source : Chemistry Libre Texts
The Cahn- Ingold – Prelog (CIP) sequence rules, named for Robert Sidney Cahn, Crystopher Kelk Ingold & Vladimir Prelog -alternatively known as CIP Rule. The purpose of CIP priority rule is to name a specific stereoisomer of a molecule. Stereoisomers are compounds having same molecular formula and same connectivity of atoms but differ in their three dimentional orientation.
The configuration of the chiral carbon is decided as follow-
The four atoms or groups attached to the chiral carbon are ranked in order of priority , priority is decided by following sequence rules.
(a) Atom of higher atomic number gets priority. Atom of higher priority is assigned no.1 & atom of least priority is assigned no.4.
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(b) If the first atom is the two groups have same atomic number then relative priority of the group is decided by a comparison of the atomic number of the next atom in the two groups.
Ex.1—- In CH3 & –CH2–CH3
In both first atom is Carbon but next atom in –– CH 3 is H , while in ––CH2––CH3 , next atom is ‘C’. So ethyl group is given higher priority than methyl group.
Ex.2 — –CHO is given higher priority than –CN because in group , C is directly attached to oxygen while in –CN group C is directly attached to N. (at. no. of oxygen is greater than N).
(c) Multiple bonds are treated as separate single bonds.
Ex.1—- –CH=O is given priority over –CH2OH
Examples Of CIP rule –
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source : schoolbag.info
source : OChemPal
source : Chemistry for everyone-wordpress.com