ON THE STEREOCHEMISTRY OF NITROGEN, 179 
Werner hypothesis, which has predicted and accounted for all the cases of 
isomerism of this kind hitherto observed. The absence of isomerides in 
some cases in which they were expected—for instance, the oximes of 
aliphatic aldehydes and ketones—is not to be regarded as a serious objection, 
since one of the compounds may be so unstable as to be almost immediately 
transformed into the more stable isomeride, or again, in such cases as the 
azo compounds, either a suitable method for preparing the isomeride may 
not be known, or the compound may not be reactive enough to undergo 
transformation by any of the methods available. 
II. QuinquEVALENT NiTRoGEN ComMPouNDs. 
Attention was first drawn to the ammonium compounds with reference 
to the discussion whether valency was fixed or variable, and from 1816 
onwards a lively controversy waged between chemists who maintained 
that ammonium chloride was a molecular compound, and those who held 
the view that it was an atomic compound in which nitrogen was quin- 
quevalent. 
Experiments made by V. Meyer and Lecco! showed that the union of 
trimethylamine with ethyl iodide on the one hand, and of ethyl-dimethyl- 
amine with methyl iodide on the other, gave rise to the same product ; 
after an objection raised by Lossen? had been answered? this was 
regarded as strong evidence in favour of the atomic nature of these 
compounds. Had the results of these experiments been different an 
erroneous conclusion would have been arrived at. At present the doc- 
trine of variable valency is accepted by most chemists, and ammonium 
aN 
chloride is usually represented as H Voce ake with a quinquevalent nitro- 
H 
H 
gen atom attached to four atoms of hydrogen and one of chlorine. 
(i) The Formation of Substituted Ammonium Compounds, 
The rate of formation of substituted ammonium compounds from 
amines and alkyl halogen compounds varies to a very great extent, and is 
found to depend both on the alkyl groups in the amine and on that in the 
halogen compound, Much work has already been carried out on this sub- 
ject, and much more will have to be done before one can hope to under- 
stand the reactions. Thus Menschutkin‘ investigated the velocity of the 
reaction between a very large number of alkyl halogen compounds and 
amines ; no general conclusions can be drawn from this work, the amines 
being divided into three classes, according as the maximum rate of 
formation occurs (1) for the salt of the tertiary amine, (2) for the salt of 
the secondary amine, or (3) for the quaternary ammonium salt. 
Tn all cases iodides reacted about seven times more rapidly than 
bromides, and these about a hundred times faster than chlorides. 
It frequently happens that the same compound is formed quite rapidly 
in one way, and quite slowly or not at all in another way.° 
1 Ber., 1874, 7, 1747; 8, 233, 936; Annalen, 1876, 180, 170. 
2 Ber., 1875, 8, 49. 8 Ber., 1877, 10, 309. 
4 Zeit. phys. Chem., 1895, 17, 191. 
5 Of. also Proc. Chem. Soc., 1901, 17, 205. 
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