ATOMIC LINKING OF ALCOHOLS

87

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 :—

51

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 = CH ̧—O—CH ̧ + NaI

3

Methyl Alcoholate

3

Methyl Iodide

3

3

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 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 ̧

3

OH

3

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, C2H,.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, C2H2O.

If we compare these formula 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.

6

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 ̧ÿO, 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 :—

CH,O
Formaldehyde

CH,O,

Formic Acid

C2H2O
Acetaldehyde

C2H2O2

Acetic Acid

C2H2O
Propionaldehyde

C3H6O2
Propionic Acid

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

ISOMERIC ALCOHOLS

89

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

Formic acid, H-CO-OH

Acetic acid, CH ̧-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.

H-CO-ONa+ HONa = H—H+NaO—CO—ONa CH, CO-ONa + HONa = CH‚—H+ NaO-CO-ONa C2H-CO-ONa+ HONa = C2H ̧-H+NaO-CO—ONa

2

3

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.

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 yield acids which contain a carbon atom, which is united to two hydrogen atoms and the hydroxyl group.

The group —CH-OH is as characteristic of these 'primary' alcohols (as they are called) as the carboxyl group is of the acids. In the same way it can be shown that the group =CH-OH is characteristic of the second class, the secondary' alcohols,

'

6

which yield acetone or kindred bodies on oxidation.

The

tertiary' alcohols which yield neither acids nor ketones, but lose carbon on oxidation, contain the group C—OH.

After ascertaining these characteristic points of difference for a large number of alcohols, the other chemical and physical properties of the alcohols were investigated. The result showed that, in a group of isomeric alcohols, the primary boil higher than the secondary, and these again higher than the tertiary, but the latter have, on the other hand, a higher melting point. The three classes of alcohols can be distinguished by means of their boiling points. Although it is unnecessary to use this method for this particular purpose, it proves of great value in discriminating between isomeric alcohols of the same class. For example, four isomeric butyl alcohols (C,H,,O) are known: two of these are primary, and consequently contain the group HO-CH2The difference between them must consist in a difference in the arrangement of their other carbon atoms. According to theory, two modes of linking are possible :— HO-CH2-CH2-CH2-CH, and HO-CH2-CH—CH ̧ CH,

3

10

Which of these formulæ belongs to the alcohol boiling at 116°, which is obtained by the reduction of butyric acid, and which formula must be ascribed to the alcohol boiling at 109° and contained in fusel oil? This problem may be solved in different ways. By depriving each alcohol of the elements of water a hydrocarbon, butylene (CH), is obtained. Each butylene unites with hydriodic acid, forming a butyl iodide (C,H,I), in which the iodine can be replaced by hydroxyl. The original alcohols are not reproduced by this process. The alcohol boiling at 116° yields a secondary alcohol (boiling point 99°), and the alcohol boiling at 109° yields a tertiary alcohol boiling at 83°. Only one formula is possible for each of these alcohols :

CONSTITUTION OF BENZENE

91

The difference from the original alcohols, can only be due to the fact, that the new hydroxyl does not take up the position previously occupied by the old hydroxyl. Imagine that they are reconverted into primary alcohols, then we get the preceding formulæ again, and we see that the first formula, in which no carbon atom is directly united to more than two others, belongs to the alcohol boiling at 116°; the other, in which one carbon atom is united to three others and one hydrogen atom-is consequently in a tertiary' position-belongs to the fusel oil alcohol boiling at 109°. The first kind of linking is termed 'normal' to distinguish it from the abnormal branched' or 'side-chain linking.

Experience has shown that the normal compounds always have a higher boiling point than those with side chains, and that the boiling point of the latter falls as the number of side chains increases. In the case of bodies having a similar constitution, the addition of CH, raises the boiling point from 18° to 22°. This fact may be used for determining the constitution or for testing and confirming the accuracy of a constitution determined by other methods.

§ 51. Aromatic Compounds.-Benzene and the so-called aromatic compounds' derived from it offer a remarkable example of the manner in which the atomic linking has been investigated. Benzene is a hydrocarbon which contains the same number of carbon and hydrogen atoms. Its composition is represented by the formula CH, where n stands for a whole number. Its molecular weight is therefore

m = n(C + H) = n (11·97 + 1) = n × 12·97.

Faraday found that the density of its vapour is 2.752 times that of air; m will therefore be approximately 79.43.

m=28.87 x 2.752 = 79.43,

or, for the corrected value,

m = 6 × 12·97 = 77·82 = C ̧H。.

'The name 'aromatic compounds' has its origin in the fact that the members of this group which were first investigated possess an aromatic odour, a property not shared by all the members.