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It would be very difficult, perhaps impossible, to include much information regarding the methods of. formation, the relative stabilities under different conditions, and generally the power of doing' of a compound, in a single intelligible and not too cumbrous formula. I call the student's attention to the kinetical aspects of the structural formulæ now used in chemistry, because I consider it of paramount importance that he should remember how little information these formulæ give in comparison with what we would desire to have, that he should not forget that the experimental methods by which these formulæ are obtained are for the most part kinetical methods, while the interpretation of the results is expressed in a language which has grown out of almost purely statical considerations, and that while he recognises the vast importance of structural formulæ, he may still refuse to bow the knee to this chemical Baal, which has been set up in these times, so aptly described by Remsen as the era of ‘formula worship.'

257. Is it possible to connect the structure of the molecules of various compounds, as this structure is expressed in formulæ based for the most part on statical considerations, with the relative affinities of these compounds, which, as we have seen, are numbers obtained by the employment of kinetical methods of research, and which tell a great deal as to the power of doing of the compounds ?

The numbers given in the table in par. 235 (chap. 111.) represent the relative affinities of a series of acids, including several carbon acids the structural formula of which have been well established. If the relative affinities of the chloracetic acids are compared with that of acetic acid, and if lactic and trichlorolactic acids are also compared, we have this result. Acid.

Relative afsinily.
Acetic

CH3-CO,H
Monochloracetic CH,Cl - CO,H
Dichloracetic CHCl, -CO,H

52
Trichloracetic CC1Z - CO,H

6-3

22

87

Lactic

CH3-CHOH-CO,H
Trichlorolactic CCI, - CHOH - CO,H

10-5 26

Substitution of Cl for H in acids therefore appears to be attended by an increase in the values of the affinity-constants of the acids.

The substitution of OH for H acts in a similar way. Thus,

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12

On the other hand, the substitution of CH, for H is accompanied by a decrease in the values of the affinity-constants of the acids examined. This is shewn by the following among other numbers. Acid.

Relative affinity.
Formic H.CO,H
Acetic CH,.COH

6.3
Propionic CHX.CH3.CO,H

595 But the relative affinities of methyl-, ethyl-, propyl-, and amyl-sulphuric acids, viz. 10094, 99.5, 99, and 98, shew that the substitution of CH, for H in the molecule SO,.OH.OC,H2n+1, is attended by only a very small decrease of affinity.

Ostwald's numbers further suggest that the values of the affinity-constants of the carbon acids are conditioned by the relative arrangements of the atoms in the molecules of these acids. Thus, comparing the relative affinities of acetic and trichloracetic acids, with those of lactic and trichlorolactic acids, we see that the difference between the values of the quantities in question is much larger in the case of the former than of the latter pair of acids. Thus,

+1

Relative affinity. Difference. 6'3

80°7

Acid.
Acetic

CH3.CO,H
Trichloracetic CCl2. CO,H
Lactic

CH,, CHOH.CO,H
Trichlorolactic CCiz. CHOH.CO,H

87

.

10:5
26

15.5

The replacement of H, by Cl, is accompanied by a much greater increase of affinity when there is direct mutual action between the Cl, group and the acid group COH, than when the two groups are separated by the group CHOH.

The influence of the distribution of the interatomic actions on the affinity-constants of acids is also illustrated by the following numbers. Acid.

Relative affinity.
Oxalic
HO,C-CO,H

43

H,
Malonic HOÁC -C- CO,H

17-5
Succinic HO,C - C - C - COH

7 H, H,

.

3

Lactic

CH3-CHOH-CO,H 10'5 Methoxyacetic CH, - OCH3 - CO,H 135 In the three acids oxalic, malonic, and succinic, we notice a rapid decrease in the value of the affinity as the mutual actions of the carboxyl groups become more indirect; and the comparison of lactic and methoxyacetic acids suggests that the presence of the group H,C -OCH, is attended with a greater affinity-value than that of the isomeric group H.C - CHOH

258. One of the general conclusions regarding the relations between the structure of carbon compounds, and the refraction-equivalents on the one hand, and the specific volumes,' (V), on the other, of these compounds, may here be recalled (see book I. chap. IV. pars. 141, 143, and 154). The refraction-equivalent, and also the value of (V), of an unsaturated compound, i.e. a compound in the molecule of which some of the polyvalent atoms act on less than their maximum number of monovalent atoms (see book I. chap. II. par. 62), are always less than the values of the same constants for an analogous saturated compound.

