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The active terpenes, C10H16 cannot be formulated as No. I. of the annexed formulæ, since it contains no asymmetrical C-atom, but only as either II. or III.

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Lastly, the asymmetrical carbon-atoms may be found in a lateral series, and then, as before, we get compounds, some active, some inactive, as the following:

Active.

Mandelic Acid

(C& H2) - *C H (O H) − (CO.0 H).

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(C6 H5) *CH Br

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[*CH Br

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Cinnamic Acid dibromide Diphenyl-succinic Acid (CH) — *CH (CO.OH) — [*CH (CO. OH) - C, H2]. By comparisons of this kind, which might easily be extended, we find that up to the present no substance can be indicated with certainty as disproving van't Hoff's theory or the following statements based thereupon :

1. That optically active substances invariably contain one or more asymmetrical carbon-atoms.

2. That substances containing no asymmetrical carbon-atoms exhibit no optical activity.

On the other hand, as van't Hoff has pointed out, and as may be seen from the above examples, the converse is not necessarily true, that "bodies containing asymmetrical carbon-atoms are always optically active." There are, in fact, numerous substances which contain asymmetrical carbon-atoms and yet exhibit no optical activity, and further research is needed to decide whether or not this inactivity can be referred to the causes before specified. But, even should further inquiry prove that asymmetrical carbon-atoms are not the sole condition but merely one of the conditions of optical activity, the foregoing statements, unless disproved by fresh discoveries, remain of great importance to chemistry, as they not only afford some sort of control over active substances, but may also yield definite indications as to the proper structural-formulæ.

§ 15. Artificial Production of Active Substances.-The carboncompounds in which optical activity has hitherto been observed, are all of them found in vegetable or animal organisms or as derivatives from these by simple decomposition. Many of these substances can be prepared artificially, but even when all the chemical attributes, 1 Presumably this should stand (C6H5) - *CH Br [*CH Br-CO.OH], the formula in the original being that for stilbene dibromide.-[D.C.R.]

2 The activity observed by Berthelot (Comptes Rend. 63, 818; 85, 1191) in styrol and metastyrol, C, H,. CH=CH2, obtained from liquid storax, which substances contain no asymmetrical C-atoms, is referred by van't Hoff (Ber. d. deutsch. chem. Gesell. 1876, 5) to the presence of another substance, probably corresponding to the formula C10 H18 O. In like manner, the supposed optical activity of iodide of trimethylethylstibin is attributed to chemical impurity. Le Bel (Buli. Soc. Chim. 27, 444).

and, consequently, the chemical constitution of such compounds, agree with those of the natural substances, a difference is nevertheless found to exist in their optical properties. Direct synthesis of substances from inactive components has hitherto resulted in the production of inactive modifications only.

As already indicated, this inactivity of artificial substances may be apparent (i.e. latent) only and dependent on the following causes, which at the same time indicate the means to its possible removal :

1. In synthesis, it is probable that an equal number of dextro-rotatory and lævo-rotatory molecules may always be formed, which mix or combine together and so produce optical neutrality. As an instance of the kind, Jungfleisch1 has shown that by converting ethylene-through the intermediate products ethylene bromide, ethylene cyanide, succinic acid, and dibromo-succinic acid-into tartaric acid, the inactive form is obtained, which by crystallization of its sodium-ammonium salt may be separated into dextro-tartaric and lævo-tartaric acids. This, at present, is the only instance that can be alleged of an artificial active substance; and even here it must be observed that the direct result of the synthesis, the para-tartaric acid, exhibits no optical power.

As the physical and chemical properties of such opposite-rotating modifications may differ but little, they cannot in general be separated without great difficulty, and the more so, when they form not merely mechanical mixtures but true chemical combinations, as is the case with para-tartaric acid.

Indeed the only substance as yet, whose inactivity depends on neutralization, which has been separated into right- and left-rotating modifications is para-tartaric acid. The separation can be effected by one of the following methods :

a. By crystallization of the sodium-ammonium salt, and separation by selection of the crystals with dextro-hemihedric from those with lavo-hemihedric planes (Pasteur3). This method has been somewhat simplified by Gernez, who found that by bringing into contact with a supersaturated solution of para-tartrate of sodium and

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1 Jungfleisch: Bull. Soc. Chim. [2] 19, 194. Comptes Rend. 76, 286.

2 Pasteur (Comptes Rend. 81, 128) rejects the instance unconditionally; he maintains that up to the present time no active substance has been derived from inactive substances, and that, therefore, optical activity affords a definite distinguishing characteristic between natural and artificial substances.

