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
called
tautomerization. This reaction commonly results in the formal migration of a
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atomThe atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
or
protonThe proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
, accompanied by a switch of a single bond and adjacent
double bondA double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
. The concept of tautomerizations is called
tautomerism. Because of the rapid interconversion, tautomers are generally considered to be the same chemical compound. Tautomerism is a special case of
structural isomerismStructural isomerism, or constitutional isomerism , is a form of isomerism in which molecules with the same molecular formula have bonded together in different orders, as opposed to stereoisomerism. There are multiple synonyms for constitutional isomers.Three categories of constitutional isomers...
and can play an important role in non-canonical
base pairIn molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a base pair...
ing in
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
and especially
RNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
molecules.
Chemistry
In
solutionIn chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
s in which tautomerization is possible, a
chemical equilibriumIn a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
of the tautomers will be reached. The exact ratio of the tautomers depends on several factors, including
temperatureTemperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
,
solventA solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
, and
pHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
.
Common tautomeric pairs are:
- ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
- enolEnols are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Alkenes with a hydroxyl group on both sides of the double bond are called enediols. Deprotonated anions of enols are called enolates...
, e.g., for acetoneAcetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...
(see: keto-enol tautomerismIn organic chemistry, keto-enol tautomerism refers to a chemical equilibrium between a keto form and an enol . The enol and keto forms are said to be tautomers of each other...
)
- ketene
A ketene is an organic compound of the form R'RC=C=O. The term is also used specifically to mean ethenone, the simplest ketene, where R' and R are hydrogen atoms.Ketenes were first studied as a class by Hermann Staudinger.-Formation:...
- ynolIn chemistry, an ynol is an alkyne with a hydroxyl group affixed to one of the two carbons composing the triple bond. The deprotonated anions of ynols are known as ynolates...
, e.g., for EthenoneEthenone is the formal name for ketene, an organic compound with formula C2H2O or H2C=C=O. It is the simplest member of the ketene class. It is a tautomer of ethynol.- Properties :...
- amide
In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...
- imidic acid, e.g., during nitrileA nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...
hydrolysisHydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
reactions
- lactam
A lactam is a cyclic amide. Prefixes indicate how many carbon atoms are present in the ring: β-lactam , γ-lactam , δ-lactam...
- lactim, an amide - imidic acid tautomerism in heterocyclic rings, e.g., in the nucleobaseNucleobases are a group of nitrogen-based molecules that are required to form nucleotides, the basic building blocks of DNA and RNA. Nucleobases provide the molecular structure necessary for the hydrogen bonding of complementary DNA and RNA strands, and are key components in the formation of stable...
s guanineGuanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...
, thymineThymine is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name suggests, thymine may be derived by methylation of uracil at...
, and cytosineCytosine is one of the four main bases found in DNA and RNA, along with adenine, guanine, and thymine . It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached . The nucleoside of cytosine is cytidine...
- enamine
An enamine is an unsaturated compound derived by the reaction of an aldehyde or ketone with a secondary amine followed by loss of H2O.The word "enamine" is derived from the affix en-, used as the suffix of alkene, and the root amine. This can be compared with enol, which is a functional group...
- imineAn imine is a functional group or chemical compound containing a carbon–nitrogen double bond, with the nitrogen attached to a hydrogen atom or an organic group. If this group is not a hydrogen atom, then the compound is known as a Schiff base...
- enamine - enamine, e.g., during pyridoxalphosphate
Pyridoxal-phosphate is a prosthetic group of some enzymes. It is the active form of vitamin B6, which comprises three natural organic compounds, pyridoxal, pyridoxamine and pyridoxine.-Role as a coenzyme:...
-catalyzed enzymaticEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
reactions.
