282 



NATURE 



[July 23, 1903 



beam of light thrown through their solutions. Such sub- 

 stances are said to be optically active, and since the deflec- 

 tion of the plane of polarisation may be either towards the 

 right or towards the left, the exhibition of optical activity 

 constitutes an enantiomorphous property ; optically active 

 substances are conveniently classified as dextro- and laevo- 

 rotatory. Van 't Hoff and Le Bel declared that the mole- 

 cules of all naturally occurring substances which exhibit 

 optical activity when in the fluid state contain asymmetric 

 carbon atoms. All substances the molecules of which con- 

 tain an asymmetric carbon atom must possess enantio- 

 morphous molecular configurations — similar to those 

 assigned to the two lactic acids — because they exhibit 

 properties of an enantiomorphous character. A very 

 beautiful experiment which the late Sir George Gabriel 

 Stokes devised may be so modified as to serve for the 

 demonstration of optical activity. Stokes's experi- 

 ment consists in passing a plane polarised beam of 

 light through a tall cylinder containing water which 

 has been rendered very slightly turbid by the addition 

 of a little alcoholic solution of resin ; a spectrum 

 is then seen spread out in the column of liquid, and spread 

 out in a way which is not enantiomorphous, the water 

 possessing no optical activity. The modification of Stokes's 

 experiment consists in replacing the non-enantiomorphous 

 water by some enantiomorphous liquid — conveniently by a 

 70 per cent, aqueous solution of the dextrorotatory cane- 

 sugar, or by a 50 per cent, solution of the laevorotatory 

 fruit-sugar ; on making this change it is seen that instead 

 of the spectrum lying in the cylinder vertically, and there- 

 fore non-enantiomorphously, it winds spirally or corkscrew- 

 wise round and round the column of the enantiomorphous 

 liquid. The two spirals or helices are clearly enantio- 

 morphous, and the two liquids of opposite optical activity 

 give rise in this experiment to oppositely wound spirals — 

 to spirals which are related to each other like the right- 

 and left-handed corkscrews shown in the lantern slide. The 

 opposite sign of the rotatory power exhibited by the cane- 

 sugar and fruit-sugar solutions is more clearly shown by 

 turning the polarising prism in its mount, when the two 

 spirals turn in opposite directions. 



Although cases of optical activity are very frequently met 

 with among chemical substances of animal or vegetable 

 origin, it must be noted that no purely laboratory product 

 or substance prepared without the use of enantiomorphous 

 operations or materials is, in the ordinary way, optically 

 active. The reason of this needs but little seeking, if the 

 solid tetrahedron models are once more consulted. Start- 

 ing with a non-enantiomorphous substance is equivalent to 

 starting with a methane derivative of the constitution 

 X X 



and replacing one of the two X groups by a fourth group Q 

 so as to obtain a compound containing an asymmetric carbon 

 atom. Obviously, unless some power of selection of 

 an enantiomorphous nature is exercised in replacing X by 

 Q, the doctrine of chance will ensure the one X group being 

 replaced the sam.e number of times as the other in an 

 enormous number of tiny molecules. Thus there will result 

 just the same amount of the right-handed optically active 

 substance as of its left-handed isomeride. When an optically 

 active substance is prepared in the laboratory, it is there- 

 fore obtained as a mixture of two enantiomorphously re- 

 lated isomerides ; such a mixture is said to be compensated, 

 because the right-handedness of the one component is just 

 counterbalanced by the left-handedriess of the isomeric 

 constituent. These compensated substances are represented 

 by the third lactic acid and by the third tetrahydroquin- 

 aldine previously referred to, but not further discussed. 



Since one of the great problems with which chemistry 

 is grappling involves the synthetic preparation of naturally 

 occurring optically active substances, it is of the utmost 

 importance that the chemist should be in possession of 

 working methods for resolving these compensated mixtures 

 into their optically active components. All the kinds of 

 methods applicable to such resolutions necessarily involve 

 the introduction of enantiomorphism — either of method or 



NO. 1760, VOL. 68] 



of material. Three types of methods were introduced by 

 Pasteur, namely, (i) spontaneous resolution by crystallisa- 

 tion ; (2) resolution by combination with optically active 

 substances ; and (3) resolution by the action of living 

 organisms. 



