Hydrogen bonding protic solvents

Parker, 1962, 1965a), which suggests that there are strong dipole-dipole interactions between individual solvent molecules, but one would not expect to find the same strongly associated structure which exists in protic solvents. Because dipolar aptotic solvents can much more readily accommodate polar solutes into their structure than can hydrogen-bonded protic solvents, the standard chemical potential of polar molecules in protic solvents is usually greater than in dipolar aprotic solvents, i.e, as noted, moelcules are less soluble in protic solvents. 

Kemp et al., 1978). The rate is slowest in an aqueous solution and is enhanced in aprotic and/or dipolar solvents. The rate augmentation of 106—108 is attainable in dipolar aprotic solvents such as dimethyl sulfoxide and hexamethylphosphoramide (HMPA). Interestingly, the decarboxylation rate of 4-hydroxybenzisoxazole-3-carboxylate [53], a substance which contains its own protic environment, is very slow and hardly subject to a solvent effect (1.3 x 10-6 s-1 in water and 8.9 x 10-6 s-1 in dimethylformamide Kemp et al., 1975). The result is consistent with the fact that hydrogen-bonding with solvent molecules suppresses the decarboxylation. 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

Water, alcohols, and carboxylic acids are polar protic solvents able to form hydrogen bonds (hydroxylic solvents). They solvate both cations and anions well. A nucleophilic reagent such as bromide ion must be accompanied by a cation, say, the sodium ion, and hydroxylic solvents dissolve salts such as NaBr by hydrogen bonding to the anion and electron donation to the cation. This is solvation by a polar protic solvent. These solvents do not ionize the salt, which already exists in the solid state as ions they separate and solvate the ions already present. 

In hydrogen-bond donor solvents such as alcohols and trichloromethane, the tautomeric equilibrium is shifted in favor of the colourless form (12a) more than in other solvents. This is obviously due to the formation of hydrogen bonds between / 12a) and these protic solvents. In aprotic solvents, A//° is negative and the reaction is exothermic. Since, however, all AG° values are positive, the negative value of KH° must be over-compensated by a positive entropy change cf. Eq. (4-4). 

Protic solvent A hydrogen bond donor solvent as, for example, water, ethanol, and acetic acid. We define hydrogen bond donors as compounds containing hydrogens that can participate in H-bonding. 

In polar protic solvents, anions are highly solvated by hydrogen bonding with solvent molecules and therefore are less free to participate in nudeophihc substitution reactions and polarizability dictates nucleophilicity. 

Protogenic Capable of acting as a proton (hydron) donor strongly or weakly acidic (as a Bronsted acid). The term is preferred to the synonym protic or the more ambiguous expression acidic by itself. Also called HBD (hydrogen-bond donor) solvent. ... 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

---