What Groups Form Hydrogen Bonds

A class must be defined in terms of observable properties and this often introduces difficulties of two kinds. The first difficulty is. . . that most properties can vary continuously. .. so that clean-cut separations. .. are often somewhat arbitrary.. . . The other difficulty. . . [w] that it is often necessary to amend definitions many times in order that the class may continue to be useful.  

A survey of any concept must be made within bounds selected by the authors of the survey. Such a selection of bounds implies, indeed it constitutes, a working definition of the concept. Hence the usefulness of the survey is enhanced by a clear and explicit statement of this definition. 

Unfortunately, a precise definition often evokes criticism from two directions, as noted by E. B. Wilson, Jr. First, different workers may disagree on the optimum placement of the bounds of the concept, since lines of demarcation in science are rarely sharp. Second, the growth of human experience may cause an evolution of the concept—a movement of the lines of demarcation. Anticipating this evolution, some scientists prefer to remain unfettered by sharply drawn lines. They wish to avoid the danger that study of the concept will be inhibited by a definition solidified in a mold formed by inadequate information. Davies has, for 

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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... 

If two molecules had a tendency to associate with each other because groups on their surfaces could form hydrogen bonds, what would be the effect of putting these molecules in water Explain. 

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