a-CAROTENE 

 Chemical Constitution 



153 



CH, CHj 



CH, CH, 



C CH, CH, CH, CH, C 



/\ I I I I /\ 



CH, C-CH=CH-C=CHCH=CH-C«CHCH=CHCH=C-CH=CHCH=C-CH=CH-CH CH, 



CH» C'CHa 



H,C-C 



CH, 



a-Carotene 

 CH, CH 



The elucidation of the constitution of a-carotene has been carried out 

 mainly by Karrer and co-workers^*^. The hydrocarbon contains 11 double 

 bonds^^". The absorption spectrum, the maxima of which are displaced by 12 

 mix towards shorter wavelengths as compared with /9-carotene, suggests that 

 not all double bonds are in conjugation. A definite proof for the formula of 

 a-carotene was given by Karrer, More and Walker^^i ^^]^Q showed that 

 ozonisation of the pure pigment (chromatographed on calcium hydroxide) 

 furnishes small quantities of isogeronic acid, besides geronic acid which is also 

 formed in the oxidation of /^-carotene. 



CH, CH, CH, CH» 



CH, 

 CH. 



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



CH 



This formula is in agreement with the fact that a-carotene is optically 

 active. Further confirmation of this constitution is provided by the complex 

 oxidation products which have recently been prepared by Karrer and co- 

 workers (cf. p. 155). 



Properties 



Crystalline form: a-Carotene separates from a mixture of benzene and 

 methanol in violet prisms and clusters and from petroleum ether in dark violet 

 prisms or polygons. 



References p. 165-170. 



