90 MOLECULAR STRUCTURE 



question whether a subsequent process comes into play, or 

 whether the displacement occurs during the reaction itself. 

 We may conclude hence, for the practice of determining 

 constitutions, that a formula arrived at only in one way is 

 uncertain, but that as such inversions are rare, when two 

 processes yield the same result with regard to constitution, 

 the uncertainty is small. The conclusion is strengthened 

 when one process is one of formation, the other of 

 decomposition, since then the possibility of the same 

 subsequent change influencing the reaction is excluded. 

 In this way the above case of pinacolin was decided ; its 

 decomposition on oxidation with formation of trimethyl- 

 acetic acid, C(CH 3 ) 3 COOH, together with formic acid, 



C 6 H ]2 O + 3 O = C(CH 3 ) 3 COOH + HCOOH, 



has led to the adoption of the formula 

 (CH 3 ) 3 CCOCH 3 . 



The result would be still more conclusive if the existence 

 of an isomer rendered impossible the choice of a second 

 formula, i. e. in the case considered, if the body 



(CH 3 ) 2 C-C(CH 3 ) 2 



O 



were known and turned out to be different from pinacolin. 

 Hence the case quoted on p. 86 of methyl mustard oil and 

 methyl thiocyanate affords the greatest certainty, for 

 formation and decomposition yield the same formula, and 

 the other constitutional formula suits the mode of formation 

 and decomposition of an isomeric compound. 



2. DETEKMINATION OF CONFIGUKATION. 

 (Stereochemistry.) 



Whilst the study of constitution is restricted to 

 determining the internal relations or bonds of the atoms in 

 the molecule, the higher problem of finding the relative 



