CAUSES OF THE POLYAD CHARACTER OF RADICLES.

69

study of polyad compounds has been prosecuted with great activity and success within the last few years, and the circumstances under which an organic radicle exhibits a polyad type have consequently attracted much attention. The foregoing observations will have prepared the reader to seize the bearing of the principal points which have been ascertained regarding this interesting subject.

A large number of the hydrocarbon radicles which have at present been studied are monads, others are dyads, and a few are triads. No isolated radicle containing an uneven number of atoms of hydrogen is known; though isolated radicles with an even number of atoms of hydrogen are of frequent occurrence.

5

I. All radicles hitherto admitted of the form H2n+1, such as methyl (EH), ethyl (Є,H), or amyl (¤ ̧H11), are monads; as is seen in chloride of ethyl (H.)Cl, and iodide of amyl (Є¡H11)I; and, in isolated ethyl, ethylide of ethyl (E,H,) (E ̧H ̧).

2n

2

2. Radicles of the form H, such as ethylene (E,H,), tritylene (H), octylene (H1), are dibasic, as in Dutch liquid (H)"Cl. Such radicles may exist in the free state.

n

3. A radicle of the form H-1 is sometimes triad, at others monad; (H), for example, is triad in glycerin, (¤ ̧H;)''') ,, and monad in allylic alcohol, H3

chloride (H.)'Cl.

allylic alcohol, Hie, and in

allyl

A radicle may as a rule be made to change its equivalency, or basic power, by the removal of hydrogen. As an example of this we may take marsh gas, which is usually regarded as a neutral body of the methyl series:* by acting upon this gas with chlorine, and 4 atoms of hydrogen may be removed in succession, and a corresponding number of atoms of chlorine substituted :

I, 3,

The third term in the series is not known with chlorine, but it has been obtained with iodine by Butlerow.

By a saturated or neutral body is understood a compound which will not unite directly with an elementary monad, or a monad radicle: whenever it forms compounds, such compounds are produced by substitution, methyl chloride EHCl, for example, being obtained from marsh gas by substituting I atom of chlorine for 1 of hydrogen.

70

INFLUENCE OF ARRANGEMENT OF ATOMS IN

Now from these substitution-products of marsh gas, compounds of four different equivalencies have been obtained, corresponding to the following different radicles, viz. :

The compound (EH)'Cl is manifestly a chloride of a monad radicle. If methylene diniodide be treated with silver acetate, methyl-glycol diacetate, the ether of a dibasic alcohol, is procured (1214)—

and if chloroform be treated with sodium-alcohol a tribasic formic ether is obtained :

Whilst, finally, if carbonic tetrachloride be acted upon by aniline, a base is obtained containing 19H17N3 :

:

The equivalency, or as it is sometimes called, the basicity, of a radicle, is therefore increased by unity for each atom of hydrogen which is withdrawn, so that a monobasic radicle becomes dibasic by the loss of an atom of hydrogen; a dibasic radicle becomes tribasic by the loss of another atom of hydrogen, and by the further removal of an atom of hydrogen the tribasic radicle becomes tetrabasic.

For the further prosecution of these speculations the reader may consult Kekulé (Org. Chemie, i. pp. 158-183).

it

(1069) Influence of Arrangement of Atoms in a Compound on its Properties. From what has been already stated it is obvious that a body does not necessarily belong to a given type merely because may contain one of the characteristic elements which contribute to form the typical compound. It would be wrong, for example, to infer that a compound was formed on the hydrochloric acid type simply in consequence of the presence of chlorine among its

A COMPOUND ON ITS PROPERTIES.

71

components; since the properties of a body are dependent, not only upon the nature and number of atoms of its component elcments, but also in a very important manner upon the molecular arrangement of those components. Indeed, in many cases the arrangement of the atoms in the compound has a greater influence upon its properties than even the nature of the elements of which it is composed. It may in fact be said that the whole theory of types is founded upon the dissimilar behaviour of atoms otherwise identical save in their relative position in the molecule of the compound. For example, the chlorine in potassic chlorate does not perform the same part that it does in potassic chloride; in the latter instance it is susceptible of displacement when mixed with silver nitrate; the soluble potassic nitrate and insoluble silver chloride being formed when solutions of the two salts are mixed

AgNO3+ KCl=KNO ̧+AgCl ;

2

whereas, when potassic chlorate and nitrate of silver are mixed, no precipitation of chloride of silver takes place. So it is in organic substances: for example, when isatin, H ̧Ñ→2 (a com pound derived from indigo) is distilled with caustic potash, a powerful volatile base termed aniline (H,,H,N) is obtained. Now derivatives of isatin may be formed in which one, two, and three atoms of chlorine may be successively substituted in each molecule of isatin for an equal number of atoms of hydrogen; and when these chlorinated compounds are distilled with caustic potash, they yield substances corresponding in composition to aniline, but in which a certain number of atoms of hydrogen in the group (H) are displaced by a corresponding number of atoms of chlorine, thus :

(HCI)NO,+4 KHO=(E,H,C1)H2N+2 K‚¤¤ ̧+H2;

(€,H,Cl2)NO,+4 KHO=(¤ ̧H ̧C12)H ̧Ñ+2 K ̧¤Ð ̧+H2.

