PRESIDENTIAL ADDRESS. 417 
the anomalous results obtained by Brihl and by Sir William Perkin *® in the 
investigation of the refractivity and the magnetic rotation of certain un- 
saturated compounds, remained without explanation until Thiele in 1899, 
by his hypothesis of partial valency, accounted for the comparative inactivity 
ot the central pair of carbon atoms in compounds of this type—compounds 
which are characterised by containing alternate single and double linkings 
in their formuie 
-CH:CH:CH:CH: + -:CHBr‘CH: CH: CHBr° 
. 
This conception of Thiele’s has both focussed attention on the distribution 
of valency within the molecule, contributing largely to the wide acceptance of 
theories of valency such as Werner’s, and given to the study of physical proper- 
ties—especially those ‘constitutive’ properties of refraction, dispersion, and 
magnetic rotation—an impetus which has by no means spent its force. Further, 
the occurrence of this anomaly, ‘ exaltation’ as it is called, is now relied on as 
evidence of the presence of this particular distribution of valency, with results 
which in Auwers’ hands have furnished important clues to the structural 
formule of terpenes and other compounds. 
As additive properties become constitutive, so the value of a knowledge 
ot the physical properties of a substance will tend to increase, but there is 
little ground for hope that the problem of the constitution of benzene will be 
solved from the physical side. The controversy which has arisen between 
Hantzsch and Auwers regarding the physical properties of cyclooctatetraene in 
relation to its chemical structure is a case in point; *° the absence of optical 
exaltation in this hydrocarbon is wholly unexpected, but, on the other hand, 
the type of compound is entirely new. With benzene also the distribution of 
valency within the molecule differs from that in any known compound; our 
knowledge of it, admittedly far from complete, has been gained from the 
chemical side, and is summarised in the various structural formule ; but the limita- 
tions of the physical method of attack can be traced from Thomsen’s endeavour 
to determine its structure from thermochemical data *’ to the more recent inven- 
tion of isorropesis. And, despite the evidence obtained from refractivities, we 
may not unreasonably demur to the suggestion that derivatives of benzene, which 
by their behaviour towards substituting agents show themselves to be wide 
apart in chemical properties, such as nitrobenzene and aniline on the one 
hand or chlorobenzene and phenol on the other, should respectively be classified 
together.** Undoubtedly, most useful information is obtained from a com- 
parison of the physical properties of two related substances, the exact con- 
stitution of one of which is uncertain, but that of the other known. ‘Therefore, 
bearing in mind the great development that has taken place recently in the 
correlation of physical properties with chemical constitution by methods based 
on refraction and absorption, every chemist will welcome the entry of Dr. 
Lowry into that field of research on the relation between magnetic rotation 
and structure, which for all time will be associated with Sir William Perkin’s 
name. 
Substitution in the Benzene Series. 
Turning now to a discussion of the problem of substitution in cyclic com- 
pounds, one important factor must not be overlooked; the even distribution of 
the residual affinity of the benzene molecule is disturbed by the introduction of 
a substituent. The study of substitution in benzene derivatives indicates that, 
as a consequence of this disturbance, a directing influence comes into play which, 
when the substituent is changed, may Vary in the effect it exercises on the course 
of substitution. 
°5 Cf. J. W. Brihl, Ber., 1907, 40, 878; Sir W. H. Perkin, Zrans. Chem. Soc., 
1907, 91, 806, for references to earlier papers. 
** A. Hantzsch, Ber., 1912, 45, 563; K. Auwers, ibid., 971. 
"7 Cf. H. E. Armstrong; Phil. Mag., 1887 [v.] 28, 73; J. W. Brihl, 
J. prakt. Chem., 1887 [ii.] 85, 181, 209. 
“Cf. J. W. Brithl, Zeit. physikal, Chem., 1894, 16, 220; Smiles, Relations 
between Chemical Constitution and Physical Properties (Longmans, 1910), p. 299. 
19138. EE 
