Absolute Configuration of Allenes and Biphenyls

In order to describe the absolute configuration of chiral allenes, the Cahn-Ingold-Prelog (CIP) rules are first used to ascertain the priorities at the each terminal carbon, as is done for alkenes at this point the similarity with alkenes ends. The method will be demonstrated with the trisubstituted allene 14a  

Look along the three-carbon allene unit from one end ( either end can be chosen), and put uppermost the higher priority group at the near end. Initially we look from the left in this example, which already has the front substituents in a vertical plane. 

Represent 14a as a projection 15, with the near substituents vertical, and the back substituents horizontal. 

The front substituents, prioritized, take precedence over the back substituents, themselves prioritized. This gives Br Me C02H H. 

The last step is an adaptation of the original protocol (see Cahn3) and the method can also be conveniently applied to chiral biphenyls. 

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