AvuGUST 20, 1914] 
NATURE 
647 
regarded as a derivative of methane in which the | 
methane carbon atom was attached to four different 
univalent atoms or groups of atoms; a methane carbon 
atom so associated is termed an asymmetric carbon 
atom. It is of interest to note that the van ’t Hoff- 
Le Bel deduction resulted from the discussion of the 
behaviour of organic substances of some molecular 
complexity; the optically active substances then 
known were mostly the products of animal or vege- 
table life, and among them none occurs which contains 
less than three carbon atoms in the molecule. Lactic 
acid, CH,.CH(OH).CO.OH, is practically the most 
simple optically active substance of natural occurrence ; 
it contains twelve atoms in the molecule, and it has 
only recently been found possible to associate optical 
activity with a much more simply constituted sub- 
stance, namely, chloriodomethanesulphonic — acid, 
CHCI.SO,H, the molecule of which contains less 
than 5 per cent. of carbon and only nine atoms, four 
more than the minimum number, five, which theo- 
retically can give rise to optical activity.? 
The working out of the practical consequences of 
the doctrine of the tetrahedral configuration of the 
methane carbon atom by von Baeyer, Emil Fischer, 
and Wislicenus is now a matter of history; the ac- 
quisition of masses of experimental data, broad in 
principle and minute in detail, placed the van ’t Hoff- 
Le Bel hypothesis beyond dispute. The rapid growth 
of organic chemistry as a classified subject contrasted 
strongly with that of inorganic chemistry, in which 
the collection of a great varietv of detailed knowledge 
incapable of far-reaching logical correlation formed 
the most striking feature; in fact, the extension of 
the conclusion, proven in the case of carbon com- 
pounds, that the Frankland-Kekulé constitutional 
formulz must be translated into terms of three- 
dimensional space, to compounds of elements other 
than carbon, did not immediately follow the applica- 
tion of the theory to this element. Twenty years ago. 
indeed, the idea prevailed that carbon compounds 
differed radically from those of other elements, and 
we were not prepared to transfer theoretical con- 
clusions from the organic to the inorganic side of our 
subject. In 1891, however, Le Bel stated that he had 
found optical activity associated wth asymmetry of a 
quinquevalent nitrogen atom; although the experi- 
mental work upon which this conclusion was founded 
is now known to be incorrect,” the conception thus 
put forward was important, as suggesting that the 
notion of space-configuration could not be restricted 
logically to methane derivatives. When it was proved 
in 1899 that benzylphenylallylmethylammonium iodide 
could exist in a right- and left-handed configuration, 
it became necessary to admit that the spacial arrange- 
ment of the parts of a chemical molecule, previously 
restricted to methane derivatives, must be extended 
. to ammonium salts.* 
The demonstration that optical activity, or enantio- 
morphism, of molecular configuration is associated 
not only with the presence of an asymmetric quadri- 
valent carbon atom, but also with that of a nitrogen 
atom attached to five different radicles, was the result 
of an improvement of technique in connection with the 
study of optical activity; previously the resolution 
into optically active components of a_ potentially 
optically active basic substance had been attempted 
with the aid of naturally occurring optically active 
weak acids of the general type of d-tartaric acid. 
The application of the strong d- and I-bromocamphor- 
sulphonic acids and the d- and J[-camphorsulphonic 
acids to such purposes rendered possible the isolation 
1 Pope and Read, Trans. Chem. Soc,, 1914, 105, 811. 
2 Jbid., 1912, 101, 519. 
3 Pope and Peachey, Trans. Chem. Soc., 1899, 75, 1127- 
NO. 2338, VOL. 93| 
of the optically active substances containing no 
asymmetric atom other than one of quinquevalent 
nitrogen. The resolution of asymmetric quaternary 
ammonium salts of the kind indicated was rapidly 
followed by the preparation of optically active sub- 
stances in which the enantiomorphism is associated 
with the presence of an asymmetric sulphur, selenium, 
tin, phosphorus, or silicon atom; compounds of the 
following constitutions were thus obtained in optically 
active modifications :— 
GiHer eG, A= CH. ACI, CH.) “GHe 
eS. rad \ vA Die 
N ; S Se i 
‘\ Vi 
GA GH. Gk \uCH, «CO; OH. Cl. .cOnmemE 
(Cols | CH, C,H; GH; 
ere Ze 
Sn 2 P 
oS tN 
C,H, I O CH; 
@oHe (Goi 
| 
SO,FL C.He. GH, 45h; O;./Si CH... Coll, (SOLED 
| | 
C3H; C3H, 
In all this work, and amongst all the varied classes 
of optically active compounds prepared, it was in». 
every instance possible to indicate one ~° particular 
quadrivalent or quinquevalent atom in the molecule 
which is separately attached to four or five different 
atoms or radicles; the enantiomorphism of molecular 
configuration may be detected, in fact, by the observa- 
tion that such an asymmetric atom is present. It 
must, however, be insisted that the observed optical 
activity is the result of the enantiomorphism of the 
molecular configuration; the asymmetry of a par- 
ticular atom is not to be regarded as the cause of 
the optical activity, but merely as a convenient geo- 
metrical sign of molecular enantiomorphism. In 1874 
van ’t Hoff realised that molecular enantiomorphism 
and optical activity might conceivably exist without 
the presence of an asymmetric carbon atom, and 
suggested that compounds of the type 
aS EE 
1Cee ac 
7H Na 
should be of this kind. Previously this particular 
case had escaped realisation experimentally, but an 
example fulfilling similar conditions was described in 
1909; in this year the d- and lisomerides of 1-methyl- 
cyclohexylidene-4-acetic acid, 
« Q-. Jel 
CHy | CHa CHa 3 
Hi? + SCHuy CH ee a NOOl OE 
were obtained.* The consideration of the constitution 
of these substances shows no carbon atom which is 
attached to four different groups, but a study of the 
solid model representing the molecular configuration 
built up in accordance with the van ’t Hoff-Wislicenus 
conclusions reveals the enantiomorphism. 
It is of Some importance to note that the configura- 
tions assigned to such optically active substances as 
have been mentioned above, on the basis of the ex- 
perimental evidence, are of as symmetrical a character 
as the conditions permit; the Kekulé formula for 
methane, CH,, in which all five atoms lie in the same 
plane, is not of so highly symmetrical a character 
as the van ’t-Hoff-Le Bel configuration in which the 
four hydrogen atoms are situate at the apices of a 
H 
4 Perkin, Pope, and Wallach, Trans. Chem. Soc., 1909, 95, 1789; Perkin 
| and Pope, Trans. Chem. Soc., 1911, 99, 1510. 
