R and S Nomenclature of Asymmetric Carbon Atoms
Any asymmetric carbon has two possible (mirror-image)
spatial arrangements called configurations.
Cahn-Ingold-Prelog Rules
• Assign a relative “priority” to each group bonded to the
asymmetric carbon.
• Consider the first atom of the group bonded to the
asymmetric carbon. Atoms with higher atomic
numbers receive higher priorities.
I > Br > Cl > S > F > O > N > 13C > 12C > Li > 2H > 1H
• In case of ties, use the next atoms along the chain of
each group, e.g. –CH(CH3
)2 > -CH2CH3
• Treat double/triple bonds as if each were a bond to a
separate atom.