416 TRANSACTIONS OF SECTION B. 
inversion, as Professor Frankland has said, ‘may mark an epoch in our views 
with regard to the mechanism of the process of substitution in general.’** 
The Structure of the Benzene Molecule. 
The abandonment of the theory of the fixed valency unit in favour of the 
view that the carbon atom has both principal and residual valencies has raised 
afresh that perennial topic of controversy—the structure of the benzene 
molecule. Probably few will contest the statement that for practical purposes 
only three formule have emerged from the long discussion of the problem, viz. 
Kekulé’s oscillation formula with fixed valency units, for which much physical 
evidence has been pleaded: Thiele’s formula, in which his theory of ‘con- 
jugated double linkings’ is applied to the Kekulé formula, with the consequence 
that the three double linkings disappear owing to self-neutralisation 
of the partial valencies, the benzene molecule thus containing six inactive 
double Jinkings ; ** and Armstrong’s ‘ centric’ formula, in which by its residual 
valency ‘each individual carbon atom exercises an influence upon each and 
every other carbon atom.’ ** The dotted lines indicate the residual valencies. 
Kekule. Thiele. Armstrong. 
The discovery of cyclooctatetraene has brought a new interest into the 
discussion,** for the structural formula assigned to this hydrocarbon shows alter- 
nate single and double linkings as in Kekulé’s symbol, and the optical behaviour 
(refractivity) corresponds with that of benzene. 
CH = CH 
2x 
CH CH 
iH il 
cH CH 
se oe 
CH = CH 
But its chemical properties are entirely different from those of benzene; it 
forms compounds not by substitution but by addition, and it has the reactivi- 
ties of a highly unsaturated compound. If these experimental results be 
accepted, then—as Willstatter shows—the peculiar properties of benzene are 
not to be explained by Kekule’s or Thiele’s formula, and the verdict is given in 
favour of the ‘centric’ symbol—that earliest embodiment of the conception of 
residual valency, which Armstrong later turned to such good account in the 
quinonoid theory of colour identified with his name. 
The reference to the optical behaviour of cyclooctatetraene may perhaps sug- 
gest the inquiry: Do not the physical properties of the carbon compounds throw 
light on the questions that have been raised? A little consideration will show 
that, on the contrary, the answer must be: It is only by chemical evidence 
that physical data can be interpreted or corroborated, and in the absence of 
such evidence the ‘ additive’ results which accrue from physical observations 
have no bearing on questions involving the determination of structure or the 
structural transformations which accompany a chemical change. For example, 
1911, 8, 67), and to illustrate Werner’s hypothesis, which is more explicit than 
Fischer’s, in a paper by W. E. Garner (Proc. Chem. Soc., 1913, 29, 200). 
2. Pp. F, Frankland, ‘The Walden Inversion,’ Presidential Address: to the 
Chemical Society (Z’rans. Chem. Soc., 1913, 108, 713). 
*4 J. Theile, Annalen, 1899, 306, 126. 
°° H. E. Armstrong, 7rans. Chem. Soc., 1897, 51, 264 (footnote). 
“4 R. Willstatter and E. Waser, Ber., 1911, 44, 3423. 
