has shown that nearly perfectly inactive mannite solutions become strongly dextro-rotatory on the addition of borax. Again, according to Müntz and Aubin,1 inactive glucose becomes strongly dextro-rotatory on the addition of sodium sulphate, and still more so of borax; addition of sodium carbonate produces, on the contrary, left-rotation. Moreover, the activity of organic acids and also of alkaloids is in most cases increased by their conversion into salts, and sometimes very considerably. The reverse also happens, as, for example, in the case of chloride of laudanine, which Hesse2 reports as inactive, although the free base is lævo-rotatory. Papaverine exhibits the same property.

4. In the preceding cases we have spoken only of the synthesis of bodies the chemical structure of which is identical with that of the natural substances, but where structural differences exist there will obviously be dissimilarity of optical power. Thus inactive paraconine from butyric-aldehyde is not an imide base like the active natural conine (Schiff3), and accordingly differs from it in its other structural properties.

§ 16. Optical Properties of Derivatives of Active Substances.It has long been observed that when optically-active substances undergo chemical changes, some of their derivatives exhibit rotatory powers whilst others do not. Where the molecular constitution remains unaltered, as in the conversion of acids into salts, ethers, amides, &c., or in the combination of alkaloids with acids, the activity is usually retained. On the other hand, where there is actual chemical decomposition, active and inactive derivatives are obtained with an apparent absence of all rule. If, however, the hypothesis of asymmetrical carbon-atoms be called into aid, the apparent irregularity vanishes, and the conditions, as van't Hoff5 has shown, resolve themselves into the following

4

1. The rotatory powers of active substances are obliterated in such of their derivatives as are wanting in asymmetrical carbon

atoms.

1 Müntz and Aubin: Ann. Chim. Phys. [5] 10, 564.

2 Hesse: Liebig's Ann. 176, 198, 201.

3 Schiff: Liebig's Ann. 157, 352; 166, 94.

+ Exceptions exist in laudanine and papaverine, which, as mentioned in § 15, when free are optically-active, but in the form of chlorides are inactive (Hesse).

5 J. van't Hoff: see the two pamphlets mentioned in note 7, p. 24 of the present work; also Ber. d. deutsch. chem. Gesell. 1877, 1620.

As examples may be cited:

1. From Active Tartaric and Malic Acids is derived Inactive Succinic Acid.

occur.

In none of the above derivatives do any asymmetrical C-atoms See formulæ on a previous page (p. 27 and following pages). 2. Derivatives of active substances, when such derivatives contain asymmetrical carbon-atoms, usually exhibit rotatory power. As examples may be taken :—

1. From Active Amyl-alcohol (by oxi- is derived Active Valerianic Acid, &c.

On the other hand, cases occur in which such derivatives, the presence of asymmetrical carbon-atoms notwithstanding, exhibit

no rotatory power, as the subjoined (compare with the formulæ previously given, page 27 and following pages) :—

1. From Active Malic Acid (with is derived Inactive Monobromo-succinic Acid.

A conversion of active substances into inactive isomers frequently results from exposure to a higher temperature. Dextrotartaric acid, heated in presence of water to a temperature of 160° Cent., is converted chiefly into inactive, undecomposable tartaric acid; whereas, at a temperature of 175° Cent., a mixture of the latter with a predominance of para-tartaric acid is obtained. Conversely, at a temperature of 160° Cent. para-tartaric acid is converted into the other inactive tartaric acid (Jungfleisch).1 Again, dextrocamphoric acid mixed with a little water, and raised to a temperature of 170° to 180° Cent., passes into inactive camphoric acid (Jungfleisch); and active amyl-alcohol when heated in sealed tubes becomes inactive (Le Bel3). The conversion into the inactive state is often assisted by the presence of other substances. Thus fermentationamyl-alcohol loses its rotatory power when distilled under ordinary pressure in presence of caustic potash, caustic soda, or chloride of calcium (Balbiano 1). By merely heating in a sealed tube for a quarter of an hour at 250° Cent. with a few drops of concentrated sulphuric acid, active valerianic acid is rendered inactive (Erlenmeyer and Hell 5). Repeated agitation with one-twentieth of its volume of concentrated sulphuric acid in the cold, and subsequent distillation, converts active turpentine-oil into inactive terebene (Riban 6). The case of tartaric acid indicates the probability that such conversions of active into inactive substances are due to the simultaneous formation of right-rotating and left-rotating modifications, resulting in optical neutrality.