The first method depends upon the fact that on crystal- 

 lising a compensated substance it sometimes deposits crystals 

 of the dextro- and of the laevo-isomeride side by side, and 

 of such size that they can be mechanically sorted. The 

 enantiomorphous factor determining the separation in this 

 kind of method is obviously the enantiomorphous intelli- 

 gence which has the power of discriminating between right- 

 and left-handedness. This sort of method is unfortunately 

 but rarely applicable, owing to the fact that two enantio- 

 morphously related substances usually crystallise together 

 in the form of a loose chemical compound. 



The second kind of Pasteur method is applicable to the 

 resolution of compensated acids and bases, and depends 

 upon the following considerations. On combining a com- 

 pensated basic substance, viz. a mixture of rf-B and Z-B 

 with an optically active acid — say with d-A — a mixture of 

 two salts, namely d-B, d-A and i-B, d-A, will be obtained. 

 These salts, however, are not enantiomorphously related, as 

 will be realised on substituting for illustrative purposes a hand 

 for the base and a glove for the acid ; the combination d-B, 

 d-A will then be represented by a right-hand in a right- 

 handed glove, whilst the combination Z-B, d-A will corre- 

 spond to a left hand in a right-handed glove. The struggles 

 of the left hand with the right-handed glove will not be 

 a factor in determining the behaviour of the appropriately 

 assorted right hand and right-handed glove. So, also, the 

 properties of the substance d-B, d-A — its solubility, melting 

 point, &c. — will be conditioned by an enantiomorphous re- 

 lationship of quite a different order from that determining 

 the corresponding properties of the salt Z-B, d-A ; the solu- 

 bilities, being determined by different factors, will naturally 

 also differ, and the two salts will therefore be separable 

 by crystallisation. The first resolution of a compensated 

 base was effected in 1885 by Ladenburg, and consisted in 

 resolving the synthetic alkaloid coniine into its optically 

 active components— one of which proved to be identical 

 with the alkaloid contained in the juice of the hemlock — 

 by crystallising it with dextrotartaric acid. Since this time 

 the methods of resolving compensated bases have been 

 materially improved by the application of optically active 

 acids derived from camphor for use in place of the dextro- 

 tartaric acid, and an experiment in illustration can now 

 be shown on the lecture table. 



On adding a solution of ammonium dextrobromocamphor- 

 sulphonate to a solution of compensated tetrahydro-/3- 

 naphthylamine hydrochloride, a white crystalline precipitate 

 of dextrotetrahydro-^-naphthylamine dextrobromocamphor- 

 sulphonate — the salt d-B, d-A — is thrown down, whilst the 

 Isevotetrahydro-^-naphthylamine remains in solution as 

 its hydrochloride. The resolution in this, and in many 

 other cases, can be very rapidly effected, and bv still further 

 applying the optically active sulphonic acids derived from 

 camphor a considerable extension of the original van 't Hoff- 

 Le Bel theory has become possible. These workers traced all 

 cases of optical activity to the presence of an asymmetric 

 carbon atom, and deduced from their work the conclusion 

 that the environment of the carbon atom in methane is a 

 tetrahedral one. It is true that all the optically active sub- 

 stances which have yet been obtained from natural sources 

 owe their optical activity to the presence of an asymmetric 

 carbon atom, but it is important to note that by applying 

 the second Pasteur method to the investigation of synthetic 

 materials, substances owing their optical activity to the 

 presence of asymmetric atoms of elements other than those 

 of carbon can be prepared. Thus, ammonium iodide has 

 the molecular composition NH^I, and, like methane, con- 

 tains in its molecule four hydrogen atoms which are re- 

 placeable by other atoms or groups of atoms ; on replacing 

 these hydrogen atoms by the four groups of atoms or radicles, 

 methyl, allyl, benzyl and phenyl, a substance is obtained 

 which is conveniently named methylallylbenzylphenyl- 

 ammonium iodide, and has the following constitution :— 



C3H/ I '\C,H, 

 I 



