The Sequence Rule for Assignment of Configurations to Chiral Centers
Assign sequence priorities to the four substituents by looking at the atoms attached directly to the chiral center.
1. The higher the atomic number of the immediate substituent atom, the higher the priority.
For example, H? < C? < N? < O? < Cl?. (Different isotopes of the same element are assigned a priority according to their atomic mass.)
2. If two substituents have the same immediate substituent atom,
evaluate atoms progressively further away from the chiral center until a difference is found.
For example, CH3? < C2H5? < ClCH2? < BrCH2? < CH3O?.
3. If double or triple bonded groups are encountered as substituents, they are treated as an equivalent set of single-bonded atoms.
For example, C2H5? < CH2=CH? < HC?C?
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