Prof.Dr. Meral Tunçbilek
ISOMERISM IN DRUG
The Cahn Ingold Prelog (CIP) System For Naming Chiral Centers
A solution to this quandary was proposed by Robert Cahn, Chris Ingold, and Vladimir Prelog in 1966. The resulting “CIP” protocol works as follows:
1. Prioritize the four groups around a chiral center according to
atomic number. The highest atomic number is assigned priority
#1, and the lowest atomic number is assigned priority #4.
2. Orient the chiral centre such that the #4 priority substituent is pointing away from the viewer. For our purposes, it’s enough for it merely to be attached to a “dashed” bond.
3. Trace the path of priorities #1, #2 and #3. (For this part you ignore #4).
1. If the path traced from 1-2-3 is clockwise, the chiral center is assigned (R) (from Latin, rectus)
2. If the path traced is counter clockwise, the chiral center is assigned (S) (from the Latin sinister)
3. Now we have a better way to describe molecules [A] and [B] shown above. Molecule [A] is named (R)-1-bromo-1-chloroethane, and molecule [B] is named (S)-1-bromo-1-chloroethane:
* -SH, -SCH3
* -OH, -OCH3, -OCH2CH3, -OCOCH3
* -NH2, -NHCH3, -N(CH3)2, -NO2
* -CH3, -C2H5, -C3H7, CH(CH3)CH2CH3, -C(CH3)=CH2, -C(CH3)2CH2CH3, -C6H5,
We should reiterate that the designations (R) and (S) bear no relationship to whether a molecule rotates
plane-polarized light clockwise (+) or
counterclockwise (-). For example the most common naturally occurring configuration of the amino acid alanine is (S), but its optical rotation (in aqueous acid solution) is (+).
• What is common in the writing of enantiomers is that both the desired absolute configuration (R/S) and the optical rotation (+/-) are reported.
(+)(R)-When there are more than one asymmetric carbon in the structure: 3S, 5R