By the action of chlorine upon aniline, another compound has been obtained, termed trichloraniline (EH,Cl,,H,N), in which three atoms of chlorine have taken the place of three atoms of hydrogen in a certain group in the molecule of aniline.

It is particularly to be remarked that the basic properties of the aniline are preserved in these compounds, though with gradually

72

PRODUCTION OF CHEMICAL METAMORPHOSES.

decreasing distinctness; and it is not until the third atom of chlorine has been introduced, that the basic power finally disappears; aniline, for example, is powerfully basic; chloraniline is distinctly basic; dichloraniline, feebly so; and trichloraniline is a neutral body.

Not one of these bodies when in solution, combined with nitric or sulphuric acid for instance, gives any precipitate of chloride when mixed with nitrate of silver; yet a single atom of chlorine, in the form of hydrochloric acid, will neutralize the basic power of an atom of any of these compounds. Thus, if hydrochloric acid be combined with aniline so as to form hydrochlorate of this base (ЄH,N,HCl), the solution will have all the characters of a neutral salt, and will occasion an immediate precipitate when its solution is mixed with one of nitrate of silver :

€2H,N,HCl + AgNO1=AgCl+¤ ̧H,N,HNO.

6

These differences may be accounted for upon the supposition that chlorine, when in combination, may discharge different functions; in the different chlorinated anilines, for example, it is evident that the chlorine has entered more intimately into the composition of the body than in the case just cited, since part of the hydrogen in the molecule of the base has been actually displaced by chlorine in the first cases; whereas, in the last, the chlorine is external to the base. The full importance of such processes of substitution will be made manifest as the reader proceeds; and in order to facilitate the acquirement of distinct notions upon this point we pass next to the consideration of a few of the principal modes in which the chemist effects the metamorphoses of the compounds which come before him.

§ III. PRODUCTION OF CHEMICAL METAMORPHOSES.

(1070) Three principal methods are employed in examining the chemical composition of organic bodies, and in tracing their relations to other compounds. These three methods are-1. That of oxidation. 2. That of reduction. 3. That of substitution, including double decomposition.

1. Oxidation. It will be unnecessary to consider minutely the effects of rapid direct oxidation or combustion, as the general result in all cases where a sufficient amount of air is supplied is to convert the carbon into carbonic anhydride, and the hydrogen into water, whilst the nitrogen is commonly liberated in the free state.

PROCESSES OF OXIDATION.

73

It is the study of the process of gradual oxidation which affords results so instructive to the chemist. When an organic body is placed in contact with a powerful oxidizing agent, such as nitric acid, chromic acid, or a mixture of sulphuric acid and black oxide of manganese, the effects which are produced will vary in different

cases.

1. The oxygen may simply remove hydrogen; as when alcohol is submitted to distillation with chromic acid. In this case water is produced, and a new body, aldehyd, containing all the carbon of the alcohol, is produced:

2. The newly-formed substance, by absorbing oxygen, may give rise to a body in which the hydrogen removed from the alcohol is represented by an equivalent amount of oxygen, as when aldehyd passes into acetic acid, 2 €2H+, becoming 2 €‚H ̧Ð ̧.

2

2

Or, 3. The complex molecule may be broken up more or less completely, in which case both the carbon and the hydrogen are partially removed in the form of carbonic anhydride and water, whilst an organic body of less complex composition is left; as when stearic acid is treated with nitric acid in this case the products vary with the strength of the acid employed, and with the time during which they are left to act upon each other; so that the following compounds, each successively simpler in composition, might be obtained from stearic acid (H3602) :—

But it is not only to the processes of oxidation effected in the laboratory that it is necessary to attend. Oxidizing actions are in constant operation unperceived on every side of us. The gradual decay of organic compounds is owing to their spontaneous slow oxidation; for decay is, in reality, only a slow process of combustion: a fact which Liebig indicated by substituting the term eremacausis for that of decay (from peμoç, gentle, Kavoiç, combustion). The decay of wood is a familiar instance of this kind; the hydrogen of the wood becomes more rapidly oxidized than its carbon, and the result is the formation of a brown powder termed ulmin or humus, in which the carbon preponderates over the hydrogen and the oxygen, to a much greater