ATOMIC LINKING AND PHYSICAL PROPERTIES

83

The formation of addition compounds is frequently used in determining atomic linking, it being assumed that the addition takes place at those points where the available affinities are situated.

§ 47. Determination of Atomic Linking from Physical Properties. Numerous observations show that the atomic linking exercises a marked influence on the properties of substances, and it often happens, that two isomeric compounds containing exactly the same constituents, possess totally different properties, solely in consequence of the different linking of their atoms; e.g. ethyl acetate has a pleasant ethereal odour and butyric acid has a rancid, offensive smell. When the constitution of a series of substances has been investigated, the influence of the atomic linking on their properties is soon apparent, and we are now able to determine the atom linking by a study of the properties. The following physical properties chiefly come into consideration: density, fusibility, volatility, colour, solubility, crystalline form, smell, taste, physiological action, &c. The relation of all these properties to atomic linking, has not yet been sufficiently investigated. These relations grow clearer day by day, and by this knowledge it is now possible to make new substances possessing certain desired properties. The recent syntheses of dyes and colouring matters offer a brilliant example of our power to accomplish this object.

The application of physical properties to the investigation of atomic linking depends chiefly on the fact that the physical properties change regularly, when a certain definite alteration in the atomic linking is repeatedly made and the rest of the compound is left unchanged. If we examine a series of organic compounds, the members of which differ from each other in their molecular weights by increments of 1C and 2H it is often observed that the boiling points of these bodies exhibit a certain fixed difference for each CH, group. This is not always the case, but only when the constitution and linking remain the same, with the exception of the slight difference in composition due to the introduction of the group CH2. When we find that there is a regular increase in the boiling point with the molecular weight, we conclude that the bodies have the same or similar atomic linking. Numerous examples

of these relations are found in the text-books on organic chemistry.

§ 48. Determination of Atomic Linking from the Chemical Behaviour. The chemical properties are even more dependent on the atomic linking than the physical properties. The influence which certain groups of atoms exert on chemical behaviour, is first determined in compounds of comparatively simple structure, such as those, for instance, in which only one arrangement of the atoms is possible or such as permit their atomic linking to be easily ascertained. More complicated compounds exhibiting similar properties are assumed to contain the same groups of atoms. If large or small deviations occur, the origin of these is investigated by increasing the number of observations and comparing them with one another. In this way a rich collection of rules has been obtained, which enable us to deduce the structure of compounds from their chemical properties.

In the case of each of the frequently occurring combinations of atoms, such as hydroxyl, OH, amide, NH„, imide, NH, &c., we have not only determined which properties of the compounds indicate their presence, but we have also ascertained the differences in their behaviour caused by the nature of the atoms or radicals with which they are combined. For example, we are not only acquainted with a whole series of tests for identifying the presence of hydroxyl, but we can also ascertain, from the peculiarities in the behaviour of the substance, whether this hydroxyl is attached to carbon or nitrogen or whether the carbon atom united to the hydroxyl is combined with hydrogen, or oxygen, or only another carbon atom. In other words, we

have the means of discriminating between the following formulæ :

CH,—OH,

2

and many others.

=CH-OH, EC-OH, —CO—OH,

If a compound contains nitrogen we can ascertain from its chemical character whether the nitrogen is directly combined with oxygen, oxygen and carbon, hydrogen and carbon, or with carbon only.

The relations between atomic linking and chemical properties are, at the present time, amongst the chief objects of investigation in the field of organic chemistry, and are generally discussed

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85

in connection with this branch of the science. Unfortunately this subject seldom meets with systematic and comprehensive treatment.1

§ 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 had been investigated and analysed increased, the number of new formulæ in use increased, and these formulæ were changed when they apparently ceased 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 increasing bitterness, 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 concord. 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 take 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)

1 The relationship is systematically treated in E. Lellman's Principien der organischen Synthese, Berlin, 1887.

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 contain 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 C„H2n+2O, in which n may represent any whole number; its 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, CHÃO, 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

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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 :

CH,—CH,—OH and CH ̧—O—CH2

3

2

Ethyl hydroxide

3

Methyl oxide

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 well-known compound (methyl oxide) which may be prepared by the following reaction :

CH,—O—Na + CH2—I

Methyl Alcoholate

3

=

Methyl Iodide

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 C2H,, 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 different manner in which the hydrogen atoms are distributed. Taking this into account, we can only have two formulæ for the alcohols :

CH‚—CH¿—CH,—OH and CH ̧—CH—CH ̧

2

OH

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, C2HO.