Keto-enol tautomerization, hydrogen transfer

Activation energies for unimolecular 1,3-hydrogen shifts connecting ketones and enols are prohibitive, so that thermodynamically unstable enols can survive indefinitely in the gas phase or in dry, aprotic solvents. Ketones are weak carbon acids and oxygen bases enols are oxygen acids and carbon bases. In aqueous solution, keto-enol tautomerization proceeds by proton transfer involving solvent water. In the absence of buffers, three reaction pathways compete, as shown in Scheme 2. 

The keto-enol tautomerization in the excited triplet state of 2-methylacetophenone is associated with hydrogen transfer in the CH O fragment ... 

Charge density analysis has been carried out for three reaction paths involving intramolecular hydrogen transfer the keto-enol tautomerism of acetaldehyde, the pinacol rearrangement of protonated ethane-1,2-diol, and the unimolecular decomposition of methanediol, reactions involving H-transfer between C O, C C, and O O atoms.288... 

In contrast to keto-enol tautomerism, such enol-enol tautomerism is characterized by extremely rapid hydrogen transfers. It was shown by ab initio calculations that structures 32a and 32b are more stable than the degenerate tautomeric forms 32c and 32e by 104.7 kJ mol as well as by 117 kl mol than symmetric structure 32d. According to these calculations, a synchronous tunneling of two protons must occur in the naphthazarine molecule 32 between the identical structures 32a and 32b with a frequency of 20 to 40 MHz, i.e. approximately 10" to 10" migrations of hydrogen from one oxygen atom to another per second take place. 

The hydrogen atom of the 0-H group in enol 39 (also acidic) is attacked by the 7i-bond of the C=C unit the hydrogen atom is transferred to the carbon with cleavage of the 0-H bond, as shown in 39, to form the ketone (6). This reaction interconverts an enol and a ketone (the keto form in the equihbrium) however, the equilibrium strongly favors the keto form, and this process is called keto-enol tautomerism. The enol is said to tautomerize to the ketone. The carbonyl form is favored over the enol unless there is some special structural feature such as the presence of a second electron-withdrawing group on an a-carbon. Therefore, if an enol is formed in a chemical reaction, assume that it will tautomerize to the carbonyl form, which is the isolated product. 

It is not necessary to protonate an enolate to obtain these two isomers. They exist in equUibrium with each other by a proton transfer reaction known as keto-enol tautoffleristn. Tautomerization describes the interconversion of two isomeric structures that differ in the location of a hydrogen atom. Tautomerization requires a change in the kinds of bonds between at least two other sets of atoms in the structures. We encountered this phenomenon in the isomerization reaction of the enol formed in the hydration of an alkyne (Chapter 7). We know that the keto form is more stable than the enol form. As we saw in the last section, this order of stabilities results primarily from the difference between the bond strengths of a carbon—oxygen double bond and a carbon—carbon double bond. 

Ultrafast studies on tautomerism concentrate on compounds that can exhibit hydrogen transfer in the electronically excited state. Hydrogen transfer is a very typical reaction for the interconversion between different tautomeric forms. It converts enol to keto, amino to imino, imino to enamino, and lactim to lactam forms, to name some examples. For time-resolved experiments, excited-state intramolecular proton transfer (ESIPT) is particularly well suited since a short laser pulse in the visible or ultraviolet (UV) spectral region can trigger this process by promoting the molecule into the electronically excited state and initiating the transfer in this way [3]. The vast majority of experiments on tautomerism with ultrafast time resolution are therefore done on compounds exhibiting ESIPT. 

The optical properties of the 8-o-PhOH-purine adducts have provided insight into their ground-state structures at the nucleoside level. These adducts have the ability to phototautomerize, through an excited-state intramolecular proton transfer (ESIPT) process, to generate the keto form. This tautomerization depends on the presence of a intramolecular hydrogen (H)-bond between the phenolic OH and the imine nitrogen (N-7). Figure 14 shows normalized absorption and emission spectra for 8-o-PhOH-dG and 8-o-PhOH-dA in aqueous buffered water and hexane. In water, 8-o-PhOH-dG shows only enol emission at 395 nm, while 8-o-PhOH-dA shows enol emission at 374 nm and phenolate emission at 447 nm. In hexane, both adducts show keto emission at 475 nm 8-o-PhOH-dA also shows a small amount of enol emission and no phenolate emission. These results show that in water, the intramolecular H-bond... 

Mltsulshi et al. described (20) that 4-OH was in the hydrazo form in polar solvents, and azo form in nonpolar solvents. It was suggested that one hydrogen atom of the hydroxide group at the 4-positlon of the naphthalene ring transferred to the 3-positlon of the azo group, and the tautomerism was established between hydrazo form of the keto type and the azo form of the enol type. In this study, we consider that the spectrum with a peak at the 480 nm... 

The solvent plays an important role in determining K. This can occur through specific solute-solvent interactions such as hydrogen bonding or charge transfer. In addition the solvent can reduce solute-solute interactions by dilution and thereby change the equilibrium if such interactions are different in enol-enol, enol-keto, or keto-keto dimers. Finally the dielectric constant of the solution will depend on the solvent and one can expect the more polar tautomeric form to be favored by polar solvents. Some of these aspects are explored in this experiment. 

The keto and enol forms of aldehydes and ketones represent a common example of tautomerism. The tautomers interconvert by an equilibrium process that involves the transfer of a hydrogen atom from oxygen to carbon and back again. 

Upon removal of an electron, the situation is very often reversed, that is, enol radical cations are more stable than their keto counterparts [3], because enols are often more easily ionized than ketones. As a consequence, enol radical cations are stable toward tautomerization, which makes it possible to investigate their reactivity extensively in the gas phase [2]. However, reports on the observation of enol radical cations in a condensed phase, where their molecular and electronic structure can be probed more easily, are comparatively scarce, probably because enols cannot be used as precursors for their radical cations (due to their usually rapid re-ketonization), and because ketone radical cations often prefer to decay by intermolecular proton (or hydrogen atom) transfer, or by a or P-cleavage (cf. the McLafferty rearrangement), rather than by enolization. 

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