46 



METHODS OF ELUCIDATING CONSTITUTION 



VI 



6. DETECTION AND ESTIMATION OF CARBONYL GROUPS 



The detection of carbonyl groups in polyene pigments often presents difficul- 

 ties as the usual carbonyl reagents (hydroxy lamine, semicarbazide, etc.) do 

 not always react. With some carotenoids, special methods of oximation have to 

 be used*. Others (e.g. capsanthin) cannot be oximated at all by the methods at 

 present available. In such cases the hydroxyl groups are first determined and 

 the Zerewitinoff determination is repeated after complete reduction of the 

 pigment. If the number of hydroxyl groups has increased, the presence of a 

 carbonyl group which has been reduced to a secondary or primary alcohol 

 grouping, is indicated. 



If the carbonyl group is conjugated with the system of conjugated ethylenic 

 bonds, it can be recognised by a strong red shift of the absorption maxima 

 (cf. p. 56). 



TABLE 10 



NATURALLY OCCURRING CAROTENOIDS CONTAINING CARBONYL GROUPS* 



* Only those carotenoids the structure of which has been completely or largely de- 

 termined, are mentioned in this table. Bixin, crocetin and azafrin which contain carboxyl 

 groups have not been included. 



7. determination of CARBOXYL GROUPS 



Carboxyl groups are determined by titration with alkalis. The carotenoid 

 is best hydrogenated before the titration which is then easier to carry out. 

 Details will be found in the papers by Kuhn and co-workers^". 



* The oximation of polyene pigments containing carbonyl groups is usually effected 

 with hydroxylamine acetate. Sometimes it is necessary, however, to employ free hydroxyl- 

 amine. Cf. R. Kuhn and H. Brockmann, Ber. 66 (1933) 828. 



References p. 51-52. 



