Constitutional isomers tautomers

The keto and enol forms are constitutional isomers Using older terminology they are referred to as tautomers of each other... 

The aldehyde or ketone is called the keto fonm, and the keto 5 enol equilibration referred to as keto-enol isomerism or keto-enol tatitomerism. Tautomers are constitutional isomers that equilibrate by migration of an atom or group, and theh equilibration is called tautomerism. The mechanism of keto-enol isomerism involves the sequence of proton transfers shown in Figure 9.6. 

Interestingly, the product actually isolated from alkyne hydration is not the vinylic alcohol, or enol (ene + ol), but is instead a ketone. Although the enol is an intermediate in the reaction, it immediately rearranges to a ketone by a process called keto-enol tautomerisni. The individual keto and enol forms are said to be tautomers, a word used to describe constitutional isomers that interconvert rapidly. With few exceptions, the keto-enol tautomeric equilibrium lies on the side of the ketone enols are almost never isolated. We ll look more closely... 

Note the difference between tautomers and resonance forms. Tautomers are constitutional isomers—different compounds with different structures—while resonance forms arc different representations of a single structure. Tautomers have their atoms arranged differently, while resonance forms differ only in the position of their electrons. Note also that tautomers are rapidly interconvertible. Thus, keto and enol isomers are tautomers, but aJkene isomers such as 1-butene and 2-butene are not, because they don t interconvert rapidly under normal circumstances. 

There are nine constitutional isomers with the empirical formula C3H60. However, since two of the compounds exist as pairs of stereoisomers, the total number of isomers equals eleven. The two methyloxiranes, A and B, are enantiomers, they are stable and very volatile. Acetone (C) is in equilibrium with its enol tautomer (propen-2-ol, D). Similarly, the two diastereomeric enols ( )- and (Z)-prop-l-en-l-ol (E and F) are in equilibrium with their tautomer propanal (or propionaldehyde, G). The other isomers are (last group of compounds from left to right) methoxyethene, prop-2-en-l-ol (commonly known as allyl alcohol), oxetane and cyclopropanol. 

Tautomers are constitutional isomers that differ in the location of a double bond and a hydrogen atom. Two tautomers are in equilibrium with each other. 

When two constitutional isomers are present in a rapidly established equilibrium, the two molecular types are designated tautomers and the phenomenon tautomerism. Tautomers differ in the position of a double bond and a flexible group. For the keto-enol tautomerism, the tautomeric forms are the keto and the enol form. Mostly, the equilibrium is on the left hand side. 

A tautomerization reaction interconverts two constitutional isomers of an organic molecule, referred to as tautomers, which differ only by the positions of hydrogen atoms. The scheme below shows how the reaction typically takes place at the ketone functional group of an organic molecule. The ketone form of the molecule is commonly referred to as the keto form of the molecule, while the alcohol form is commonly referred to as the enol form of the molecule. 

Tautomers Constitutional isomers that interconvert by migration of an atom or group. 

Keto-enol tautomers are not resonance structures. They are constitutional isomers in equilibrium (generally favoring the keto form). 

Tautomers (Section 18.2) Constitutional isomers that are easily interconverted. Keto and enol tautomers, for example, are rapidly interconverted in the presence of acids and bases. 

Keto and enol forms are examples of tautomers—constitutional isomers that are in equihbrium with each other and that differ in the location of a hydrogen atom and a double... 

Tautomers Constitutional isomers that differ in the location of hydrogen and a double bond relative to 0, N, or S. 

The keto and enol forms of 2-butanone are said to be tautomers. Tautomers are constitutional isomers that are in equilibrium with each other and differ only in the location of a hydrogen atom or another atom and a double bond relative to a heteroatom, most commonly O, N, or S. This type of isomerism is called tautomerism. Because the type of tautomerism we are dealing with in this section involves keto (from ketone) and enol forms, it is commonly called keto-enol tautomerism. We discuss keto-enol tautomerism in more detail in Section 16.9. 

The enol and ketone are said to be tautomers, which are constitutional isomers that rapidly interconvert via the migration of a proton. The interconversion between an enol and a ketone is called keto-enol tautomerization. Tautomerization is an equilibrium process, which means that the equilibrium will establish specific concentrations for both the enol and the ketone. Generally, the ketone is highly favored, and the concentration of enol will be quite small. Be very careful not to confuse tautomers with resonance structures. Tautomers are constitutional isomers that exist in equilibrium with one another. Once the equilibrium has been reached, the concentrations of ketone and enol can be measured. In contrast, resonance structures are not different compounds and they are not in equilibrium with one another. Resonance structures simply represent different drawings of one compound. 

These constitutional isomers rapidly interconvert in the presence of even trace amounts of acid and are therefore said to be tautomers of each other. Draw a mechanism for this transformation. 

Enols and ketones are tautomers, which are constitutional isomers that rapidly interconvert via the migration of a proton. 

Tautomers are constitutional isomers related by switching the locations of a multiple bond and an atom or group. In keto-enol tautomerism these are the double bond of the carbonyl group and a proton on the carbon adjacent to C=0 (the a carbon). Tautomerism describes the relationship between a keto and enol form, tautomerization refers to their interconversion. 

How is each compound related to A Choose from tautomers, constitutional isomers but not tautomers, or neither. 

Tantomerization (Section 18.2) An isomerization by which tautomers are rapidly interconverted, as in keto-enol tantomerization. Tautomers (Section 18.2) Constitutional isomers that are easily interconverted. Keto and enol tautomers, for example, are rapidly interconverted in the presence of acids and bases. Terminal residue analysis (Section 24.5) Methods used to determine the sequence of amino acids in a peptide by reactions involving the N- and C-terminal residues. 

A ketone and an enol differ only in the location of a double bond and a hydrogen. A ketone and its corresponding enol are called keto-enol tautomers. Tautomers ( taw-toe-mers ) are constitutional isomers that are in rapid equilibrium. The keto tautomer predominates in solution, because it is usually much more stable than the enol tautomer. Interconversion of the tautomers is called keto-enol interconversion or tautomerization. 

---