ISOMERISM AND THE VISUAL PIGMENTS 



occur. In such cases, if the doubly-bound atoms are also connected 

 to dissimilar atoms or groups, the whole molecule can exist in 

 different non-labile forms. 



An example of isomerism arising from these causes is that of 

 maleic and fumaric acids, 



maleic acid fumaric acid 



In maleic acid the two carboxyl groups lie on the same side of the 

 molecule; in fumaric acid, on opposite sides. The two isomers 

 differ from each other in both physical and chemical properties. Thus, 

 because the carboxyl groups in maleic acid are adjacent, the elements 

 of water can be readily stripped from the molecule, e.g. by heating, 

 with the formation of a stable anhydride. But no anhydride of 

 fumaric acid is known. 



The type of isomerism typified by maleic and fumaric acid is known 

 as cis-trans isomerism. Compounds in which similar groups are on 

 the same side of the molecule are called cis forms ; those in which 

 they are on opposite sides being called trans forms. 



Although the most famihar examples of cis-trans isomers involve 

 orientation about a double bond, cis-trans isomerism can also occur 

 about a single bond, provided free rotation about that bond is 

 prevented — as, for example, when the atoms form part of a rigid ring 

 structure. 



CIS-TRANS ISOMERISM IN COMPOUNDS CONTAINING MORE THAN 

 ONE DOUBLE BOND 



When the molecule contains more than one double bond — for 

 example, in substances such as the vitamins A, the retinenes and 

 the diphenylpolyenes — cis-trans isomerism may give rise to a 



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