44 METHODS OF ELUCIDATING CONSTITUTION VI 



hydrogenated in suspension; under these conditions the reaction takes con- 

 siderably longer than usual. Another characteristic feature of the hydrogenation 

 of carotenoids is that a relatively large amount of catalyst is required for 

 complete reduction. 



The microhydrogenation of polyene pigments described by KuHN and 

 MoELLER^ is extremely valuable in view of the small amounts of material 

 required. Using this method, numerous carotenoids which occur in only very 

 small quantities in nature have been examined for the number of double 

 bonds present in the molecule. Details regarding the apparatus and reagents 

 required will be found in the memoir already cited*. 



It was shown by Zechmeister and Tuzson^ that polyenes add bromine in 

 chloroform solution. The titration method based on this reaction has the dis- 

 advantage, however, that not all double bonds participate. Thus, according 

 to Zechmeister, carotene and xanthophyll absorb only 8 molecules of bromine 

 instead of ii. A more suitable reagent, which in most cases reacts with all the 

 double bonds present, is iodine chloride as described by Pummerer and Reb- 

 MANN, and Pummerer, Rebmann and Reindel^. 



According to the last-named authors^, the double bonds can also be saturated 

 by the addition of oxygen. In practice, this method consists of reacting the 

 carotenoid with perbenzoic acid in chloroform solution and back-titrating the 

 excess perbenzoic acid after oxidation is complete. However, oxidation with 

 perbenzoic acid again does not always result in the reaction of all double bonds 

 present. Thus, the only completely reliable method of determining the number 

 of double bonds in a carotenoid is catalytic hydrogenation. 



2. DETERMINATION OF SIDE-CHAIN METHYL GROUPS 



The first method employed for the determination of side-chain methyl 

 groups is due to Kuhn, Winterstein and Karlovitz'' and consists of oxidation 

 by means of potassium permanganate in alkaline solution. Under these condi- 

 tions a side-chain methyl group and the carbon atom to which it is attached 

 give rise to one molecule of acetic acid. This method was later replaced by the 

 oxidation with chromic acid, which was found to be more reliable^. 



Karrer, Helfenstein, Wehrli and Wettstein^ showed that oxidation 

 by means of alkaline permanganate only degrades unsaturated groupings of 

 the type =CH-C= 



CH3 



whereas more saturated groupings such as 



— CH2— C= 



CH3 

 References p. §1-^2. 



