Aldehydes keto-enol tautomerism

Phosphorylation, reduction, and tautomerization. Reaction of 3-phos-phoglycerate with ATP generates the corresponding acyl phosphate, which is reduced by NADH/H to an aldehyde. Keto-enol tautomerization of the aldehyde gives dihydroxyacetone phosphate, the same reaction as step 5 of glycolysis (F ure 29.4). 

The aldehyde or ketone is called the keto form and the keto enol equilibration referred to as keto-enol isomerism or keto-enol tautomerism Tautomers are constitu tional isomers that equilibrate by migration of an atom or group and their equilibration IS called tautomerism The mechanism of keto-enol isomerism involves the sequence of proton transfers shown m Figure 9 6... 

Enols are related to an aldehyde or a ketone by a proton transfer equilibrium known as keto-enol tautomerism (Tautomensm refers to an mterconversion between two struc tures that differ by the placement of an atom or a group)... 

A single Kekule structure does not completely descnbe the actual bonding in the molecule Ketal (Section 17 8) An acetal denved from a ketone Keto-enol tautomerism (Section 18 4) Process by which an aldehyde or a ketone and its enol equilibrate... 

Carbonyl compounds are in a rapid equilibrium with called keto-enol tautomerism. Although enol tautomers to only a small extent at equilibrium and can t usually be they nevertheless contain a highly nucleophilic double electrophiles. For example, aldehydes and ketones are at the a position by reaction with Cl2, Br2, or I2 in Alpha bromination of carboxylic acids can be similarly... 

There is a distinct relationship between keto-enol tautomerism and the iminium-enamine interconversion it can be seen from the above scheme that enamines are actually nitrogen analogues of enols. Their chemical properties reflect this relationship. It also leads us to another reason why enamine formation is a property of secondary amines, whereas primary amines give imines with aldehydes and ketones (see Section 7.7.1). Enamines from primary amines would undergo rapid conversion into the more stable imine tautomers (compare enol and keto tautomers) this isomerization cannot occur with enamines from secondary amines, and such enamines are, therefore, stable. 

When a terminal alkyne is treated with an excess of hydrogen halide the halogens both end up on the more substituted carbon (Fig. F). This is in accordance with the Markovnikov s rule which states that the additional hydrogens end up on the carbon which already has the most hydrogens. The same rule applies for the reaction with acid and mercuric sulphate which means that a ketone is formed after keto-enol tautomerism instead of an aldehyde (Fig. G). 

In addition to its mechanistic importance, keto-enol tautomerism affects the stereochemistry of ketones and aldehydes. A hydrogen atom on an a carbon may be lost and regained through keto-enol tautomerism such a hydrogen is said to be enolizable. 

An isomerism involving the migration of a proton and the corresponding movement of a double bond. An example is the keto-enol tautomerism of a ketone or aldehyde with its enol form. (p. 1047)... 

There is an equilibrium between the keto group (aldehyde or ketone) and the enol group that is known as keto-enol tautomerization. 

Aliphatic nitro compounds show a number of reactions which parallel those of carbonyl chemistry. Primary and secondary nitro compounds exhibit tautomerism paralleling keto-enol tautomerism (Scheme 3.94a). Aliphatic nitro compounds dissolve in aqueous sodium hydroxide with the formation of sodium salts. The resultant anions behave as carban-ions and will condense with aldehydes. An example involves the formation of m-nitrostyrene from nitromethane and benzaldehyde (Scheme 3.94b). 

Alpha hydrogens are hydrogens on carbons directly attached to a carbonyl group. They are weakly acidic and can be abstracted by base to form a carbanion. The carbanion is called an enolate ion and is resonance stabilized. Neutralization of the enolate ion results in an enol, a compound in which an alcohol group is directly bonded to a carbon involved in a carbon-carbon double bond. The enol is in equilibrium with the original aldehyde or ketone in an equilibrium referred to as keto-enol tautomerism. The equilibrium usually favors the keto form. 

Butlerov found out that in alkaline medium (calcium hydroxide), formaldehyde HCHO polymerizes to form about 20 different sugars as racemic mixtures, Butlerov 1861. The reaction requires a divalent metal ion. Breslow found a detailed mechanism of reaction that explains the reaction products, (Breslow 1959). He found that glycol-aldehyde is the first product that is subsequently converted into glyceral-dehyde (a triose), di-hydroxy-acetone, and then into various other sugars, tetrose, pentose, and hexose. The formose reaction advances in an autocatalytic way in which the reaction product is itself the catalyst for that reaction with a long induction period. The intermediary steps proceed via aldol and retro-aldol condensations and, in addition, keto-enol tautomerizations. It remains unexplained how the phosphorylation of 3-glyceraldehyde leads to glycral-3-phosphate (Fig. 3.6). Future work should study whether or not ribozymes exist that can carry out this reaction in a stereo-specific way. 

The process by which enols are converted to aldehydes or ketones is called keto-enol isomerism (or keto-enol tautomerism) and proceeds by the sequence of proton transfers shown in Figure 9.6. Proton transfer to the donble bond of an enol occurs readily because the carbocation that is produced is a very stable one. The positive charge on carbon is stabilized by electron release from oxygen and may be represented in resonance terms as shown on the following page. 

Keto—enol tautomerism (Section 18.4) Process by which an aldehyde or a ketone and its enol equihbrate ... 

The next stage of oxidation for organic compounds is to aldehydes and ketones. In acid solutions, aldehydes and ketones are often hydrolyzed to produce ketohydrates (two OH groups attached to the same carbon atom). They also undergo keto-enol tautomerism to convert into more acidic species which are then more capable of forming ionic bonds with metal ions. The principal kinetic question is whether the keto, enol, or ketohydrate form is the one involved in the oxidation reaction. The results of the most complete investigations of ceric oxidation of ketones and aldehydes are contained in table 4. 

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