-l.VD Ml'.TIinDX or IDENTIFICATION 235 



urally wonders whether any of the known compounds of the same 

 empirical formula hear any relation to carotin, hut it must be admitted 

 that they throw no light on its constitution. The simplest empirical 

 formula for carotin, i.e., (C r ,H 7 ) 8 suggests a possible relation to the 

 trrpene derivatives, the cymenes, or to the toluene derivatives, durcne 

 and the propyl toluenes, all of which have the empirical formula 

 (\,,H 14 . The cymenes. however, are structurally isopropyl benzene-, 

 and durene is tetramethyl benzene. The former is a colorless oil and 

 the latter a colorless solid, m.p., 79-80 C. There seems to be no rea- 

 son for believing that carotin is related in any way to these substances 

 or to the propyl toluenes. 



The problem of the structure of carotin is of special interest because 

 pigmented hydrocarbons are something of a novelty. Those which 

 have been described will be mentioned briefly, inasmuch as their con- 

 stitution at least indicates the probably chromatophor group in caro- 

 tin, namely, >C:C<. 



Apparently the first pigmented hydrocarbon to be mentioned in the 

 literature is acenaphtylene, C 12 H 8 , probably having the structure 



It was first described by Behr and van Dorp (1873) and 



HC:CH 



Blumenthal (1874) and later by Graebe (1893). It forms leaflets 

 of a golden yellow color, soluble in alcohol, ether and benzene, which 

 melt at 92-93 C. De la Harpe and van Dorp (1875) and later 

 Graebe (1892) have described the hydrocarbon di-biphenylenathene, 

 C 26 H 16 , which crystallizes as intensely yellowish red needles and 

 scales, m.p., 187-188 C. (corrected), and forming yellow solutions. 

 According to Graebe this hydrocarbon adds hydrogen readily, going 

 over into the colorless compound C 2B H 1S . Thiele (1900a) found that 

 the hydrocarbon fulven, C 2 H 4 .C:C.CH, forms brilliantly colored de- 

 rivatives. He mentions dimethyl fulvene, C 8 H 10 , a bright orange col- 

 ored oil, methyl phenyl fulvene, C 13 H 12 , a red oil, and diphenyl ful- 

 vene, C 1S H M , which crystallizes from petroleum ether as deep red 

 prisms, m.p., 82 C. Thiele (1900b) has also described the hydro-' 

 carbon cinnamylidenindene, C G H 5 .CH:CH.C CH:CH, which crys- 



V 7 



C, ; H 5 



tallizes as yellowish-red needles, m.p., 190 C., and is easily soluble 

 in most organic solvents. More recently Pummerer (1912) has dis- 