In turpentine-oil a gradual conversion of the right-rotating into the left-rotating form has been noticed. If the oil is placed

1 Jungfleisch: Ber. d. deutsch. chem. Gesell. 1872, 985; 1873, 33.

2 Jungfleisch: Jahresb. für Chem. 1873, 631.

3 Le Bel: Jahresb. für Chem. 1876, 347.

4 Balbiano: Jahresb. für Chem. 1876, 348.

5 Erlenmeyer and Hell: Liebig's Ann. 160, 302.

6 Riban: Jahresb. für Chem. 1873, 370.

in a retort provided with an upright condenser, and, whilst the retort is kept filled with carbonic acid at the ordinary pressure of the atmosphere, the contents are raised to boiling, whereby a temperature of 160° to 162° Cent. is attained, no change of rotatory power can be detected at the end of sixty hours. But such change occurs when the temperature passes beyond 250° Cent. in a sealed tube. A sample of English right-rotating turpentine-oil, which gave an original deviation of a;= + 18.6° in a layer of 100 millimetres, showed the following angles of rotation at higher temperatures:

At a certain stage an inactive mixture of right-rotating and left-rotating molecules must therefore have been present (Berthelot1). Polymerization occurs simultaneously with the above changes. This can readily be produced without any change of temperature, by treating with antimony trichloride, whereby, for example, left-rotating terebenthene is converted into right-rotating tetraterebenthene (Riban 2).

In certain cases, two isomeric derivatives possessing opposite rotatory powers but of unequal intensity are simultaneously formed from an active substance, the immediate product being therefore also active. By heating mannite to 150° Cent., or treating it with muriatic acid, we obtain a right-rotating mixture, which on evaporation separates out the crystallizable lævo-mannitan, leaving amorphous dextro-mannitan in the mother liquor (Bouchardat 3). Again, cane-sugar is transformed by the action of ferments or dilute acids into left-rotating invert-sugar, which can be split into rightrotating glucose (dextrose) and left-rotating glucose (lævulose). On the other hand, cane-sugar exposed with one-twentieth part water, in a sealed tube, for the space of two or three minutes, to a temperature of 160° Cent., gives an inactive glucose, which appears to be indivisible (Mitscherlich,1 Müntz and Aubin 5).

1 Berthelot: Ann. Chim. Phys. [3], 39, 10.

2 Riban: Bull. Soc. Chim. [2], 22, 253. Jahresb. für Chem. 1874, 451.

3 Bouchardat: Jahresb. für Chem. 1875, 792.

4 Mitscherlich: Lehrbuch der Chem. 4 Aufl. I, 337.

5 Müntz and Aubin: Ann. Chim. Phys. [5], 10, 564.

The direction of rotation in derivatives, in relation to that in the parent substance, follows no fixed rule. Most derivatives exhibit their rotatory power in the same direction as the parent substance, particularly when no real alteration of molecular constitution has taken place. This is seen in the conversion of active acids and alkaloids into their salts. Still there are exceptions here, as we find that malic acid, which is left-rotating when free, is right-rotating in its neutral salts, especially in the double salt of antimony and ammonium, from which, after precipitation of the antimony with sulphuretted hydrogen, a left-rotating solution of acid malate of ammonia is obtained (Pasteur1). Again, right-rotating para-lactic acid has left-rotating zinc and calcium salts (Wislicenus2). But even in more profound reactions the original direction is generally maintained. Thus, dextro-tartaric acid gives dextro-malic acid; from dextro-camphor we get dextro-camphoric acid; from lævo-camphor, lævo-camphoric acid; dextro- and lævo-borneols give respectively dextro- and lævo-camphors; and left-rotating amygdalin gives leftrotating amygdalic and mandelic acids (Bouchardat3). Nevertheless, some derivatives do exhibit rotatory power in a direction contrary to that of the parent substance, as in the following instances:Dextro-camphoric acid gives left-rotating camphoric anhydride (Montgolfier1); dextro-para-lactic acid gives left-rotating ether-anhydrides (Wislicenus); the active compounds derived from lævo-amylalcohol (amyl chloride, amyl iodide, amyl cyanide, diamyl, ethyl-amyl, amylamine, amyl valerate, valerianic aldehyde, valerianic acid, and capronic acid) are all right-rotating; left-rotating santonic acid, with nascent hydrogen passes into right-rotating hydrosantonic acid (Cannizarro3); left-rotating terebenthene hydrochlorate, heated with stearate of soda, gives left-rotating terecamphene. By saturating the latter with hydrochloric acid we get right-rotating camphene hydrochlorate, from which, by heating with water, left-rotating camphene, but with rotatory power much feebler than at first, can be recovered (Riban"). Again, mannite, with weak lævo-rotatory power, gives both right- and left-rotating derivatives. Of these nitro-mannite, mannite hexacetate, amorphous mannitan, nitro1 Pasteur: Ann. Chim. Phys. [3] 31, 67.