Inductive effects hydration reactions

Because the equilibrium constants are neither too large nor too small, the hydration reaction provides an excellent opportunity to examine the effect of the structure of the carbonyl compound on the equilibrium constant. Let s consider inductive effects first. 

The additions of iodine azide (equations 99, 100) and the mercury catalysed hydration (equation 101) were rationalized on the basis of Ad reactions with cyclic intermediates having relatively little car-bonium ion character. Orientation is then dependent on the inductive effects of the methyl and bromo substituents with little orientational effect of the phenyl group. In contrast, the orientation of the acid... 

Aldol addition reactions of ketones are rarely successful, since they are usually endoergonic. For example, the base-mediated aldolization of acetone provides only a few percent of the aldol, diacetone alcohol (equation 26). However, the conversion may be accomplished in 75% yield by refluxing acetone under a Soxhlet extractor containing calcium or barium hydroxide. - On the other hand, di-methoxyacetone dimerizes under basic conditions to the aldol, with an equilibrium constant significantly greater than unity (K = 10 dm mol equation 27). The difference in equilibrium constants of equations (26) and (27) parallels the equilibrium constants for hydration of the two ketones, and results from the inductive effect of the methoxy groups. 

Cyclic ketones cannot usually be dimerized to the corresponding -hydroxy ketones under protic conditions. An exception to this generalization is shown in equation (2S)P The 2,3-dioxopyrrolidine is quantitatively dimerized by being warmed briefly with ethanolic pyridine, or by attempted recrystallization from warm toluene. In this case, as in equation (27), the favorable equilibrium constant is no doubt related to the inductive effect of the amide C=0, which favors hydration and other addition reactions of the ketonic carbonyl. 

A similar trend may be seen in a study of the reaction of chloral with unsymmetrical ketones (equation S3 Table 3). Reactions were carried out in glacial acetic acid with or without added sodium acetate as catalyst. Several control experiments showed that the isomer ratios obtained were kinetic. The lack of reversibility in this reaction implies that AG is much more negative than for the simple aldol reactions discussed previously. This is presumably because of the inductive effect of the chlorines, which is known to favor hydration and other nucleophilic additions to chloral. 

The addition of water to the carbonyl group in aqueous solutions can lead to the formation of hydrates. The reactions and stability of hydrates depend primarily on the inductive effect of substituents. Hydration of formaldehyde readily yields methylene glycol (methanediol), which polymerises to linear oligomers and also to polymers known as paraformaldehyde. Hydrates of a-dicarbonyl and a-hydroxycarbonyl compounds spontaneously dimerise into various cyclic 1,4-dioxanes (see Figure 4.62). These hydrates are intermediates of other a-dicarbonyl and a-hydroxycarbonyl compounds in the oxidation-reduction reactions and precursors of carboxylic acids. Dialkyl ketones do not form hydrates. 

The delay, t0, preceding the onset of the main reaction may include contributions from (i) the time required for the sample to attain reaction temperature, h, (ii) additions to fh resulting from changes within the reaction sample, e.g. water removal (endothermic) from a hydrate, td, phase transitions, etc. and (iii) slow processes preceeding establishment of the main reaction, which are to be regarded as the true induction period, The effective values of th, td and may show different temperature coefficients so that the magnitude of t0(=th + ta + i) may vary with temperature in a complex manner, perhaps differently from that of the subsequent rate process. 

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