§ 49. History of the Development of the Theory of Atomic Linking. Our present knowledge of atomic linking has not been gained by a peaceful, gradual development. On the contrary, in proportion as the number of organic compounds which were investigated and analysed increased, there was a corresponding growth in the number of new formulæ employed, and these formulæ were changed when they appeared no longer to answer their purpose. Much was left to individual caprice, and formulæ were used of which it is scarcely possible at the present day to discover the meaning. It is only natural that under these conditions differences of opinion were frequent, and disputes arose, which were carried on with bitterness rising in proportion as the difficulty of proving the correctness of one view or the other increased. Gradually these points of difference were smoothed away, and widely divergent views were brought into accord. At the present time all chemists, with very few exceptions, agree in recognising as correct those formula which have been established in accordance with the laws of the theory of atomic linking. As a result of this gradual development, the correct expression of the composition of most substances was discovered, before the theory, on which the formulæ are based, was known. The theory is still of great value, and is used in determining the formula of every newly discovered substance; it is also useful in testing and correcting those constitutional formulæ already in use; and, finally, it forms the philosophical basis for the theories with which the experienced chemist is so familiar that he scarcely notes the foundations on which they rest, but which prove difficult for the beginner to understand if he has not received systematic instruction in these matters. We will now study a few examples of the method pursued in determining the constitution of an organic compound. § 50. Examples of the Determination of the Atomic Linking. We will assume that the constitution of that class of organic bodies termed alcohols' (from the Arabic name for spirits of wine) is unknown and has to be determined. These bodies are composed of carbon, hydrogen, and oxygen, and are characterised by certain properties common to all members of the class, more particularly by their power of reacting with acids, with elimination of water, to form ethereal salts. On hydrolysis the ethereal salts yield the original acid and alcohol. The alcohols are mono-, di-, or poly-acid, that is, they can combine with one, two, or more equivalents of an acid. The mono-acid contains at least one, the di-acid two, the tri-acid three oxygen atoms, from which we may conclude that there is a close connection between the equivalence of the alcohol and the quantity of oxygen it contains. We will confine our attention to the mono-acid alcohols, and only consider those mono-acid alcohols containing the maximum number of hydrogen atoms, which have the general formula CH2+90, in which n may represent any whole number; its C,H2n+2 value generally lies between 1 and 30. Some of these alcohols are liquid at the ordinary temperature, others are solid but easily fusible. They are all volatile, and can be distilled: in the case of the higher members of the series the distillation must be carried on under reduced pressure. The volatility generally diminishes as the molecular weight increases, but a larger molecular weight is not always accompanied by a higher boiling point. Isomeric alcohols have, without exception, different boiling points. In order to determine the atomic linking it is best to begin with the lowest member of the series--that is, with the member having the lowest molecular weight. This is wood spirit, CH,O, in the formula for which n=1. It is obvious that only one arrangement of the atoms is possible in this case: H H-C-O-H H As no other mode of combination is possible, the alcohol must be a compound of methyl (CH) and hydroxyl (HO). This view is confirmed by the behaviour of the compound, e.g. CH,—OH+H—I=CH,—I+ HOH As this and analogous reactions are common to all alcohols, it is probable that they all contain hydroxyl. If n=2, the formula is that of spirits of wine, C2HO. In this case two different modes of arranging the atoms are possible: But the alcohol must have the first formula, because the radical ethyl, C2H,, can be expelled unchanged from this compound by the action of acids and other substances, and this shows that the two carbon atoms are united together. The second combination represents the formula of a wellknown compound (methyl oxide) which may be prepared by the following reaction: 3 3 3 3 CH,—O—Na + CH2—I = CH,–O–CH, + NaI. There is not a second alcohol isomeric with ethyl alcohol; but when n=3, two isomeric