Cahn Ingold Prelog Priority rules - Essential CHEM TUTE Rules PDF

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Summary

Notes to help with Tutorial 2 in week 4/5...

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Method of Ranking Groups (Cahn Ingold Prelog rules) The ranking of a group is based on the atomic number of the atom used to attach the group: For example: in the following molecule, concentrating on the central carbon The 4 groups attached to that carbon are CH3

Cl

Cl

CH2

H

H

O

C H3C

O

CH3

H C

CH3 H

These are the atoms attached to the carbon The higher the atomic number of these atoms, the higher the ranking so Cl (17) > O (8) > C (6) > H (1)

CH3 3

Cl 1 CH2 C 2

H3C

H 4 O

When the attached atoms are the same, then you move down the chain of atoms to the first point of difference, then apply the atomic number rule to the atoms of first point of difference For example:

H2 C

H2 C F

NH2

The first point of difference between these two groups is the N or F attached to the CH2 F (9) > N(7) so

H2 C

> F

H2 C NH2

A

B

H2 C

The first point of difference between these two groups is the atoms attached to the second CH3 carbon In A it is (H,H,H) In B it is (C,H,H) The carbon of the set from B is higher =>

H2 C CH3

C H2

H2 C

H2 C

CH3

>

C H2 A

B

CH3 C H

The first point of difference between these two groups is the atoms attached to the first carbon CH3 In A it is (C,C,H) In B it is (C,H,H) The carbon of the set from A is higher =>

H2 C C H2

CH3

CH3

CH3 C H

H2 C

>

CH3 C H2

CH3

To summary first point of difference Chain with branching

Ranked higher than

Rank chains with branching based on 1. Position of branching 2. Amount of branching 3. Length of chain

chain with no branching Rank chains without branching based on 1. Length of chain

Example:

H2 C

CH3 C H

CH3

H2 C

CH3 C

>

>

Has branching

Most number of branches

H2 C

H2 C

H2 C C H2

C H2

Longer chain with no branching

CH3

H C CH3

CH3

Branching on lowest carbon no.

>

CH3

H2 C

H2 C CH3

>

C H2

CH3...