Protic solvents, hydrogenation with

In nonpolar and weakly polar aprotic solvents, the salts of Nu are present as ion-pairs (or ion-clusters) in which the nearby cations diminish the reactivity of the anion. Since, with a given cation, ion-pairing is strongest with the smallest ion, F , and weakest with the largest ion, I", the reactivity of X- decreases as the size of the anion decreases. In polar protic solvents, hydrogen-bonding, which also lessens the reactivity of X, is weakest with the largest ion,... 

The results for the Stokes radii of anions are more eomplex and are eonsidered separately for protic and aprotie solvents. In the ease of aprotie solvents, the value of fsT decreases in the aprotie solvents with inerease in solvent radius (fig. 6.12). Since the interaction between the anion and solvent is weak, the number of solvent molecules which move with the ion decreases with inerease in solvent size. This trend is weaker for the larger CIO4 ion than for the smaller CP ion, demonstrating that the CP anion is more strongly solvated in solvents of the smallest molecular size. In the case of protic solvents, hydrogen bonding is involved in anion solvation. Data are available in only a few solvents so that a detailed analysis is not possible at present. 

The metal also plays a role. The presence of a highly covalent 0—Li bond favors the trans product, but a more ionic O—Na or O—K bond, in alcoholic solvents, favors the cis.- " The lithium species is less bulky, due to poorer solvation in the aprotic solvent. The protic solvent hydrogen bonds extensively with the ionic sodium species, leading to a large increase in the relative size of the alkoxide moiety. This bulky group influences the geometry of the intermediate and leads to an increase in the cis product.- ... 

The acetoxy dienone (218) gives phenol (220). Here, an alternative primary photoreaction competes effectively with the dienone 1,5-bonding expulsion of the lOjS-acetoxy substituent and hydrogen uptake from the solvent (dioxane). In the case of the hydroxy analog (219) the two paths are balanced and products from both processes, phenol (220) and diketone (222), are isolated. In the formation of the spiro compound (222) rupture of the 1,10-bond in the dipolar intermediate (221) predominates over the normal electron transmission in aprotic solvents from the enolate moiety via the three-membered ring to the electron-deficient carbon. While in protic solvents and in 10-methyl compounds this process is inhibited by the protonation of the enolate system in the dipolar intermediate [cf. (202), (203)], proton elimination from the tertiary hydroxy group in (221) could reverse the efficiencies of the two oxygens as electron sources. 

Hydrogen bonding with protic solvents or reagents occurs widely in azines even when they are not appreciably basic and the protic compounds are very poor acids. The latter do not have to be present... 

A similar effect is produced by cocrystallization with protic solvents capable of forming a hydrogen bond-stabilized environment. Thus, dihydropyrimidine 47 (R = R = aryl,R = R = COOR, R = H) cocrystallizes with water (1 1) exclusively as the 1,6 tautomer (98T9837). 2,4,6,6-Tetraphenyldihydropyrimidine 47 (R = R = R = R = Ph, R = H) exists as the 1,6 tautomer in its solvate with... 

Aldehydes and ketones with an a hydrogen atom undergo a base-catalyzed carbonyl condensation reaction called the aldol reaction. For example, treatment of acetaldehyde with a base such as sodium ethoxide or sodium hydroxide in a protic solvent leads to rapid and reversible formation of 3-hydroxybutanal, known commonly as aldol (aidehyde + alcohol), hence the general name of the reaction. 

Another marked physical difference between sulphides and sulphoxides (or sulphones) is that sulphoxides (and lower alkyl sulphones) are hygroscopic and dissolve quite readily in water or protic solvents such as alcohols, and even more so lower alkyl or alkyl aryl sulphoxides are almost freely miscible with water. This can be accounted for by the formation of the strong hydrogen bond between the S—O bond in the sulphoxides and water molecules. Moreover, lower alkyl sulphoxides and sulphones such as dimethyl sulphoxide (DMSO) or sulpholene can dissolve a number of metallic salts, especially those of alkali and alkaline earth metals, and hence these compounds have been widely utilized as versatile and convenient solvents in modern organic chemistry26 (Table 3). 

Aprotic polar solvents such as those listed in Table 8.1 are widely used in electrochemistry. In solutions with such solvents the alkali metals are stable and will not dissolve under hydrogen evolution (by discharge of the proton donors) as they do in water or other protic solvents. These solvents hnd use in new types of electrochemical power sources (batteries), with hthium electrodes having high energy density. 

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