i6o CAROTENOID HYDROCARBONS OF KNOWN CONSTITUTION X 



are formulated as 5 : 6-dihydro-a-carotene and 5 : 6-dihydro-/3-carotene, re- 

 spectively. It is of interest that both these hydrogenation products are obtained 

 from a-carotene as well as /3-carotene. 



Under the usual conditions of isomerisation (cf. p. 39) both dihydrocaro- 

 tenes undergo reversible cis-trans isomerisation. The absorption spectra 

 exhibit the characteristic "as-peak", 



\V \V 



C CHo CHo CHo CHo C 



/\ I I I \ /\ 



CH2 CH-CH=CH-C=CHCH=CH-C=CHCH=CHCH=C-CH=CHCH=C-CH=CH-C CH, 



I I II I " 



CHj CH"CH3 HsC'C CHj 



\ / 5 :6-Dihydro-^-carotene \ / 



CH2 CHj 



CHo CHo CHo CHo 



\V . \V 



C GHo CHo CHo CHo C 



/\ r I I I /\ 



CH2 CH-CH=CH-C=CHCH=CH-C=CHCH=CHCH=C-CH=CHCH=C-CH=CH-CH CHj 



CH2 CH'CH3 H3C*0 CH2 



\ / 5:6-Dihydro-a-carotene "%/ 



CHj CH 



5 : 6-Dihydro-/?-carotene crystallises from mixtures of carbon disulphide and 

 ethanol, benzene and methanol, or chloroform and methanol, in characteristic 

 plates or wedges. The highest m.p. which has been recorded is 164°, but in a 

 number of cases it was considerably lower, e.g. 155 and 160°. The solubility of 

 dihydro-y5-carotene and its behaviour in the partition test corresponds to that 

 of /3-carotene. In the chromatogram the pigment is adsorbed below /9-caro- 

 tene, but above 5 : 6-dihydro-a-carotene. 



Solvent: Absorption maxima: 



Carbon disulphide 509.5 476 m/n 



Benzene 489 458 m/j. 



Chloroform 489 457 m/i 



Petroleum ether 477.5 447.5 m^ 



Ethanol 477.5 448 m/n 



5 : 6-Dihydro-a-carotene crystallises from a mixture of carbon disulphide 

 and ethanol in microscopic rectangular yellow leaflets (cf . the original communi- 

 cation). From a mixture of benzene and methanol it is obtained in crystals 

 which resemble those of natural a-carotene, and melt at 202-203° (with 

 previous sintering). 5 : 6-Dihydro-a-carotene is somewhat more soluble than 

 5 : 6-dihydro-jS-carotene. 

 References p. iG^-ijo. 



