Hydrogen bonding anion solvation

Polar protic solvents also possess a pronounced ability to separate ion pairs but are less favorable as solvents for enolate alkylation reactions because they coordinate to both the metal cation and the enolate ion. Solvation of the enolate anion occurs through hydrogen bonding. The solvated enolate is relatively less reactive because the hydrogen-bonded enolate must be disrupted during alkylation. Enolates generated in polar protic solvents such as water, alcohols, or ammonia are therefore less reactive than the same enolate in a polar aprotic solvent such as DMSO. 

Anions are less solvated in dipolar aprotic solvents than in water. In water and protic solvents, the anions solvate by ion-dipole interactions on which strong hydrogen bonding is superimposed. In the cases of the dipolar aprotic solvents, the anions solvate also by ion-dipole interactions, but without the influence of hydrogen bonding. Instead, solvation is aided by less energetic interactions arising from the mutual polarizability of the anions and the solvent molecules (Parker, 1962). 

Polar, aprotic solvents also dissolve salts by ion-dipole interactions, albeit not as well as prohc solvents. Because they cannot form hydrogen bonds, they solvate anionic nucleophiles relahvely weakly. The consequences are twofold. First, compared to prohc solvents, the reactivity of the nucleophile is raised, sometimes dramahcally. For example, bromomethane reacts with... 

Many replacement reactions are often much faster in polar aprotic solvents than in hydroxylic solvents. Thus, the reaction of methyl bromide with iodide is about 500 times faster in acetone than in methyl alcohol. In addition, methyl iodide reacts with chloride about a million times faster in DMF than in methyl alcohol. This happens because the OH group of hydroxylic solvents solvates anions, forming the hydrogen bond (ROH-"X -HOR). The solvated anions are therefore much less reactive. On the other hand, aprotic solvents having no hydrogen are unable to form hydrogen bonds. Anions in polar aprotic solvents are therefore more free, more reactive, and thus better nucleophiles. Another example of a strong solvent effect on a 8 2 reaction is the bimolecular replacement of bromide by azide in 1-bromobutane ... 

Polar solvents solvate ionic species and thus favor ion formation and separation in S l reactions. Part of the effect is due to dielectric constant, which facilitates charge separation (Table 9.6), and part to the ability of the solvent to solvate both anion and cation. Dipolar protic solvents are chosen since they are able to solvate both anions and cations (Figure 9.20). Solvation of cations involves interaction between lone pairs and the positive center and solvation of anions the formation of hydrogen bonds. The solvation of any type of ion increases with its charge and decreases with its size. Typically, S l reactions are carried out in polar solvents such as water and alcohols. 

A comparison of phenol acidity in DMSO versus the gas phase also shows an attenuation of substituent effects, but not nearly as much as in water. Whereas the effect of ubstituents on AG for deprotonation in aqueous solution is about one-sixth that in the gas phase, the ratio for DMSO is about one-third. This result points to hydrogen bonding of the phenolate anion by water as the major difference in the solvating properties of water and DMSO. ... 

In media such as water and alcohols, fluoride ion is strongly solvated by hydrogen bonding and is neither very basic nor very nucleophilic. On the other hand, the poorly solvated, or naked, fluoride ions that aie present when potassium fluoride dissolves in benzene in the presence of a crown ether aie better able to express their anionic reactivity. Thus, alkyl halides react with potassium fluoride in benzene containing 18-crown-6, thereby providing a method for the preparation of otherwise difficultly accessible alkyl fluorides. 

A hydrogen-bonded cyclic transition state can be postulated for a nucleophile like ethanolamine or ethylene glycol anion whose hydrogen bonding to an azine-nitrogen in aprotic solvents can facilitate reaction via a cyclic transition state such as 78, cf. Section II, F. Ethanolamine is uniquely reactive with 2-chloronitrobenzene by virtue of a cyclic solvate (17) of the leaving group, a postulate in line with kinetic evidence. 

A possible explanation comes from X-ray analyses of the sulfonic acids [45]. All X-rayed crown ether crystals contained water and the sulfonic acid moiety was dissociated. Therefore in crystals of [45], macrocyclic ben-zenesulfonate anions and hydronium ions (sometimes hydrated) are present. The ions are bound to each other by hydrogen bonds. The size of the included water-hydronium ion cluster (varying by the number of solvating water molecules) depends on the ring size. In the 15-membered ring, HsO" was found, whereas in a 21-membered ring HsO and in the 27-membered ring were present. This means the sulfonic acid functions in [45] are... 

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