4 DETERMIN ATION OF HYDROXYL GROUPS 45 



are only incompletely oxidised to acetic acid or not attacked. A micro-method 

 for the determination of side-chain methyl groups has been described by KuHN 

 and RothI"^. 



3. DETERMIXATIOX OF 7S0PR0P YLIDENE GROUPS 



For the determination of zsopropylidene groups (CHaJgC^C . . , a method 

 introduced by Karrer, Helfenstein, Pieper and Wettsteix^^ and somewhat 

 modified by Kuhn and Roth^^^ fg used. 



The z'sopropylidene grouping is degraded by ozonisation to acetone which 

 is determined iodometrically. The micro-determination of Kuhn and Roth 

 depends on the same principle, but ozonisation is followed by oxidation with 

 permanganate, to improve the yield of acetone. 



4. determination of HYDROXYL GROUPS 



By determination of active hydrogen by the method of Zerewitinoff, it 

 was established by Karrer, Helfenstein and Wehrli^^ that the oxygen atoms 

 present in xanthophyll are not present in the form of ether groupings as had 

 previously been assumed, but as hydroxyl groups. An apparatus for the 

 Zerewitinoff determination has been developed by Flaschentrager^^ and 

 a somewhat modified procedure has been described by Roth^^. The following 

 empirical facts must be taken into account in interpreting the results of 

 Zerewitinoff determinations : If several (4-6) hydroxyl groups are present in 

 the polyene molecule, they may not all react. In the presence of two hydroxyl 

 groups, on the other hand, somewhat high values are sometimes obtained^®. 

 Ketones which have a tendency to enolise also give rise to the production 

 of methane, and thus simulate hydroxyl groups^'. 



The determination of the position of the hydroxyl groups in the polyene 

 molecule is more difficult than the determination of their number. In this 

 connection, compare the investigations discussed on page 201. A clue to the 

 position of the hydroxyl groups is often provided by the absence of certain 

 oxidation products. 



The question as to whether two hydroxyl groups are present in neighbouring 

 positions can sometimes be decided by the method of Criegee^^, e.g. in the 

 case of azafrin (cf. page 282). 



5. determination of methoxyl groups 



Up to the present time, rhodoviolascin is the only known naturally occurring 

 carotenoid containing methoxyl groups. These can be determined in the usual 

 way by the method of Zeisel^^. 

 References p. 51-52. 



