200 REPORT—1904. 
9. The Ultimate Product of Isomeric Change is a Mixtwre.—It is 
necessary to emphasise the fact that except when isomeric change is 
complete the ultimate product is not a definite compound or a new 
isomeride but merely a mixture. Thus if the ultimate product of the 
isomeric change of glucose were a definite third isomeride, as has been 
suggested by Lippmann,! by Tanret? and more recently by Simon,? it 
would be impossible by mere crystallisation to reconvert it into the labile 
a-glucose. So also if ammonium thiocyanate and thiourea were converted 
completely into the compound 3AmCNS,CS(NH,),, postulated by Reynolds 
and Werner, it would be impossible by mere cooling to crystallise from 
the fused mass anything but the compound in question. 
10. The Classification of Dynamic Isomerides.—Following Butlerow’s 
example dynamic isomerides have usually been divided into two classes 
according as isomeric change takes place ‘spontaneously’ or only under 
special conditions. The ‘spontaneous’ changes have often been called 
‘tautomeric,’ whilst the other group have been distinguished as ‘ ordinary ’ 
isomeric changes. Such a distinction may be convenient but cannot be 
defended on any other ground, for the spontaneity of the change is not an 
inherent property of the substance but depends on the presence or absence 
of a catalytic agent in the dirt that normally accompanies it. It must 
also be remembered that many isomeric changes which are not spontaneous 
at ordinary temperatures appear to become so when the temperature is 
raised, and the classification in question thus unconsciously involves an 
arbitrary temperature limit. But although no satisfactory classification 
of dynamic isomerides can be made on the basis of the readiness with which 
they undergo change, it is nevertheless possible to distinguish two principal 
groups, of which the second includes the majority of the so-called spon- 
taneous changes. 
These two groups are as follows : 
A. Isomeric changes in which two radicles are interchanged, of which 
neither can be split off alone as an ion. Closely related to this 
group are a number of cases in which a double bond is shifted in 
an unsaturated compound. 
B. Isomeric changes in which a single radicle is transferred which 
is capable in at least one of the isomerides of acting as an ion ; the 
transference is accompanied by a rearrangement of bonds in the 
molecule, 
The fact that changes of these two types usually take place under 
different conditions and in presence of different catalytic agents, goes far 
to show that the distinction is not arbitrary but fundamental. These two 
groups of changes are dealt with in the two following sections. Optical 
inversion, which includes examples from both groups, is discussed under a 
separate heading. 
III. Lsomeriec Changes in which Two Radicles are Interchanged. 
1. Interchange of Radicles in Aliphatic Compounds.—Butlerow’s 
observation that the isodibutylenes are accompanied by isomeric alcohols 
is of importance not only as affording an illustration of this type of 
1 Ber. 1896, 29, 203. 2 Bull. Soc. Chen. 1896, iii. 16, 195. 
3 0. R. 1901, 182, 487. 