3 Pasteur: Ann. Chim. Phys. [3] 24, 442; 28, 56; 38, 437.

4 Gernez: Liebig's Ann. 143, 376.

ammonium a crystalline fragment of the dextro-tartrate, the latter alone crystallizes out, the lævo-tartrate remaining in solution, and vice versa.

b. By causing the para-tartaric acid to combine with some other active substance, whereby two modifications with unequal solubilities are produced. Thus if cinchonicine (dextro-rotatory) be dissolved in para-tartaric acid, lævo-tartrate of cinchonicine separates out first on evaporation. On the other hand, from a solution of quinicine (dextro-rotatory) in para-tartaric acid, crystals of the dextro-tartrate of quinicine are first separated (Pasteur).1

c. By mixing a solution of para-tartrate of ammonia with ferment (yeast-extract), the dextro-tartrate is eliminated by fermentation, while the lavo-tartrate remains unchanged (Pasteur).2

Mixtures of right- and left-rotating molecules are probably produced in the synthesis of all substances containing asymmetrical carbon, the molecules of which are not made up of two similarly constituted parts. This is probably the case with the malic acid obtained from monobromo-succinic acid, CO. OH-CH-CH.O H -CO. OH, with ethylidene-lactic acid from propionic acid, CH, ̧ -CH.OH-CO. OH, with mandelic acid prepared by treating benzoic aldehyde with prussic and muriatic acids, C, H.-CH. OH -CO. OH, &c. These products are all inactive, whereas the malic acid of plants, the ethylidene-lactic acid of muscle juice, and mandelic acid obtained by decomposition of active amygdalin with muriatic acid, despite the similarity of chemical constitution, all exhibit rotatory power. No attempts to dissociate any of these inactive preparations have yet been made.

2. The molecule produced by synthesis may be inactive by virtue of internal compensation-that is, it may be made up of equal halves with opposite rotatory powers. In such cases it may reasonably be expected that optical inactivity will disappear when the symmetry of the molecule is disturbed, as for instance, in the case of inactive indivisible tartaric acid, by converting it into benzotartrate of ethyl or simply into an acid tartrate, thus:—

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In these derivatives the halves being dissimilarly constituted, there ought to be rotatory power. Experiments in this direction have not yet been made, and it is quite possible that even such derivatives may prove inactive. This might easily be the case if the substitution took place in the right-rotating and left-rotating groups respectively of the two halves of the molecule, the result being an optically neutral mixture.

Substances containing several asymmetrical C-atoms are, according to van't Hoff's view, capable of forming by partial internal compensation certain isomers, with very feeble rotatory powers. Such a substance is mannite, CH, . OH−(*CH . O H),—C H2 . O H, which exhibits an extremely slight lævo-rotation. Converted into mannite dichlorhydrin, mannite hexacetate, mannite hexanitrate or into mannitan we obtain strongly dextro-rotatory substances. By conversion, for example, into hexanitrate (nitro-mannite), C H2. O. N O2 - (*CH. ONO2)4-CH2.O.NO2, the symmetry of the chemical formula is not destroyed, but some alteration apparently takes place in the four asymmetrical groups, whereby they, or at any rate three of them, assume the right-rotating position.

In this way Müntz and Aubin3 converted an inactive substance, viz., the glucose obtained by exposing cane-sugar with a little water to a temperature of 160° Cent., into an active substance, by transforming it first into mannite (with sodium-amalgam and water), and then into nitro-mannite.

So, likewise, inactive dulcite exhibits rotatory powers when converted into the diacetate, or into dulcitan diacetate (Bouchardat).

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3. Some of the resulting products do possess rotatory power, but only of a very feeble kind; thus, for example, in the case of mannite, a layer 3 to 4 metres in depth is requisite for its determination. In such cases aid may be furnished by an observation of Biot, who found that the rotation of tartaric acid was considerably increased by the addition of boric acid. Similar results have been observed in other substances. Thus, Vignon1

1 Loir: Jahresb. für Chem. 1861, 729. Tichanowitsch: Jahresb. 1864, 582. Grange: Jahresb. 1869, 752. Schützenberger: Liebig's Ann. 160, 94 (1871). Krecke: Arch. Néerland, vii. (1872). Vignon: Jahresb. 1874, 884. Bouchardat: Jahresb. 1875, 790, and Comptes Rend. 84, 34 (1877). Müntz and Aubin: Jahresb. 1876, 149; Ann. Chim. Phys. [5] 10, 553 (1877).

2 Müntz and Aubin: Ann. Chim. Phys. [5] 10, 553.

3 Bouchardat: Ann. Chim. Phys. [4] 27, 68, 145.

4 Vignon: Ann. Chim. Phys. [5] 2, 433.

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