42.0 TRANSACTIONS OF SECTION B. 
place, the agent promoting the substitution being chlorine arising from the 
following series of reactions :— 
NClAc NHAc NHAc 
+H == oO Cee + HCl 
Cl 
As bromination has been shown to follow the same course, it is evident that no 
secure foundation now exists for the view, formerly widely held, that the 
reactivity of amines is intimately connected with the variable valency of 
nitrogen leading to initial substitution in the side chain. 
Even were this view, now discredited, still applicable to the amines, it could 
not be extended with the same certainty to the phenols. Hence, in explanation 
of the rigid adherence to the ortho-para- law observed among the mono- substitu- 
tion derivatives of these two groups of compounds, it is noteworthy that Thiele,” 
for the phenols, suggests that the reactivity may be due to these substances 
being stable enolic forms of ketodihydrobenzenes, and that Orton,** for the 
amines, conjectures that it may arise from the formation of dynamic isomerides 
of quinonoid structure : 
O NAc 
OH . NHAc oe 
(\ = | en ‘ee Se he ee 
\F wwe \F 
How far these suggestions may open up a new field of inquiry into the 
‘mechanism’ of substitution remains to be seen; it is at least interesting that 
their extension to the naphthalene series shows that not only does the reactivity 
of the naphthols and of a-naphthylamine recall that of phenol and aniline, but 
the orientation of their mono- substitution derivatives ** in almost every case is 
the same as that of one or other of the six naphthaquinones, the existence of 
which has been predicted by Willstatter.°° 
Symmetric and Asymmetric Syntheses. 
It must not be supposed that the ‘mechanism’ of substitution can be ex- 
plained by reference only to the examples of this type of reaction which have 
been mentioned, or that the summary attempted in the restricted field of the 
replacement of hydrogen by halogen is a complete picture of all the different 
views advanced to account for this chemical change. Rather, the effort has 
been made to indicate in broad outline the difficulties that beset any explora- 
tion of that debateable region which lies between the two sides of a chemical 
equation. But, as the wonderful story of carbon chemistry shows, the failure 
to comprehend the processes operative in substitution does not impede rapid 
progress in other directions. The study of the mobility of radicals, desmotropy 
being only one of many examples of this phenomenon, continues to present fresh 
problems, of which that raised by Thorpe ** in connection with the mobile 
hydrogen atom of glutaconic and aconitic acids may be mentioned, as it revives 
a question of old standing : Do free units of valency exist in carbon compounds? 
The syntheses of caffeine and certain alkaloids, of sugars and peptone-like poly- 
52 J. Thiele, Annalen, 1899, 306, 129. 
53 British Association Report, 1910, p. 96. 
4 Of. W. P. Wynne, art. ‘Naphthalene,’ Thorpe’s Dictionary of Applied 
Chemistry, second edition, vol. 3 (Longmans, 1912). 
55 R, Willstatter and J. Parnas, Ber., 1907, 40, 1406. 
5° N. Bland and J. F. Thorpe, Zrans. Chem. Soc., 1912, 101, 871, 1490. 
