Sequence Rules

Sequence Rules

It is the spatial arrangement of atoms within the chiral molecules that is described in terms of absolute configuration (R) and stereochemistry (S). The Sequence Rules were developed by Cahn, Ingold, and Prelog to assign priority orders to the atoms or groups directly attached to stereocenters. These rules determine whether a stereocenter's absolute configuration is (R) or (S).

Rule 1 - The atom with the highest atomic number receives priority over the atom with the lowest atomic number

For example,

I > Br > Cl > F >O > N > C > H.

Rule 2 - Priority is given to isotopes with higher atomic weights.

For example,

3H (tritium) > 2H (deuterium) > 1H (hydrogen)

Rule 3 - Whenever two or more atoms are directly attached to the same stereocenter, the next atom on the chain determines the priority order.

For example,

–CO2CH3 > –CO2H > CONH2 > COCH3 > CHO > CH2OH

Rule 4 - The amount of a double bond taken into account when an atom is single bonded to two atoms is the same as if it were a single bond. In the case of a triple bond between two atoms, treat this atom as if it were 13 bonded to a single atom. To order the priority of the unsaturated groups attached to the stereocenter, convert the unsaturated groups into saturated groups, e.g.,



The numbers should be assigned to functional groups in priority order based on the above sequence rules. The atom/group with the lowest priority should be viewed with maximum space separation from the center of the generic tetrahedral molecule. The decreasing order (R) corresponds to a clockwise decreasing order while the decreasing order (S) corresponds to an anti-clockwise decreasing order.


Rule 5 - There may not be a difference between a smaller group and a longer one in priority.

For example,

The priority of –CH2Cl is greater than that of –CH2CH2CH2CH3.

Rule 6 - Assign the lowest priority group at the front of the molecule, when it is facing the plane. S is anticlockwise, and R is clockwise.



Rule 7 - The stereocenter of the whole molecule can be defined by using a number that identifies the stereocenter preceding configuration (e.g., 1R, 4S) when there are multiple chiral carbons.

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