234 CAROTINOIDS AND RELATED PIGMENTS 



Arnaud (1886) which would correspond to the formula C 10 H 56 I 3 , ac- 

 cording to our present accepted composition of the hydrocarbon. This 

 iodide is formed by adding iodine to a petroleum ether solution of caro- 

 tin crystals in less than the required amount to combine with all the 

 carotin. The iodide crystallizes out as dark violet leaflets with a cop- 

 per colored reflection, which melt sharply at 136-137 C. Willstiit- 

 ter and Escher obtained the same iodide by adding double the amount 

 of iodine crystals to benzene-carbon disulfide and carbon disulfide- 

 ether solutions of carotin. The other iodide, described by Willstatter 

 and Mieg and also by Willstatter and Escher (1910), corresponds to 

 the formula C 40 H 5(i I 2 , and is prepared by adding crystalline iodine to 

 an ether solution of carotin in an amount equal to only one-third the 

 weight of carotin present. The form and color of these crystals are 

 the same as those of the tri-iodide, but differ from it by showing no 

 definite melting point, the crystals slowly decomposing between 140 

 and 170 C. 



The analyses which have been made of the two iodides, one contain- 

 ing two atoms and the other three atoms of the halogen, show ex- 

 cellent correspondence with the theoretical amount of iodine in com- 

 pounds showing this composition. It is not clear, however, just what 

 structure of the carotin molecule would permit the formation of an 

 iodide containing three atoms of iodine. 



Carotin also forms a bromine derivative which is at once both an ad- 

 dition and a substitution product, which conforms with the constitution 

 C 40 H 36 Br 22 . The formation of this product is stated by Willstatter 

 and Escher to take place on the addition of 0.5 grams of powdered 

 carotin in small portions with shaking to 16 grams of bromine at C., 

 the bromine being protected from moisture in the reaction flask by a 

 CaCl 2 tube. The reaction is completed by standing at room tempera- 

 ture and the precipitate filtered off on glass wool and washed with hot 

 anhydrous formic acid, giving a brittle colorless product without defi- 

 nite crystalline form. The bromide has no melting point but de- 

 composes at about 171 -174 C. It dissolves easily in benzene and 

 carbon disulfide, fairly easily in ether, but with difficulty in even hot 

 alcohol or petroleum ether; it is insoluble in glacial acetic and an- 

 hydrous formic acid. The bromine cannot be completely removed with 

 zinc dust and glacial acetic acid, or with silver acetate. 



The structure of the carotin molecule has proved to be a difficult 

 problem to solve. Escher's (1910) attempt resulted only in the pro- 

 duction of amorphous products of high molecular weight. One nat- 



