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

2

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, 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 EC-OH.

After ascertaining these characteristic points of difference for a large number of alcohols, the other chemical and physical properties of the alcohols are investigated. The results show that, in a group of isomeric alcohols, the primary boil higher than the secondary, and these again higher than the tertiary, but in the case of solid isomeric alcohols, the tertiary have the highest 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 (CHO) are known; two of these are primary, and consequently contain the group HO-CH,-. The 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—CH,—CH2-CH2-CH, and HO-CH2-CH-CH, CH3

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 :

CH3

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 alcohol boiling at 109° obtained from fusel oil. 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 compounds1 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

'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.

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=C2H ̧.

A very large number of combinations is possible for the twelve atoms contained in such a molecule, and, at first sight, it appears perfectly hopeless to attempt to investigate the constitution of benzene.

An ingenious interpretation of the behaviour of this substance led Kekulé to propose an hypothesis which explains all its peculiarities in the simplest way. This hypothesis has maintained its position to the present day, in spite of all the criticism to which it has been exposed for a quarter of a century. The hypothesis is based on the observation that, when an atom of hydrogen in benzene is replaced by another atom or radical, only one single derivative is produced. It does not matter which of the hydrogen atoms is replaced; isomeric compounds are never formed. That it is not always one and the same hydrogen atom that is replaced, but that in reality different hydrogen atoms are displaced, can be shown in the following way. Nitro-benzene is formed by the action of nitric acid on benzene : HO-NO2 = CH-NO2 + H2O

C ̧H +

Benzene

Nitric acid

2

Chloro-benzene (C,H,CI) can be prepared in several ways from this nitro-benzene, in which the nitro- group (NO2) has replaced an atom of hydrogen. We may either reduce the nitro- group to NH, and replace the latter by chlorine, or we may replace a hydrogen atom in nitro-benzene by chlorine (in this case it is evident a different hydrogen must be displaced). The nitro- group is eliminated from the chloro-nitro-benzene

6

(CH,NO,CI) and replaced by hydrogen. Experiments of this kind have been conducted in widely different forms, but they all yield one and the same chloro-benzene. From this behaviour of benzene we conclude that all the hydrogen atoms in benzene are equivalent to one another in every respect, and that each hydrogen atom is combined in exactly the same way as each of the others. These conditions are satisfied by Kekulé's hypothesis that all six carbon atoms are united together forming a closed ring,' and the hydrogen atoms are uniformly distributed amongst the carbon atoms as is shown by the following formula:

In order that all four affinities of each carbon atom may come into play, it is assumed that the linking in the ring is alternately by one and by two affinities. In recent times this comparatively minor detail of the hypothesis has given rise to much discussion.

In

Kekulé's formula explains the whole behaviour of benzene and its derivatives in a remarkably satisfactory manner. the first place it indicates that each mono-substitution product' obtained by the replacement of one of the hydrogen atoms can exist in one form only, and that isomerides cannot exist. But if a second atom or radical is substituted for a hydrogen atom, the perfect symmetry of the molecule is changed, and the second atom may take one of three positions, namely, next to the first or separated from it by one or by two carbon atoms. If the first atom takes the position at

The ring formula does not indicate that the atoms are arranged in a plane circle, but only that they form a closed chain.

1, the second may occupy the position at 2 and 6 (which are identical), or at 3 and 5, or finally at 4.

Strictly speaking, the positions 2 and 6 are not absolutely identical on account of the double linking; this has led some authors to assume that the fourth affinities are free or else united to the opposite carbon atoms. These modifications of the hypothesis have not up to the present acquired any particular practical significance.

In spite of the repeated endeavours of many chemists to discover four isomeric di-substitution products of benzene, e.g. di-chloro-benzenes, not more than three have ever been discovered. It is this circumstance which has gained for Kekulé's hypothesis a general recognition of its value. The three isomeric di-substitution products are distinguished by the prefixes ortho,' ' meta,' and 'para.'

The problem now arises, Which of the three positions 1.2, 3, and 1. 4, or which of the three formulæ

is to be assigned to each of these compounds?

Owing to the inherent difficulties of the problem, this