To arrange organic compounds in series, that is, to determine the laws according to which the properties in a given type are modified by substitution of an element or group of elements for other elements, this is the constant purpose of the chemist philosopher. These thousands of compounds which he produces in his laboratory are for him, however, the terms which serve him to construct his series. Today, in the imperfect state of the science, there is still need for many terms; but later, knowledge of certain series will eliminate direct study of many other terms whose properties he will be able to predict with the same certainty as he predicts today the properties of propionic or valeric alcohols, even though he has not yet obtained these alcohols.
In the state of the science, organic compounds can be related to three or four types, each capable of giving series which resemble those represented by formic and stearic acids, potash, and sulfuric acids; these types are
By exchanging their hydrogens among certain groups, these types give rise to acids, to alcohols, to ethers, to hydrides, to radicals, to organic chlorides, to acetones, to alkalis.
Water H2O Hydrogen H2 Hydrochloric acid HCl Ammonia H3N
The series formed by each type has its extremes, which can be called the positive, or left, side and the negative, or right, side. An organic group, substituting for hydrogen, which places itself on the positive side will produce compounds placed on the same side; the groups ethyl CH3, methyl C2H3, amyl C5H11, for example, will give by this substitution alcohols resembling water, aldehydes or radicals resembling hydrogen, ethers resembling hydrochloric acid, alkalis resembling ammonia. The groups of which I speak resemble, in fact, potash or other reputedly electropositive metals; the oxides (the alcohols) and the alkalis to which they give rise behave like bases, in that they can combine with acids placed at the other extreme of the series.
Other organic groups, for example, cyanogen CN, acetyl C2H3O, benzoyl C7H5O, on substituting for the hydrogen of the types mentioned give rise to those compounds which are farther removed than the preceding from compounds formed with hydrogen, to compounds which are placed more to the right, toward the negative extreme. The oxides formed by these groups resemble sulfuric acid more than potash. ...
The water type, changing half its hydrogen for a hydrocarbon group CH3, C2H5, etc., gives rise to an alcohol; changing all its hydrogen for a similar group, it produces the corresponding ether.
The same type, in changing half its hydrogen for a group containing at once carbon, hydrogen, and oxygen, produces a hydrated monobasic acid, resembling acetic acid. When the substitution is effected by the same group on the two atoms of hydrogen of water, the product is the corresponding acid anhydride; Mr. Williamson has already made the same comparison, and its exactitude seems to me today to be perfectly demonstrated by my experiments. Finally, when the substitution of the two hydrogen atoms of water is made half by a hydrocarbon group like ethyl or methyl and half by an oxygenated group resembling that which is found in a monobasic acid, the ester of this acid is obtained.
The hydrogen type can undergo the same substitutions as the water type and produce as many combinations.
The compounds resembling marsh gas, known as hydrides, are evidently related to hydrogen as alcohols are to water; the ethyl and methyl radicals correspond to the ethers of these alcohols. Aldehydes are to hydrogen as monobasic acids are to water; acetyl, benzoyl, and other oxygenated radicals correspond to acid anhydrides; the acetones, finally, as M. Chancel has already remarked, represent the esters of the aldehydes and consequently are to hydrogen as the esters of monobasic acids are to water.
The hydrochloric acid type gives rise, on the one hand, to hydrochloric ethers, that is, to chlorides resembling chloride of potassium or chlorides of electropositive elements, when the substitution is effected by hydrocarbon groups; and, on the other hand, to electronegative chlorides corresponding to monobasic acids, like acetyl chloride or benzoyl chloride, when the same substitution is effected by groups contained in these monobasic acids.
Finally, the ammonia type produces alkalis able to combine with acids, or amides able to combine with bases (oxide of silver, mercury, copper, etc.), according to whether the substitution on the hydrogens of the ammonia is effected by groups which give rise to bases (alcohols, organic oxides), or by groups which produce organic acids. The bodies resembling the hydrate of oxide of ammonia are represented at the other end of the series by acid amides.
It can be seen by this rapid summary how the application of the notion of series permits simplification of the general theory of organic compounds. They no longer terrify by their number and variety, for, instead of being formulated by special theories which lack any connection, as they are called ethers, amides, alkalis, or acids, they become simply terms whose properties can be predicted according to the place they occupy in the series. And what certainly adds to the advantage of such a system is the similarity of method of formation or decomposition which it expresses for all the bodies which it contains. Experiment shows, in fact, that organic compounds are almost all the result of double decompositions resembling those which we effect in mineral chemistry.