Скачать или смотреть Stereochemistry: Helical and Axial Chirality

Some chiral molecules don’t have a stereogenic centre, for example a helix. A helix can be viewed down the axis and of the four substituents that create the helix the nearer two are assessed for priority just between themselves. Then the further two groups are evaluated for priority again just between themselves. Now we calculate the path from highest priority near to highest priority far, if clockwise its called P for plus and if anti-clockwise it’s M fro minus. P and M are helical descriptors. Some molecules that are helical chiral can also have axial chirality, these include biphenyls, spiranes and allenes. Descriptors for axial chirality are different and do not use P and M. It uses a methodology similar to stereogenic centres with one addition, nearer substituents are always a higher priority than the farther substituents. The priority for all four substituents are assigned together. The molecule is viewed with the lowest priority furthest from the viewer, if going from highest priority to third highest priority is a clockwise movement it’s aR and if anti-clockwise it’s aS where the a is optional and tells you it refers to axial chirality. Where the chirality is both helical and axial M and aR then P and aS are associated with each other. Sometimes a molecule has helical chirality but not axial chirality in such molecules as helicenes. Start at the nearer part of the structure and move towards the further part of the structure is this movement is clockwise it’s P and if anti-clockwise it’s M. All these systems have non-superimposable mirror image alternatives so have enantiomers.