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| R and S Notation of Optical Isomers |
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| The scheme of representing optical isomers using the prefix R (derived from Greek word Rectus) and prefix S (derived from Greek Sinister) was proposed by R.S.Cahn, C.K.Ingold and V.Prelog. |
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| Step 1 |
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| Priorities are assigned to the four different groups or atoms attached to the chiral carbon atom. One such rule is based on atomic numbers. An atom with higher atomic number gets higher priority. Foe e.g., in bromochloroiodomethane the sequence of priority is iodine, bromine, chlorine and hydrogen. |
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| Step 2 |
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| The group of lowest priority is directed away from us and arrangement of other groups is observed. If our eye travels in a clockwise direction when moving from the group of highest priority to the group of 2nd priority and then to the 3rd priority group, the configuration is R. For e.g., the configuration of bromochloroiodomethane given below is R, because when our eye travels from iodine (highest priority) to hydrogen (lowest priority) it follows in a clockwise direction. Similarly the mirror image of the R configuration has S configuration. The eye moves in an anti clockwise |
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| direction while moving from iodine to hydrogen. The racemic modification is specified by the prefix RS. For e.g., racemic lactic acid is written as (RS) lactic acid. |
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| Note: R and S configuration does not give any idea of optical rotation. |
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