Now Brühl has tried to shew that an increase of the refraction-equivalent is connected with a loosening of the attractions

1 For more details see Ostwald, 7. für prakt. Chemie, (2). 18. 362: 28. 479, 488, 492 : 29. 403.

2 Ber. 14. 2533.

between the atoms in the molecules of compounds. Brühl's reasoning seems to me to be very unsatisfactory! But a similar conclusion has been arrived at by Schiff from his measurements of the values of (V) for a large series of compounds, and from some general considerations drawn from the kinetic theory of gases'. Schiff's comparison of (V) for the normal acids

CLOH

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C.H. CO with(V)forthe normal alcohols C.H.co

1

OH' shews that the value of (V) for the monovalent oxygen atom in the molecules of the acids increases as the series is ascended. But, if we admit the general conclusion arrived at by Brühl, and also by Schiff, this increase is accompanied by an increased loosening (Lockerung) of the group Co. OH. Such a loosening would, we should expect, be attended by a decrease in the acidity of the acids of the series. This conclusion is in keeping with Ostwald's values for the relative affinities of the three acids, formic, acetic, and propionic, viz. 12, 6-3, and 5:5.

Whether Schiff's conclusion is accepted or not must depend upon the results of further investigations. There is one point especially worthy of note in the nature of the argument adopted by Schiff, namely, it is based on measurements of the changes in the values of (V) which accompany definite chemical operations. This, it seems to me, is as it should be. As we require determinations of the changes of energy which accompany this or that chemical change, so we must have determinations of the variations in such physical constants as (V), (R.), &c., which proceed along with definite, and if possible simple, chemical processes.

There appears to be a definite connection between the course of a chemical operation and the ratio of the original to the final volume of the reacting system. W. Müller-Erzbach has endeavoured to express this relation in the so-called 'law of smallest volumes,' which states, that the smaller the volume occupied by the products of a chemical change, the greater is the loss of energy during the change, and therefore the more probably will the change occur'.

1 In connection with this see Thomsen, Ber. 16. 67. ? Annalen, 200. 321. 3 For data see Schiff, loc. cit. 314-315.

259. Ostwald's researches suffice, I think, to establish the existence of a connection between the structures and the affinities of molecules. In other words, these researches put an instrument into the hands of chemists by the use of which they may hope to gain a more complete answer than has hitherto been possible to some of the questions which lie at the root of chemical science. A structural formula, which is the result of an extended investigation, summarises a great many facts about the composition, and also tells something of the reactions, of the compound formulated; the number which represents the relative affinity of the same compound is obtained by comparing the power of doing of the substance with that of other substances, and enables us, to some extent, to predict the course and the results of the chemical changes that will occur in given systems of which the substance forms a member.

The structural formula is based on the molecular and atomic theory, and, in so far as it has been obtained by assuming the theory of valency, it includes in its expression the older views regarding equivalency. It may be possible, some day, to indicate by this formula the relative loss or gain of energy which has occurred in the passage from some standard state to the state expressed by the formula. The relative affinity, on the other hand, is based on a kinetic theory of chemical action, and is the outcome of the study, for three-quarters of a century, of chemical change. We begin to see how the formula and the affinity may be merged into a common expression, which shall tell us, not only the composition, but also the function of the substance, and in doing so will reconcile the two schools which have so long existed in chemistry, the school of Bergmann, Berzelius, and Dalton, with that of Berthollet, Davy, and Dumas.

1 Annalen, 218. 113. See also Ber. 14. 217 and 2212 ; 16. 758: 17. 198. Also Donath and Mayrhofer, Ber. 16. 1588. Compare also Spring's work on the connection between allotropy and volume ; see foot-note on p. 137.

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