alcohols are possible. Both contain the group of atoms C,H,, propyl; but the compounds which these two radicals form are only isomeric, and not identical. It is evident, then, that the radicals are not identical, but merely isomeric. Now, three carbon atoms can only be linked together in one way; the difference must therefore be due to the manner in which the hydrogen atoms are distributed. Taking this into account, we can only have two formulæ for the alcohols : CH-CH2-CH2-OH and CH3-CH-CH, он The hydroxyl is either united to one of the end carbon atoms, which is also combined with two hydrogen atoms, or it is attached to the middle carbon atom, which is united to one hydrogen atom. Now the question arises, which formula is to be ascribed to each of the two known alcohols, C,H,.OH. One is formed together with ethyl alcohol in the process of fermentation, and occurs in fusel oil, and boils at 97°C.; the other boils at 83°, and was first prepared by Friedel by the action of nascent hydrogen on acetone, CHO. If we compare these formule with that of ethyl alcohol we find that the formula of ethyl alcohol bears a closer resemblance to the first than it does to the second formula, for the hydroxyl is attached to a carbon atom which is united to two hydrogen atoms, and only one other carbon atom. It is therefore probable that the first formula belongs to that alcohol which bears the closest resemblance to ethyl alcohol. This is without doubt the fermentation propyl alcohol boiling at 97°. Without going into detail, we may point out that, like ethyl alcohol, this alcohol on oxidation loses two atoms of hydrogen, forming an aldehyde (alcohol dehydrogenatus), and this by taking up oxygen is converted into an acid. On oxidation isopropyl alcohol, boiling at 83°, yields the acetone C,HO, from which it was obtained by reduction, but it does not yield any acid. Other isomeric alcohols exhibit a similar difference of behaviour on oxidation. It is consequently important to ascertain the atomic linking of the aldehydes, acids, and acetone. The aldehydes and acids have the molecular weights represented by the following formulæ : CH2O CH,O, Formic acid C2H2O C2H2O2 Acetic acid C2H2O C2H2O2 As these bodies are derived from alcohols, all their carbon atoms must be united together. But as the number of hydrogen atoms does not attain the maximum value for n, given in § 45, the question arises: Are we to assume the existence of unsaturated affinities or of double linking? This question is difficult to decide experimentally in the case of the aldehydes. With regard to the acids, the answer is decidedly in the negative. We consider that the hydroxyl of the alcohol remains in the acid, as it is easy to simultaneously expel from an acid one atom of oxygen and one of hydrogen. If we make use of this supposition, only one of the theoretically possible formulæ for formic acid derived from methyl alcohol is available, viz. H-CO-OH. The radical HCO, which is combined with the hydroxyl, is called 'formyl;' it is composed of carbonyl, CO, and hydrogen, H. On comparing this formula with that of methyl alcohol, it is seen that in addition to the hydroxyl, the acid contains an atom of oxygen attached to the carbon atom instead of two atoms of hydrogen in the alcohol. The close resemblance which exists between acetic and propionic acids and formic acid makes it highly probable that these acids are formed from the alcohols in a similar way, and contain instead of two atoms of hydrogen, one atom of oxygen, united to the carbon atom to which the hydroxyl is attached. According to this hypothesis the formulæ for the acids would probably be 6 Formic acid, H—CO—OH Propionic acid, CH-CH-CO-OH If these formulæ are correct the group of atoms termed carboxyl,' CO-OH, is characteristic of these acids and determines their properties. Formic acid is therefore hydrogen carboxyl; acetic acid, methyl carboxyl; and propionic acid, ethyl carboxyl. This supposition is confirmed by the behaviour of the acids; for in reactions in which formic acid and its salts give off hydrogen, acetic acid yields methyl and propionic acid yields ethyl. All three acids are decomposed by heating with an excess of caustic soda, yielding sodium carbonate and hydrogen, methyl hydride or methane, ethyl hydride or ethane respectively. H-CO-ONa+ HONa H-H+NaO-CO-ONa = CH, CO-ONa+ HONa= CH,-H+ NaO-CO-ONa CH-CO-ONa+ HONa =C,H,-H+ NaO-CO-ONa These views are confirmed by many reactions of these acids, and the investigation of many other acids proves that wherever the carboxyl group of atoms occurs the compound has the properties characteristic of an acid. When this was once recognised it became evident why some alcohols do not yield acids. Only those alcohols can |