Arthur Rudolf Hantzsch Chemistry

The following selection is in Zeitschrift für Physikalische Chemie 134, 406–412.

The Constitution of Acids and Salts and Their Chemical Alterations by Solvents

A. HANTZSCH

My studies of acids and salts, which have lasted for many years, have led me to certain conclusions. For a better understanding of the changes seen in the following work by means of molecular refraction, I will give here in compact form a summary of the most important results obtained up to now and, to anticipate the most essential general conclusions, I will explain the alterations most simply in a purely chemical way instead of assuming a physical state to explain the changes. Naturally I will not deny the effect of the latter, but it produces only secondary changes following the primary chemical alterations.

The starting point for the chemical theory comes partly from my study of the connection between constitution and color of colored salts and acids, partly from the work of K. Schaefer on the ultraviolet absorption of nitric acid and its derivatives. By extinction measurements on salts such as

and also on acids such as and bases such as we recognized optical constants independent of great differences in degree of dissociation, so long as the colored complex cations and anions remained unaltered or at any rate, like most nonelectrolytes, were altered only slightly by solvate formarion in different solvents, as for example, chloroplatinic acid and its salts, as well as the recently thoroughly studied salts of the acid Thus here the dissociation is an optically indifferent process.

The optical analysis of nitric acid, its salts and esters has also yielded the basis for the following considerations.

Constitution of Homogeneous Acids and Their Alterations by Solvents

Nitric acid is homogeneous and indifferent nonionizing solvents like ether is optically very similar to its esters

but is completely different from its alkali salts and thus according to A. Werner, is not a "true" acid with ionogen bound hydrogen but a "pseudoacid" with hydroxyl hydrogen However, in water solution with increasing dilution it becomes increasingly similar optically to its salts, and with great dilution, identical with them; it is then (as was assumed at first) apparently changed into the true acid but actually, as with ammonia, it has been converted by water, a very weak basic anhydride, into a hydroxonium salt comparable to the ammonium salt This salt-forming action of water is confirmed by the fact that the solid monohydrate of the strongest acid, perchloric acid, is actually a hydroxonium perchlorate and has a crystal lattice corresponding to ammonium perchlorate However, the other strong acids appear in the homogeneous condition, like nitric acid, not to be heteropolar complexes of true acids with ionically bound hydrogen atoms as given by A. Werner, but to be homopolar hydroxyl compounds whose acid hydrogen ion is thus fixed to a single oxygen atom. Thus, in general, free H ions do not exist, but only hydroxonium ions. Accordingly, all monomolecular acids are pseudoacid nonelectrolytes, but they appear as "pseudo-electrolytes" because in water they are apparently true acids, but actually they are dissolved more or less completely as their hydroxonium salts. From this we arrive at an important theoretical definition and a practical expression of their different acidities or strengths. These are not determined physically by their different dissociation constants as is stated by the dilution law of W. Ostwald for the strong acids, nearly equally strongly dissociated in water, but purely chemically by the differing strengths of their tendencies to salt formation. Indeed, since the apparently simplest salt formation by substitution

can be exactly measured only with difficulty because of its heterogeneous system, the measurement is most simply done by means of additive salt formation with basic anhydrides (for example, amines

) and especially with water, by determination of the state of equilibrium in solution

Thus the previously unknown "degrees of acidity" of the strongest acids can be determined qualitatively. This is first done statically by an indicator method, namely by study of the equilibrium

Second, kinetically, by measurement of the rate of decomposition of the diazoacetic ester through the nondissociated acid which is also produced primarily through additive salt formation of an aliphatic diazonium salt. The latter then decomposes spontaneously, most simply according to the following equation

Third, by measurement of the rate of inversion of sugars at the greatest concentration of acids possible.

Thus we can arrange the strong inorganic acids according to decreasing acidity as follows:

For all the above acids, however, important results have been clearly shown only by the above chemical theory and they are incompatible with the results of the dissociation theory: the nonionized strong acids act most strongly because of their great tendency to salt formation; on the other hand, in water solution the ions that are present and are the main most active components are the weakest because they dissolve as hydroxonium ions (though these are very unstable). The fact that this situation is different in weak acids, and why this is so, will be explained later.

All acids in water solution ultimately become completely saltlike, and the greater the dilution, the weaker they are. This peculiar "mass action" of water can only be explained chemically, however. as the anions of very weak silicic acid further add acid anhydride molecules,

to become more strongly negative polysilicate ions, so the very weak positive "cation from water" that results from water and the hydrogen of the acid forming a hydroxonium ion becomes a strongly positive "polyhydroxonium cation" by further addition of water. Thus, of the acids mentioned above, which are almost equally strong (dissociated) according to the dissociation theory, only perchloric acid and apparently some of the strongest sulfonic acids like form a solid hydroxonium salt with one mole of water, whereas the weakest acid, nitric acid (which according to the dissociation theory appears strongest) is only changed completely into the polyhydroxonium nitrate by some 90 moles of water, and this then exists only in solution. This peculiarity which distinguishes hydroxonium from ammonium must also be explained chemically. The strong association of liquid water, which only becomes monomolecular at higher temperatures, as opposed to the very weak association of liquid ammonia which is not stable even at ′34°, is also transmitted to the simple hydroxonium cations formed from water and acid, and this explains the existence of positive polyhydroxonium cations which, as will be shown later, sometimes also exist in the solid condition as hydrates of certain strong acids like and etc.

