ISOMERISM AND THE VISUAL PIGMENTS 



branched molecules ; for example, when one or more of the hydro- 

 gens in the ethylenic groups — CH=CH — are replaced by bulkier 

 groups such as methyl ( — CHg). 



The mutual interference between different parts of the same mole- 

 cule is called steric hindrance. It complicates the theory of isomerism. 

 Sometimes steric hindrance, through preventing free rotation about 

 a single bond, 'locks' what would normally be interchangeable forms 

 into non-labile isomers. For example, terphenyl derivatives which 

 are substituted in positions ortho to the pivot bonds joining the 

 phenyl groups and which are unsymmetrically substituted in the 

 terminal phenyl groups can exist in cis-trans forms (Stanley and 

 ADAMS, 1929). 



cis trans 



Independent rotation about the pivot bonds is, in these cases, 

 prevented by the steric hindrance of the A and X, and A and Y 

 groups. 



In other molecules steric hindrance may result in a reduction of 

 the number of cis-trans isomers. As an example we may consider 

 vitamin A^, which has two methyl groups attached to the conjugated 

 chain. 



Since vitamin Aj possesses four ethylenic groups (the double bonds 

 of which are marked 2, 3, 4 and 5 in the formula above) 2*, that is, 

 sixteen cis-trans forms are theoretically possible. Not all these forms 

 are realizable, however. 



129 



