Most of these substances belong to the rhombic, monoclinic, or triclinic systems, and form optically biaxial crystals, which do not exhibit the phenomenon of rotation.

§ 56. Asymmetrically linked Carbon Atoms. In investigating the cause of the rotation of light due to the nature of the molecules, it is important to notice that this peculiar phenomenon is only observed in organic compounds, and only exhibited by a comparatively small number of carbon compounds. This observation led to the hypothesis that the phenomenon is due to a peculiarity in the linking of the atoms. In fact, in 1874 two different investigators, Van t' Hoff and Le Bel, independently discovered the connection existing between the rotation of light and atomic linking and offered a perfectly satisfactory explanation of this optical isomerism.

As stated in § 43, the four affinities of a carbon atom are symmetrically arranged in space, and consequently the four atoms united to the carbon atom are arranged round it like the four corners of a tetrahedron round its centre. If these four atoms all differ from each other either in their nature or in being combined with different atoms, then two forms of combination are possible. These are sketched in perspective and numbered I and II.

The four atoms or radicals, a, b, c, d, are attached to the carbon atom in such a way that the two figures are non-superposable, and one is the reflected image of the other. Imagine your eye is placed in the position of one of the atoms, say a,

and directed towards the other three atoms; then it sees b c d in I in the direction in which the hand of a clock moves, but in II in the reverse direction.

A carbon atom in this condition is said to be an unsymmetrically linked carbon atom, or briefly an asymmetric carbon atom. A careful examination of all those compounds which in the liquid state can rotate polarised light shows that each of these bodies contains at least one asymmetric carbon atom; several contain more than one. The property of rotation depends on the presence of an asymmetric carbon atom. Let a H, b=HO, c = COOH, d=CH,; these groups are contained in malic and tartaric acid: both of these acids exist in two symmetrical forms.

=

According to the formulæ generally in use, only one form of malic acid is possible, viz.

HỌ—CO–CH–CH, CO–OH

он

2

But if we take into consideration the fact that the carbon atom attached to the HO group is asymmetric, then two formulæ are possible. Starting from the hydroxyl, the sequence of the other atoms or radicals is in the direction of the hands of a clock in one formula

H,CO-OH,CH,-CO-OH

and in the reverse direction in the other

H,CH,-CO-OH,CO-OH.

The two formulæ are non-superposable.1

1 To make this point perfectly clear, divide the surface of two wooden balls of the same size into eight equal spherical triangles or quadrants by means of three circles cutting each other at right angles. Bore a hole down to the centre of the globe in the middle of each alternate quadrant. Insert four rods of equal length, one in each hole: these indicate the direction of the

§ 57. Active and Inactive Forms. The rotatory power of a compound ceases when the asymmetric carbon atom disappears; for example, malic acid is converted by reduction into succinic acid

HO—CO–CH,CH, COOH

which is inactive.

2

The rotatory power also ceases when equivalent quantities of both modifications unite and crystallise together. For example, the two optically active malic acids unite and form an inactive acid because the rotatory power of the one neutralises that of the other. In such cases the components may be separated by means of suitable agents; for example, one constituent may combine more readily with other dextrogyrate bodies; the other may unite more easily with other lævogyrate compounds. We are acquainted with two optically active malic acids which unite together, forming an inactive modification.

If a compound contains two asymmetric carbon atoms which are united to similar atoms or radicals, then there can exist two optically active and two inactive forms. This is the case with tartaric acid; we have dextro- and lævo-tartaric acid.

HỌ—CO—CH–CH—CO–OH.

он он

One inactive acid (racemic) is a compound of the two active forms, but the second acid owes its inactivity to the fact that the position of the other atoms attached to one carbon atom is unsymmetrical with the dextrogyrate and at the other carbon atom unsymmetrical with the lævogyrate modification.

This second form cannot be split up into two active modifications. If the two asymmetric carbon atoms are united to different atoms and radicals, the effect of one is not, as a

forces of affinity. Fix four balls of different colours to the free ends of the rods, and you have a representation of an asymmetrical carbon atom. According to the sequence of the coloured balls, the groups will be either identical or symmetrical, i.e. the reflected image of each other.

rule, counterbalanced by that of the other, and consequently all four modifications may be optically active, but in different degrees.

The number of possible isomerides increases with the number of asymmetric atoms. A large number of isomerides can exist in the series of sugars, and the terpene derivatives.

§ 58. Physical Isomerism, with Double Linking.-When an asymmetric carbon atom loses one of the four atoms or radicals with which it is combined, and attaches itself to a neighbouring atom by a double linking, the optical activity of the compound is lost, but the possibility of physical isomerism still continues. Malic acid (CHO) affords one of the best known examples of this kind. It loses water (HO+H=H2O), forming the isomeric, fumaric, and maleic acids (C,H,O,). The latter again loses water, yielding the anhydride C1H2O ̧, but fumaric acid does not form an anhydride. There is only one formula1 for the two acids in the system in general use, viz.

HỌ—CO–CH–CH,–COOH

он malic acid

HỌ—CO–CH=CH–COOH

Fumaric and maleic acids

But if we take into consideration the arrangement of atoms in space, then we have two different formulæ,

which Van t' Hoff and Wislicenus have shown are perfectly

The only other formula, HO-CO-C-CH2- CO-OH, cannot be correct, as fumaric and maleic acid unite with Br2, forming one and the same dibromosuccinic acid, HO-CO-CHBr-CHBr-CO-OH. The discussion of the formula HC: CH.C.OH

CO.Ó OH

proposed by R. Anschütz is deferred.

capable of explaining the difference in the behaviour of the two acids.

It is obvious that the first formula represents maleic acid, as the proximity of the carboxyl groups -CO-OH facilitates the formation of an anhydride.

In fumaric acid the carboxyl groups are diametrically opposite each other.

Both acids combine with the elements of water, forming inactive malic acid, which can be split up into two optically active isomerides. The addition of the elements of water takes place in each of the two possible ways.

The development of stereochemistry (from σTepeós)—that is the introduction of the idea of a difference in the arrangement of the atoms in space into the constitutional formulæ of organic compounds-has provided a satisfactory explanation for numerous cases of isomerism which formerly could not be accounted for. It has also led to the discovery of numerous relations between the arrangement of the atoms and the properties of compounds. The hypothesis of asymmetrically linked carbon atoms was first propounded in 1874, and it now ranks as one of the most firmly established of the doctrines of chemistry.

d

Recently allotropic modifications of certain nitrogen compounds have been discovered and investigated by Hantzsch and others. The cause of the difference of the two stereo-isomeric forms is probably due to the existence of the group CON-OH.

e

Nitrogen is pentavalent, and as it is impossible to arrange five points symmetrically in space (§ 43), it is probable that the five affinities are not exactly alike. We may consider that the lines of force of three of the affinities