SEPTEMBER 18, 1913]| 
NATURE Th 
which, if the views formed independently by Fischer,*° 
Werner,”’ and Pfeiffer?® may be accepted, are in- 
_ explicable unless a preliminary addition, effected as 
it is supposed by means of residual valencies, precedes 
this replacement of the eliminated radical by the sub- 
stituent. 
The Walden inversion may be illustrated by a brief 
statement of some of the facts discovered in connec- 
tion with the conversion of optically active chloro- 
succinic acid into malic acid 
R.CH(OH).CO,H—>R.CHCI.CO,H—> 
R.CH(OH).CO.H. 
Walden found that laevo-chlorosuccinic acid, obtained 
from dextro-malic acid, furnished either dextro- or 
laevo-malic acid, according to the reagent used to 
effect of the replacement of the Cl by the OH radical. 
Le ., ¢(+ Ag,O)—>/-malic acid 
Lchlorosuccinic acid t (4 KOH) malic acteh 
And as the corresponding inversion was found to 
occur with dextro-chlorosuccinic acid under similar 
conditions, a complete cycle of changes can be brought 
about.” That preservation of optical activity, and 
not racemisation, should accompany the replacement 
of a radical, attached to the asymmetric carbon atom, 
by another is a fact of much theoretical interest, as 
has already been indicated; that a change in the 
sign of rotation should occur when an exchange of 
the same radicals is achieved by one reagent and not 
by another is a mystery, that deepens rather than 
diminishes with each addition to the list of inversions, 
already long, in which it has been observed.*’ In all 
probability the discovery of the Walden inversion, as 
Prof. 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 benezene 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 “conjugated double linkings” is applied to 
the Kekulé formula, with the consequence that the 
three double linkings disappear owing to self- neutral- 
isation of the partial valencies, the benzene molecule 
thus containing six inactive double linkings;*? and 
Armstrong’s “centric ’’ formula, in which by its resi- 
dual valency ‘‘each individual carbon atom exercises 
an influence upon each and every other carbon 
atom.’"’** The dotted lines indicate the residual 
valencies. 
°6 E. Fischer, Annalen, ror1, ccclxxxi, 123. 
27 A. Werner, Ber., 1911, xliv, 873. 
28 P. Pfeiffer, Annalen, 1911, ccclxxxiii, 123. 
29 P. Walden, Ber., 1896, xxix, 133; 1897, xxx, 3145; 1899, xxxii, 1833, 
1855- 
80 Without the aid of a model it is not possible to show that the pro- 
duction of the dextyo- or devo acid may he accounted for by the hypothesis 
that an intermediate additive compound is formed, which undergoes scission 
in one or other of two ways. Diagrams of models will be found in Fischer's 
paper (doc. cit. cf. “‘ Annual Reports on the Progress of Chemistry” (Gurney 
and Jackson, 1911, viii, 67), and to illustrate Werner's hypothesis, which is 
more explicit than Fischer's, ina paper by W. F. Garner (Proc. Chem. Soc., 
1913, XXIx, 200) 
3t Pp. F. Frankland, ‘‘ The Walden Inversion,” Presidential Address to 
the Chemical Society (Trans. Chem. Soc., 1913, ciii, 713). 
#2 J. Theile, Annalen, 1899. cccvi, 126. 
43H. E. Armstrong, ‘I'rans. Chem, Soc., 1897, li, 264 (footnote), 
NO. 2290, VOL. 92] 
Kekule. Thiele. Armstrong, 
The discovery of cyclooctatetraene has brought a 
new interest into the discussion,** for the structural 
formula assigned to this hydrocarbon shows alternate 
single and double linkings as in Kekulé’s symbol, 
and the optical behaviour (refractivity) corresponds 
with that of benzene. 
cH—cH 
a \ 
CH CH 
I I 
CH CH 
CH=CH 
But its chemical properties are entirely different from 
those of benzene; it forms compounds not by substi- 
tution but by addition, and it has the reactivities 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 Kekulé’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 cyclo- 
octatetraene may perhaps suggest the inquiry: Do not 
the physical properties of the carbon compounds throw 
light on the questions that have been raised? AA 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, the 
anomalous results obtained by Briihl and by Sir 
William Perkin** in the investigation of the refrac- 
tivity and the magnetic rotation of certain unsaturated 
compounds, remained without explanation until Thiele 
in 1899, by his hypothesis of partial valency, accounted 
for the comparative inactivity of the central pair of 
carbon atoms in compounds of this type—compounds 
which are characterised by containing alternate single 
and double linkings in their formulz :— 
{GM CH.CH<CH. +. CHBr. CH: CH CHEE: 
This conception of Thiele’s has both focussed atten- 
tion on the distribution of valency within the mole- 
cule, contributing largely to the wide acceptance of 
theories of valency such as Werner’s, and given to 
the study of physical properties—especially those ‘* con- 
stitutive’’ properties of refraction, dispersion, and 
magentic 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 dis- 
tribution of valency, with results which in Auwers’s 
34 R. Willstatter and E. Waser, Ser., rorr, xliv, 3423. 
35 Cf J. W. Briibl, Ber., 1907, xl, 878; Sir W. H. Perkin, Trans. Chem. 
Soc., 1907, xci, 806, for references to earlier papers. 