The fact that the simplest organic derivatives of water, the alcohols and ethers, are similar to water, but as weaker basic anhydrides can form alkylated hydroxonium salts only with the strongest acids and can thus serve for distinguishing their degree of acidity, is shown by giving one example only: HBr in ether actually absorbs light in the homogeneous condition just as in water and like the alkali bromides in water solution; thus it is dissolved in ether as the diethyl oxonium salt

while nitric acid conversely in ether, like its ester, absorbs only as a dissolved pseudoetherate nitric acid is thus here a very much weaker acid than HBr, though because of its conductivity it should be the strongest acid in water solution. These facts must also be explained chemically and can be referred to two causes. The first is that the unsaturated hydrogen of the hydroxonium is more positive than the saturated alkyls of the alkylated hydroxonium salt, and the second is that (on the same grounds) water is the most strongly associated, alcohol less so, and ether not at all. Accordingly, the monoalkyl hydroxonium ions become only a little stronger by addition of alcohol, and the dialkyl hydroxonium salts cannot add ether at all, and thus cannot become more positive. Thus nitric acid forms only one monoetherate (isolated), while its monohydrate goes over with water into a polyhydroxonium nitrate.

The constitution of the homogeneous liquid oxygen acids is also similar to that of liquid water. Like their mother substance they are known to be associated, as hydroxyl compounds, and actually, as is well recognized, by means of the unsaturated oxygen and hydrogen atoms of the hydroxyl, and, as in associated water, traces of H-ions and OH-ions are present, or, more correctly, just as dimolar associated water forms a somewhat ionized hydroxonium hydrate by displacement of an H atom:

so the associated strong oxygen acids also isomerize, as can again be most clearly recognized in nitric acid, partly by an analogous "disproportionation" to a heteropolar saltlike electrolyte where the dimolar associated nitric acid partly goes over to a "nitronium nitrate" and the following equilibrium results:

This nitronium nitrate is dissociated in the unaltered acid, just as is hydroxonium hydrate in water; thus we have explained chemically that the homogeneous nitric acid is a relatively good electrolyte.

In the same way we also explain the apparent optical anomaly of nitric acid that it absorbs very differently and more weakly than its ester through its content of nitronium nitrate, since, as I have recognized by optical analysis and molecular weight determinations, in sulfuric acid solution this is present only as a weakly absorbing, dissociated nitronium sulfate. In addition, not only do we isolate from perchloric acid, as the strongest acid, a solid nitronium perchlorate

but also, as will shortly be published, the saltlike nature of this addition compound can be recognized by the fact that in nitromethane solution it behaves as a normal electrolyte and by electrolysis nitric acid actually migrates as a cation to the cathode.

Conclusion

Homogeneous liquid oxygen acids are "pseudohomogeneous" substances; they exist mainly as nonconducting homopolar hydroxyl compounds which are more or less strongly associated. Secondarily by intramolecular rearrangement they contain formed and dissociated "acidium salts" whose concentration can be determined approximately from the size of their conductivity.

The following considerations are valid for a consideration of the transformations of the acids in water solution:

As long as water was viewed only as a passive solvent in the purely physical theory of electrolytic dissociation, and as long as the H ion in water solution was assumed naturally to exist in water solutions of "free" acids, which broke up into acid ions and hydrogen ions, for just so long we had to assume also an esterlike pseudo acid and a saltlike true acid as an "equilibrium acid." However, since all the studies show that the available acids must be recognized as pseudo-acids

exclusively, and "free" (homogeneous) true acids almost surely do not exist, the division of acids into two different classes is groundless. Therefore the term "true" acid with the complex formula previously assigned to it must disappear because only its oxonium salt exists in its place and the term "pseudoacid" is misleading because it depends on the assumption of the existence of a true acid. Thus, we must once again call all of them, even the previous "pseudoacids," simply "acids" and define them as follows: all oxygen acids are hydroxyl compounds of negative atoms or atom complexes whose hydroxyl hydrogens can be replaced by positive metals with formation of true salts, but also by unsaturated substances like ammonia and amines, and also, which is theoretically more important, even apparently neutral oxygen compounds and "solvents" like water (in the strongest acids, alcohol and ether also) add to each other and thus form ammonium and oxonium salts. They can be defined more simply electrochemically: all homogeneous acids (halogen hydrides, oxygen acids and thio acids) in the monomolecular state are nonelectrolytes, but as evidence of their "strength" they have an increasing tendency to formation of salts, thus of electrolytes, first by replacement of hydrogen with formation of metal salts; second by addition of unsaturated compounds (such as ammonia, water, alcohol, and ether) to their hydrogen with formation of onium salts.