- anomer
In carbohydrate chemistry, an anomer is a special type of epimer. It is one of two stereoisomers of a cyclic saccharide that differs only in its configuration at the hemiacetal or hemiketal carbon, also called the anomeric carbon. Anomerization is the process of conversion of one anomer to the other...
s of reducing sugarA reducing sugar is any sugar that either has an aldehyde group or is capable of forming one in solution through isomerisation. This functional group allows the sugar to act as a reducing agent, for example in the Tollens' test or Benedict's test.-Chemistry:...
s in solution interconvert through an intermediate open chainIn chemistry, an open-chain compound or acyclic compound is a compound with a linear structure, rather than a cyclic one....
form.
Prototropy
Prototropy is the most common form of tautomerism and refers to the relocation of a proton.
Prototropic tautomerism may be considered as a subset of acid-base behavior. Prototropic tautomers are sets of isomeric
protonationIn chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...
states with the same
empirical formulaIn chemistry, the empirical formula of a chemical compound is the simplest positive integer ratio of atoms of each element present in a compound. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. The empirical formula is used as standard for most ionic...
and total
chargeIn physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...
.
Tautomerizations are
catalyzedCatalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
by:
- base
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
(1. deprotonation; 2. formation of a delocalized anion (e.g., an enolate); 3. protonationIn chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...
at a different position of the anion).
- acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s (1. protonation; 2. formation of a delocalized cation; 3. deprotonation at a different position adjacent to the cation).
Annular tautomerism: a type of prototropic tautomerism wherein a proton can occupy two or more positions of a heterocyclic system, for example, 1
H- and 3
H-
imidazoleImidazole is an organic compound with the formula C3H4N2. This aromatic heterocyclic is a diazole and is classified as an alkaloid. Imidazole refers to the parent compound, whereas imidazoles are a class of heterocycles with similar ring structure, but varying substituents...
; 1
H-, 2
H- and 4
H-
1,2,4-triazole1,2,4-Triazole is one of a pair of isomeric chemical compounds with molecular formula C2H3N3, called triazoles, which have a five-membered ring of two carbon atoms and three nitrogen atoms. 1,2,4-Triazole is a basic aromatic heterocycle...
; 1
H- and 2
H-
isoindoleIsoindole in heterocyclic chemistry is a benzo fused pyrrole. The compound is an isomer of indole and its reduced form is an isoindoline.In solution its tautomer without full aromaticity over the whole ring system is predominant:...
.
Ring-chain tautomerism: occurring when the movement of the proton is accompanied by a change from an open structure to a ring, such as the
open chainIn chemistry, an open-chain compound or acyclic compound is a compound with a linear structure, rather than a cyclic one....
and
pyranIn chemistry, a pyran, or oxine, is a six-membered heterocyclic, non-aromatic ring, consisting of five carbon atoms and one oxygen atom and containing two double bonds. The molecular formula is C5H6O. There are two isomers of pyran that differ by the location of the double bonds...
forms of
glucoseGlucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
and
furanFuran is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. The class of compounds containing such rings are also referred to as furans....
form of
fructoseFructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
.
Valence tautomerism
Valence tautomerism is a type of tautomerism in which single and/or double bonds are rapidly formed and ruptured, without migration of atoms or groups. It is distinct from prototropic tautomerism, and involves processes with rapid reorganisation of bonding electrons. An example of this type of tautomerism can be found in
bullvaleneBullvalene is a hydrocarbon with the chemical formula C10H10 with the unusual property that the chemical bonds making up the molecule are constantly rearranging as in fluxional molecules...
. Another example is open and closed forms of certain heterocycles, such as
azideAzide is the anion with the formula N3−. It is the conjugate base of hydrazoic acid. N3− is a linear anion that is isoelectronic with CO2 and N2O. Per valence bond theory, azide can be described by several resonance structures, an important one being N−=N+=N−...
-
tetrazoleTetrazoles are a class of synthetic organic heterocyclic compound, consisting of a 5-member ring of four nitrogen and one carbon atom . The simplest is tetrazole itself, CN4H2. They are unknown in nature...
or
mesoionicMesoionic chemical compounds are dipolar five- or six- membered heterocyclic compounds in which both the negative and the positive charges are delocalized...
münchnone-acylamino ketene. Valence tautomerism requires a change in molecular geometry and should not be confused with canonical resonance structures or mesomers.