Wendell Mitchell Latimer Worth H. Rodebush Chemistry
The following selection is from pages 1430–1432 in The Journal of the American Chemical Society 42, 1419–1433 (1920).
Polarity and Ionization from the Standpoint of the Lewis Theory of Valence
WENDELL M. LATIMER AND WORTH H. RODEBUSH
Associated Liquids
The phenomenon of association in liquids has long been recognized as related to dielectric constant and ionizing power as a solvent. According to one view, a so-called polar solvent contains dipoles of considerable moment, that is, positive and negative charges separated by a considerable distance. The high dielectric constant of such a liquid is considered to be due to the orientation of these dipoles in an electric field. Likewise association is supposed to take place because of the attraction of two dipoles for each other. This explanation is open to serious objections. In the first place it is hard to see why the compounds of very high dielectric constant should be chiefly hydrogen compounds. Also hydrogen chloride should contain dipoles of greater moment than water or hydrogen fluoride, yet it has a much lower dielectric constant both in the vapor and liquid. Nor does hydrogen chloride appear to be associated. It seems then that the explanation is to be sought along other lines.
Let us compare again the compounds ammonia, water and hydrogen chloride. Ammonia adds a hydrogen readily but has little tendency to give one up. Hydrogen chloride, on the other hand, shows just the opposite tendencies. Water occupies an intermediate position and shows tendencies both to add and give up hydrogen, which are nearly balanced. Then, in terms of the Lewis theory, a free pair of electrons on one water molecule might be able to exert sufficient force on a hydrogen held by a pair of electrons on another water molecule to bind the two molecules together. Structurally this may be represented as
Such combination need not be limited to the formation of double or triple molecules. Indeed the liquid may be made up of large aggregates of molecules, continually breaking up and reforming under the influence of thermal agitation.
Such an explanation amounts to saying that the hydrogen nucleus held between 2 octets constitutes a weak "bond."1 Ammonium hydroxide
is an example in which the union is fairly strong, This is contrary to the view commonly held that the weak basicity of ammonium hydroxide is due to the fact that it consists largely of a solution of ammonia in water. The idea also departs somewhat from the view taken by Lewis in regard to the polarity of
However, there seems to be no reason for believing that gradations may not exist all the way from the case of ammonium chloride, where the hydrogen is definitely transferred from the chlorine to the ammonia, to the case in the association of water where the hydrogen is still held quite firmly to the original water molecule. Ammonium chloride is probably as nearly completely polar with respect to the separation of charges as sodium chloride, and the crystal, at ordinary temperatures, is held together in a similar manner by forces only slightly less. As we raise the temperature, however, the hydrogen of the bond, being a heavy particle, acquires vibrational energy very rapidly. This breaks up the crystal structure and causes ammonium chloride to dissociate and volatilize without melting. Hydrogen chloride forms a compound of this type with water just as it does with ammonia, but it is even less stable than ammonium chloride for obvious reasons.
It is not assumed that all association is of this type. In acetic acid, for instance, the association doubtless takes place with the formation of definite polymers of 2 molecules. This kind of association however will produce properties radically different from those of a liquid like water.
If our picture of the association of water is correct, a hydrogen nucleus may be held between two oxygen octets by forces which, for quite a distance, obey Hooke’s law. Such a hydrogen would be capable of considerable displacement by an electric field. This is just the mechanism postulated by the mathematical theory of dielectric polarization as giving rise to high dielectric constant. It is quite possible in the case of the hydrogen compounds of the most electronegative elements, hydrogen fluoride, for example, that the simultaneous attraction of a highly concentrated octet of electrons, and the repulsion of the powerful positive nucleus for the hydrogen, may result in the hydrogen in the single molecule being held elastically. The association of the molecules is, however, very probably the factor that produces the extremely high dielectric constant.
1 Mr. Huggins of this laboratory in some work as yet unpublished, has used the idea of a hydrogen kernel held between two atoms as a theory in regard to certain organic compounds.