The term enol is an abbreviation of alkenol, a portmanteau deriving from “-ene”/”alkene” and the “-ol”. Many kinds of enols are known. Keto-enol tautomerism refers to a chemical equilibrium between a “keto” form (a carbonyl, named for the common ketone case) and an enol. The interconversion of the two forms involves the transfer of an alpha hydrogen atom and the reorganisation of bonding electrons. The keto and enol forms are tautomers of each other. Organic esters, Supraketo.org ketones, and aldehydes with an α-hydrogen (C−H bond adjacent to the carbonyl group) often form enols. 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. C−OH connectivity. Deprotonation of organic carbonyls gives the enolate anion, which are a strong nucleophile. ≈ 3×10−7). In 1,3-diketones, Supraketo Official Site such as acetylacetone (2,4-pentanedione), the enol form is more favored.
Keto by Supraketo form proceeds by proton transfer from O to carbon. The process does not occur intramolecularly, but requires participation of solvent or other mediators. 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 diastereomers or enantiomers. C double bond. Normally such compounds are disfavored components in equilibria with acyloins. 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 reductones. Such species are important in glycochemistry, e.g., the Lobry de Bruyn-Van Ekenstein transformation. Ribulose-1,5-bisphosphate is a key substrate in the Calvin cycle of photosynthesis. In the Calvin cycle, the ribulose equilibrates with the enediol, which then binds carbon dioxide.
The same enediol is also susceptible to attack by oxygen (O2) in the (undesirable) process called photorespiration. Phenols represent a kind of enol. For some phenols and related compounds, the keto tautomer plays an important role. Many of the reactions of resorcinol involve the keto tautomer, for example. Naphthalene-1,4-diol exists in observable equilibrium with the diketone tetrahydronaphthalene-1,4-dione. Keto-enol tautomerism is important in several areas of biochemistry. The high phosphate-transfer potential of phosphoenolpyruvate 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 enolase catalyzes the dehydration of 2-phosphoglyceric acid to the enol phosphate ester. Metabolism of PEP to pyruvic acid by pyruvate kinase (PK) generates adenosine triphosphate (ATP) via substrate-level phosphorylation. The terminus of the double bond in enols is nucleophilic. Its reactions with electrophilic organic compounds is important in biochemistry as well as synthetic organic chemistry.
In the former area, the fixation of carbon dioxide involves addition of CO2 to an enol. Deprotonation of enolizable ketones, aldehydes, and esters gives enolates. Enolates can be trapped by the addition of electrophiles at oxygen. Silylation gives silyl enol ether. Acylation gives esters such as vinyl acetate. 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 phenols. Another stabilizing factor in 1,3-dicarbonyls is intramolecular hydrogen bonding. Both of these factors influence the enol-dione equilibrium in acetylacetone. Smith MB, March J (2001). Advanced Organic Chemistry (5th ed.). New York: Wiley Interscience. Clayden, Jonathan; Greeves, Nick; Warren, Stuart (2012). Organic chemistry (2nd ed.). New York: Oxford University Press. Manbeck, Kimberly A.; Boaz, Nicholas C.; Bair, Nathaniel C.; Sanders, Allix M. S.; Marsh, Anderson L. (2011). “Substituent Effects on Keto-Enol Equilibria Using NMR Spectroscopy”. J. Chem. Educ. 88 (10): 1444-1445. Bibcode:2011JChEd..88.1444M. Guthrie, J. Peter; Povar, Igor (2013). “Equilibrium constants for enolization in solution by computation alone”. Journal of Physical Organic Chemistry. 26 (12): 1077-1083. doi:10.1002/poc.3168. Schank, Kurt (1972). “Reductones”. Angewandte Chemie International Edition. Berg, Jeremy M.; Tymoczko, Stryer (2002). Biochemistry (5th ed.). New York: W.H. Freeman and Company. Manfred Braun (2015). Modern Enolate Chemistry: From Preparation to Applications in Asymmetric Synthesis. Mukaiyama, T.; Kobayashi, S. Org. G. Roscher (2007). “Vinyl Esters”. Ullmann’s Encyclopedia of Chemical Technology. Journal of the American Chemical Society. Clayden, Jonathan (2012). Organic Chemistry. Oxford University Press. pp. Zhou, Yu-Qiang; Wang, Nai-Xing; Xing, Yalan; Wang, Yan-Jing; Hong, Xiao-Wei; Zhang, Jia-Xiang; Chen, Dong-Dong; Geng, Jing-Bo; Dang, Yanfeng; Wang, Zhi-Xiang (2013-01-14). “Stable acyclic aliphatic solid enols: synthesis, characterization, X-ray structure analysis and calculations”.
